HUMAN EXPOSURE TO VOLATILE ORGANIC POLLUTANTS ...

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Annu. Rev. Energy Environ. 2001. 26:269–301

Annu. Rev. Energy. Environ. 2001.26:269-301. Downloaded from arjournals.annualreviews.org by Texas A&M University- Corpus Christi on 09/28/05. For personal use only.

HUMAN EXPOSURE TO VOLATILE ORGANIC POLLUTANTS: Implications for Indoor Air Studies∗ Lance A. Wallace US Environmental Protection Agency, Reston, Virginia 20191; e-mail: [email protected]

Key Words personal monitors, breath analysis, indoor air quality, VOC ■ Abstract Over the past 20 years, a new scientific discipline based on direct measurement of human exposure to environmental pollutants has developed. The fundamental principle of the new science is to “measure where the people are.” This has required developing small, lightweight, quiet personal monitors for volatile organic compounds and other pollutants. A second principle has been to measure body burden, particularly exhaled breath, whenever possible to determine the relationship between exposure and dose. Studies employing the new monitors and breath measurements have overturned accepted ideas about the sources of most volatile organic pollutants. The main sources turn out surprisingly often to be small, close to the person, and completely unregulated. These findings should result in major changes in our approach to environmental regulation; however, powerful forces of resistance would need to be overcome. CONTENTS INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PRINCIPLES OF MEASURING HUMAN EXPOSURE . . . . . . . . . . . . . . . . . . . . . . METHODS OF MEASURING HUMAN EXPOSURE . . . . . . . . . . . . . . . . . . . . . . . . Sampling Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of Sampling Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . METHODS OF MEASURING BODY BURDEN . . . . . . . . . . . . . . . . . . . . . . . . . . . . Breath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Blood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mothers’ Milk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dermal Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAJOR FINDINGS OF HUMAN EXPOSURE STUDIES . . . . . . . . . . . . . . . . . . . . SICK BUILDING SYNDROME, MULTIPLE CHEMICAL SENSITIVITY, AND IRRITATION POTENTIAL . . . . . . . . . . . . . . . . . . . . . . . . . . . VOCS OF RECENT INTEREST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MTBE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

270 270 273 273 277 277 277 282 282 282 283 284 287 290 290

∗

The US government has the right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper.

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WALLACE Disinfection By-Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291


INTRODUCTION The study of human exposure to environmental pollutants has only recently attained the status of a separate scientific discipline (1–3). An International Society of Exposure Analysis has been created, and its official journal (the Journal of Exposure Analysis and Environmental Epidemiology) recently completed its tenth year of publication. Therefore it seems timely to summarize the major advances made over the past two decades in measuring human exposure. Although total human exposure includes inhalation, ingestion, and dermal uptake, this paper focuses mainly on inhalation exposures to volatile organic compounds (VOC). Also, although considerable work has been done recently on indoor air quality models, including terms for source emissions and sinks, models are not dealt with in this paper, which focuses on measurement methods and their use in major studies of human exposure.

PRINCIPLES OF MEASURING HUMAN EXPOSURE The central principle of measuring human exposure is easily stated: Measure where the people are. Although obvious, this principle was universally ignored for years. The US Environmental Protection Agency (EPA), for example, from its inception in 1970, measured air on the tops of buildings or in industrial smokestacks and measured water in streams or discharge pipes. A National Academy of Sciences publication (4) first pointed out the lack of any monitoring of people in 1977. To the Agency’s credit, it took the Academy recommendation seriously, sponsoring a workshop on personal air quality monitors. Later the EPA developed some personal air quality monitors (5), and ultimately, spurred on by Congress, it undertook a pioneering set of studies, known as the TEAM studies (for total exposure assessment methodology), that measured human exposure directly for some thousands of people representing several million residents of a dozen or so US cities and towns (6–10). The reason this principle is so powerful may be illustrated by the example of benzene. For many years, it was believed that benzene exposures would be greatest in areas of concentrated petroleum refining, petrochemical manufacture, and petroleum storage tanks. Calculations of the tons of benzene emitted showed that areas such as northern New Jersey, the Gulf Coast, the Houston ship channel, and Los Angeles have far greater emissions, and therefore should have far greater concentrations in ambient air, than most other regions of the country. However, when the first TEAM study compared exposures of 150 persons living close to the heavy petroleum refining and petrochemical areas of Bayonne-Elizabeth in northern New Jersey with those of 150 people living well away from these areas,

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no difference in exposures could be found (11). In fact, the exposures from indoor air were two to three times greater than exposures from outdoor air sources. And measurements of exhaled breath showed that smokers had levels of benzene in their blood six to ten times that of nonsmokers. Even nonsmokers living with a smoker were receiving more benzene from that source than from the major industrial sources just down the street. When the total amount of benzene emitted by cigarettes is compared with the total amount emitted by stationary sources, the ratio is about 1 to 150 (Figure 1a). Yet cigarettes produce far more exposure for smokers (about 90% of their annual total) and even outweigh all stationary sources for passive smokers (Figure 1b). The reason for this is the greater efficiency of delivery. The 50 µg of benzene

Figure 1a and b Benzene emissions are dominated by automobiles and industry, but benzene exposures are dominated by active smoking (35). Even passive smoking [environmental tobacco smoke (ETS)] produces more exposure to benzene than all stationary industry in the United States.

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in mainstream smoke (12) are delivered to the smoker with little lost, and the 400 µg in sidestream smoke (13) are diluted in a room or in the volume of a house (50–400 m3) rather than in the expanse of atmosphere between a plant or storage tank and the nearest house. These somewhat amazing facts lay undiscovered for years while complicated models were created to prove that nonoccupational exposures would be greatest in petroleum refining and petrochemical areas. Only measurements of personal exposure, using both personal monitors and measurements of exhaled breath, were able to pierce this veil of ignorance. Once it is understood that exposure is not necessarily closely related to emissions, it becomes clear that reliance on such emissions inventories as the Toxics Release Inventory to inform people of their potential exposures could be, and often is, profoundly misleading (14, 15). Similarly, providing pollution credits by emissions trading may have little relevance to human exposure, as shown repeatedly by Smith et al. (16, 17). Instead, greatly increased efficiency in reducing exposure could be obtained by a process of exposure trading (18). A second principle of human exposure studies is to use population-based probability samples whenever possible (19). It is an unfortunate corollary of the generally low level of funding for environmental studies that the great majority of such studies use groups of subjects that are both too small and too unrepresentative to be widely applicable. It may be recognized by social and behavioral scientists that this principle, which underlies polling and surveys of all types, is rarely applied in environmental studies. This is a fundamental contribution of the new discipline of human exposure: the recognition that physical/chemical methods must be supplemented by sociobehavioral methods if an adequate characterization of human exposure is to be made. Besides the TEAM studies, which were funded at the million-dollar level by government, there have been only a few other studies combining measurement of human exposure with population-based selection of participants (Table 1) (20–33). A few of the more recent studies have been funded by private sector companies, a promising indication that the relevance of such studies is being recognized. A third principle is to engage the participant as a recorder of his activities. This is generally done by means of a questionnaire administered by the investigator or a diary kept by the participant while going about his or her daily activities. (An exception occurs when the participant is unable to record his activities, as for example when the participant is a child or is engaged in an attention-requiring activity, such as driving a truck or bus.) In such cases, automatic or manual videotaping has been employed. The activities and times recorded on the questionnaire or diary can then be correlated with measured exposures. In some cases, these analyses will lead to identification and sometimes quantification of sources of exposure. In another example involving benzene, the participants in the first TEAM study provided estimates of the time spent driving a car. Even though only 12-h integrated samples were collected, the regression of benzene exposures on time spent in a car suggested that such exposures were in the range of 40–50 µg/m3 (11). Shortly thereafter,

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a study of benzene exposures of 200 California commuters found an average exposure of 13 ppb (40 µg/m3) while driving to and from work in the San Francisco Bay region (34). Because the activities of people have been investigated mainly in terms of socioeconomic questions (e.g., extent of time watching television), with questions of interest for human exposure sometimes omitted, a new exposure-centered activity pattern survey was instituted by the EPA and also by the Air Resources Board, State of California, in the mid-1980s. The EPA National Human Activity Pattern Study (35) obtained information from some 9300 persons from the 48 contiguous states. This information continues to be widely used in estimating exposure based on Monte Carlo analyses of time spent in various microenvironments (e.g., the kitchen, the shower) with specific sources. The California study extended the EPA study to 1762 adults in California (36). The fourth and last main principle is to employ body burden measurements whenever possible. These measurements act as a check on the completeness of the concurrent exposure measurements. For example, if the level in the body is greater than expected on the basis of the exposure measurements, it may be that an unsuspected route of exposure is responsible. Exactly this effect was observed in the first TEAM study, when benzene in the exhaled breath of smokers was found to be consistently six to ten times greater than in nonsmokers (37). (Because the personal monitor was unable to measure mainstream smoke, it was not possible to estimate the impact of smoking without the body burden measurement.) Of course, the measurement of body burden requires knowledge of the distribution and residence times in the various body compartments of the target chemicals— thus pharmacokinetics is another discipline that must be wedded to the disciplines of the engineer, physicist, chemist, and behavioral scientist to create the new scientific discipline of measuring human exposure.

METHODS OF MEASURING HUMAN EXPOSURE If the fundamental principle of human exposure studies is to measure where people are, it immediately follows that the method of choice is the personal monitor. Although a bulky, noisy, 6-ft-tall, several-ton sampler is perfectly okay for rooftop sampling, a personal monitor must meet stringent requirements of size, weight, noise, and battery power. A number of approaches have been tried and are summarized below.

Sampling Methods ACTIVATED CHARCOAL Historically, the measurement method of choice for occupational exposures (generally 1–100 ppm) for a given VOC has been to pump a sample of air across a sorbent (usually activated charcoal) in order to concentrate the VOCs. They are then recovered by a solvent, such as carbon disulfide. It is interesting that the focus from the beginning in occupational sampling was to measure

Place Beaumont, TX; RTP, NC Bayonne-Elizabeth, NJ; RTP, NC Bayonne-Elizabeth, NJ Antioch-Pittsburg, CA; Los Angeles, CA Greensboro, NC Devils Lake, ND Baltimore, MD Los Angeles, CA Elizabeth, NJ Washington, DC Denver, CO

TEAM

TEAM pilot

TEAM

TEAM

TEAM

TEAM

TEAM

TEAM

TEAM

TEAM

TEAM

1983

1983

Personal air, outdoor air

Personal air, outdoor air

Personal air, indoor air, outdoor air, drinking water



Personal air, outdoor air, drinking water

CO

CO

32 VOCs

32 VOCs

25 VOCs

25 VOCs

25 VOCs

32 VOCs

25 VOCs

Breath

Breath

Breath

Breath

Breath

Breath

Breath

Breath

Breath

Breath, hair, blood, urine

Breath

Biological media

450

812

11

50

150

25

25

198

355

9, 3

11, 6

Number of persons

7

7

25

24

23

6

6

6, 22

6

21

20

References

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1987

1987

1987

1982




25 VOCs, 6 metals,24 pesticides, 12 PAHs

25 VOCs

Chemicals

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1984

1981–1983

Personal air; outdoor air; drinking water, dust

Personal air

Environmental media

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1979

Year

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TABLE 1 Probability-based studies of human exposurea


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West Germany Germany US upper midwest Arizona Baltimore Toronto

GerES I

GerES II

NHEXAS

NHEXAS

NHEXAS

MMT Study

Personal air, indoor air, outdoor air

None

Blood, urine, hair

Blood, urine, hair

Blood, urine, hair

750

60

179

249

4287

488

178

18

50

33

32

31

30

29

28

9

27

26

8

8

TEAM, total exposure assessment methodology study; RTP, Research Triangle Park, NC; VOC, volatile organic compounds; PAH, polyaromatic hydrocarbons; PM, particulate matter; PTEAM, Particle Team; GerES, German Environmental Survey; NHEXAS, National Human Exposure Assessment Survey; MMT, methylcyclopentadienyl manganese tricarbonyl.

PM-2.5, PM-10, 4 metals including Mn

Metals, pesticides

VOCs, metals, pesticides

VOCs, metals, pesticides

Blood, urine, hair

Blood, urine, hair

None

None

None

150

100

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1996—1997 Personal air, indoor air, outdoor air

1995—1997 Personal air, indoor air, dust, drinking water, food



74 VOCs, metals

57 VOCs, metals

PM-10, PM-2.5, 14 metals, 12 PAHs, phthalate esters

PM-10, PM-2.5, 14 metals

Benzene

None

None

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1990—1992 Personal air, dust, food, drinking water

1985—1986 Indoor air, dust, food, drinking water

1990

Personal air, indoor air, outdoor air

32 pesticides

32 pesticides

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a

Riverside, CA

PTEAM

1989

1986

Azusa, CA

Valdez, AK

Personal air, outdoor air, indoor air, dust Personal air, outdoor air

Personal air, outdoor air, indoor air, dust

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PTEAM

1984

Jacksonville, FL

TEAM

1984

Springfield, MA

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where the person (worker) is. Probably one reason this was not adopted from the beginning in environmental sampling was the inability to sample environmental concentrations, which are typically three to four orders of magnitude lower than occupational concentrations. In the early 1980s, passive badges employing activated charcoal were developed for use in occupational sampling. The badges operate on the principle of diffusion and are often operated over an 8-h workday to provide an integrated average exposure for comparison to the occupational standards (e.g., the threshold limit value). Unfortunately, the manufacturing process for these badges leaves residues of VOCs on the activated carbon that make the badges unsuitable for short-term sampling at environmental concentrations, which are usually at parts-per-billion levels. However, the high background contamination on the badges can be overcome by extending the time of sampling to a week or more, and several studies of indoor air pollution have adopted this technique (38). Recently, attention has been refocused on employing these badges for environmental sampling. TENAX The background problems associated with activated charcoal, as well as problems in obtaining reliable recoveries of sorbed chemicals, led to a search for a more suitable sorbent. A polymer known as Tenax was widely adopted during the 1970s as a more reliable sorbent than charcoal for parts-per-billion levels (39). Tenax, properly cleaned, has low background contamination. It is also stable at temperatures up to 250◦ C, allowing thermal desorption instead of solvent desorption. (Solvent desorption involves a redilution of the VOCs, thus partially negating the concentration made possible by the sorbent.) Although expensive, Tenax can be reused many times. Drawbacks include artifact formation of several chemicals (e.g., benzaldehyde, phenol) and an inability to retain very volatile organic chemicals (e.g., vinyl chloride, methylene chloride). Although most uses of Tenax sorbent have been with active (pumped) samplers, a passive badge containing Tenax has also been developed (40). MULTISORBENT SYSTEMS In the late 1980s, attempts were made to combine the best attributes of charcoal and Tenax into a multisorbent system. Newer types of activated charcoal (Spherocarb and Carbosieve) were developed to provide more reliable recoveries. Tandem systems employing Tenax as the first sorbent and activated charcoal as the second, or backup, sorbent were employed. The Tenax collected the bulk of the VOCs, and the activated charcoal collected those more volatile VOCs that broke through the Tenax. Systems were also developed using three sorbents, such as Tenax, Ambersorb, and Spherocarb or Carbosieve (41). All such systems allow collection of a broader range of chemical types and volatilities. A multisorbent sampler was used to measure 57 VOCs in the EPA’s 100-building BASE study (42). DIRECT (WHOLE AIR) SAMPLING The direct (whole air) sampling method, first developed in the 1970s for upper-atmosphere sampling, avoids the sorption-desorption

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step, which theoretically should allow less chance for contamination. However, it requires great sensitivity on the part of the detection instruments. The method may involve real-time sampling in mobile laboratories, with direct injection of the air sample into a cold trap attached to a gas chromatograph, or sampling in evacuated electropolished aluminum canisters for later laboratory analysis (43).


Comparison of Sampling Methods No single method of sampling VOCs in the atmosphere or indoors has become a standard or reference method. In the United States, there are two preferred methods: use of Tenax and use of evacuated canisters. These methods were compared under controlled conditions in an unoccupied house (44). Ten chemicals were injected at nominal levels of about 3, 9, and 27 µg/m3. The results showed that the two methods were in excellent agreement, with precisions of better than 10% for all chemicals at all spiked levels. In Europe, the two most common methods are use of Tenax and use of activated charcoal. One study employing both methods side by side (45) found consistently higher levels of total VOC on the charcoal sorbent. The difference may be due to very volatile organics, such as pentane and isopentane, which are collected by charcoal but which break through Tenax readily. The sorbent methods lend themselves to personal monitoring—a small batterypowered pump is worn for an 8- or 12-h period to provide a time-integrated sample. Until recently, the whole-air methods employed bags or canisters that were too bulky or heavy to be used as personal monitors; however, a small canister sampler has been modified for use as a personal sampler or as a sampler for exhaled breath (46–48). A comprehensive review of sampling methods for VOCs is provided by Lewis & Gordon(49).

METHODS OF MEASURING BODY BURDEN Measurement of body burden provides a direct indication of the total dose through all important environmental pathways. Coupled with adequate pharmacokinetic models (50, 51) (including knowledge of how the VOC partitions between blood, air, and fat) measurement of a single body fluid such as breath or blood can give an indication of the total body burden and sometimes even an indication of the mode of exposure, as in the measurements of benzene in the breath of smokers, described above.

Breath Analysis of human breath as a means of relating exposure to dose of volatile organic compounds (VOCs) has been steadily improving over the past quarter century (52, 53).

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WALLACE INTEGRATED METHODS For many VOCs, the most sensitive method of determining body burden is measurement of exhaled breath. Detection limits using Tenax sorbents or evacuated canister samples are usually well under 1 µg/m3. For comparable sensitivity, blood measurements would have to detect levels as low as 10 ng/liter, a sensitivity that was out of range until recently. With the advent of improved sorbents, such as Tenax, and more specific and sensitive detection methods, such as gas chromatography (GC)/electron capture detection and GC/mass spectrometry (MS), it has become possible to detect hundreds and quantify scores of VOCs in the breath of persons exposed to normal environmental concentrations (54–57). Because exhaled alveolar breath contains only those compounds that have been given up to the alveoli by the blood as it passes through the lungs, the breath provides a noninvasive way of estimating blood concentrations. (The most prevalent use of breath analysis is in the ubiquitous highway Breathalyzer to detect alcohol in the blood.) Breath analysis is also accepted in the industrial hygiene field as the recommended compliance method (with standards known as Biological Exposure Indices based on threshold limit values) for certain common VOCs, such as styrene. Breath analysis has also been studied as a possible early warning marker for certain diseases (53). Although it is true that breath measurements reflect only the most recent exposure, it is also true that if the time of taking the breath measurement is carefully chosen, it can reflect long-term exposure. That is, persons who have been in their normal environment for a few hours are not likely to be far out of equilibrium with their surroundings (provided the exposure of their previous environment was not vastly different). Also, some studies using room-size chambers (58, 59) or specially designed spirometers (60) have determined the residence times of more than a dozen VOCs in various compartments of the body: blood (2–10 min), tissues rich in blood vessels (30 min to 2 h), and vessel-poor tissues (4–8 h). An effort to reconstruct previous exposures from a breath measurement together with a physiologically based pharmacokinetic (PBPK) model was described recently by Roy & Georgopoulos (61). The successful application of breath measurements in the TEAM studies led naturally to a series of more focused studies, aimed at answering the following important questions:

1. For each prevalent VOC, what fraction of the parent compound appears in exhaled breath, and how long does it last in the various compartments (blood, organs, muscle, fat) of the body? 2. Can we estimate previous exposure using breath measurements? Alternatively, can we predict body burden (dose) from any given exposure profile? To answer the first question on uptake and distribution, a series of chamber studies was undertaken by the EPA (58–60). At that time, no chambers were capable of being used to measure concentrations smaller than parts per billion— all existing chambers were being used for studies at 50- to 100-ppm levels and had background concentrations at the 1- to 10-ppb level, typical of most indoor

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environments. Therefore, the EPA contracted with the IIT Research Institute to create the first chamber in the world capable of such measurements. After 6 months of effort, the Institute arrived at the concept of a chamber within a chamber, with the inner chamber actually creating its air on the spot from cylinders of liquid oxygen, nitrogen, and CO2 (with the addition of charcoal-filtered water vapor to provide the proper humidification). The inner chamber was kept at positive pressure to ensure no contamination from the outer chamber, which itself was provided with filtered air to reduce the contamination from the remainder of the building. Once the chamber was completed, four subjects were exposed to common consumer products overnight before entering the clean-air chamber for observation for the next 8 h (58). Decay curves were obtained for a number of common VOCs, including tetrachloroethylene. These decay curves allowed estimation of two important parameters—the residence times in two compartments of the body and the relative amounts of each chemical distributed into each compartment (59). However, because of the 5-min collection time and 20-min cycle time of the breathcollection method, and because of the very short half-life suggested—but unable to be quantified—in the initial decay, a new method with shorter collection and faster turnaround times was deemed necessary. Such a method was developed over the next few years under EPA sponsorship (56, 62, 63). The principle of the method is beautifully simple. A person exhales through a valve into a 1-m-long tube open to the atmosphere. The early part of the breath (including the dead space air) exits the tube, leaving only alveolar air in the tube. While the person is taking the next breath, an evacuated canister sucks the alveolar air back through the tube. The method has a 1-min sampling time and a 5-min turnaround time. The method is portable (contained in a suitcase), has no moving parts, and is able to sample about 98% alveolar air with no restrictions or training needed for the person supplying the exhaled air. This method was employed in a number of subsequent field studies (taking advantage of the portability) and chamber studies, on a large number of nonpolar VOCs (64–66). Because of the short turnaround time, it was possible to take sufficient samples in the first few minutes to arrive at estimates of the half-life of the first compartment (normally in the range of 2–4 min) for a total of 15 VOCs. Half-lives for the second and third compartments were in the range of 30–60 min and 3–5 h, respectively (67). However, the half-life of the fourth compartment (associated with adipose tissue) was undetermined because of the difficulty of keeping people in a perfectly clean background area for much longer than 8 h. This problem was overcome with the next chamber study, which involved a 10-h exposure followed by a 24-h decay period. This study adequately estimated the half-life of the fourth compartment for nine VOCs (typically about 1–3 days) (68). Together with the half-lives, the exponential fits to the decay curve also determine the capacities of the various compartments. The results of this chamber study provided the first complete set of data covering all major compartments for a set of nine VOCs measured at sub–parts-per-billion levels.

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An interesting result of this study was that the parameters of the model were similar for each chemical in a given class (aromatics, aliphatics, and chlorinated hydrocarbons) and not greatly different across classes. Even more interesting, following normalization for average exposure, the parameters were similar across the range of subjects employed. It was possible to use model parameters averaged across chemical class and the five subjects to produce estimated concentrations for any given subject and chemical in that class to within about 20% for any point on the decay curve (68). This suggests that the parameters may be applicable to a large number of other nonpolar chemicals in each of the three classes studied. With these values for the compartmental half-lives and capacities in hand, it became possible to approach question 2 above–the relationship between exposure and dose. That is, given a reasonable model relating exposure and breath concentration, one can predict the resulting breath concentration, as well as concentration in blood and other tissues, from any given exposure profile, possibly to within 20%, based on the chamber studies summarized above. Moreover, it is possible to use the breath concentration to estimate previous exposure in certain cases (e.g., a sudden industrial release with well-known time of release or, conversely, a longtime, constant low-level exposure). Such a simple linear model was developed and applied to several simple situations, such as a sudden constant high exposure (step function) or a linearly increasing exposure (as in a shower) (67). The predictions of the model have also been used to help design new chamber studies, particularly the number and spacing of samples to be taken in both the uptake and decay phases. Several VOCs not included in the previous chamber studies have been tested since [methyl-tert-butyl ether (MTBE), chloroform] and results have been found consistent with the model predictions (69–71). More complicated models are also available and have been used to estimate exposure and dose for various VOCs, such as styrene (49) and chloroform (61, 72). Recently, a single-breath method has been developed (46–48). The subject breathes directly into a 1-liter evacuated cylinder through a straw-like attachment; after wasting the first (dead space) portion of his breath, the subject opens the valve on the cylinder to allow collection of the second (alveolar) portion. The cycle time is reduced to approximately 1 min. The new method allows for immediate collection following exposure, thus documenting the maximum breath (and therefore blood) concentration attained during the exposure period. However, like all previous discrete breath sampling methods, this one has the drawback of having a high cost per sample, limiting the number of samples that can be collected in any one field study. NEW CONTINUOUS BREATH SAMPLING METHOD The integrated breath sampling methods described above have several drawbacks. Each sample is an integrated sample collected in an evacuated canister for later laboratory analysis. Thus a field study requires a large number of canisters, creating logistical difficulties. In addition, each canister is analyzed separately, at considerable cost per analysis. These factors have combined to create severe restrictions on the number of samples

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investigators have been able to collect in field and chamber studies. Typically, only a maximum of four samples have been collected during uptake and 12 during decay. For a four-compartment model, this has limited investigators to about three samples during each decay phase, which has reduced precision in estimating half-lives. The situation is even worse for the uptake curve, for which sufficient data do not exist to check published parameter estimates against reality. A far more accurate picture of the uptake and decay curves could be obtained if a continuous set of samples could be collected for the entire exposure and decay periods. Several years of EPAsponsored development have resulted in a system that has now been used in two small studies. It consists of an ion trap mass spectrometer equipped with a sensitive atmospheric sampling glow discharge ionization source. Either a large laboratorybased ion trap mass spectrometer or a compact (2 ft3), lightweight (500 µg/m3 in a day, and the breath levels of both inhabitants increased steadily over 3 days from nondetectable to >70 µg/m3. For methylene chloride, the dominant source of exposure is use of paint strippers and other solvent cleaners. One study indicated that 8 h of furniture stripping in a garage led to exposures of 100,000 µg/m3, or the equivalent of a lifetime of exposure to normal ambient levels of about 1–5 µg/m3 (100). For environmental tobacco smoke (ETS), the dominant source is of course the nearby smoker. The proportion of the adult population who continue to smoke appears to have bottomed out at about 25%, and the proportion of homes with children exposed to a smoker is close to 40%. Homes with smokers have respirable

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suspended particle levels about 30 µg/m3 higher than homes without smokers; this often corresponds to a doubling of the typical level (and the increase alone is twice the recently proposed annual standard of 15 µg/m3 for particles smaller than 2.5 µm in aerodynamic diameter). A greatly increased level of exposure to polyaromatic hydrocarbons (PAH), products of incomplete combustion, some of which are considered human carcinogens, ensues. For truly enormous exposures, however, one must look to the undeveloped and developing countries. As Smith has pointed out (16), the burning of biomass (lowest rung on the “energy ladder”) for both cooking and heating, most often in homes without chimneys, leads to exposures (to particles and PAHs) hundreds or thousands of times those encountered in Europe or the United States. All these example chemicals or mixtures (except styrene) are either recognized human carcinogens (benzene, ETS, some PAH such as benzo-a-pyrene) or known animal carcinogens and therefore suspected human carcinogens. For example, benzene is considered a known leukemogen (101), and chloroform has been linked to bladder cancer (102). Although risk estimates are notoriously uncertain, the relative risk of these local indoor sources compared with outdoor sources is two to five times as great. Compared with risk levels calculated for Superfund sites, the indoor risks are on the order of 100–1000 times greater. Yet as we know, the amount of money available for regulating outdoor air or for cleaning up hazardous waste sites dwarfs the amount spent on reducing risk from indoor air sources. This is so despite the findings of several national task forces that indoor air sources are among the top environmental problems (103–105). The reasons for this disparity include the following. 1. The Love Canal episode led Congress to create the Superfund program. However, even at this site, scientific studies were unable to identify a toxic agent responsible for the reported sicknesses of residents. 2. Environmental organizations are not receptive to indications that large companies are not necessarily responsible for some important sources of pollution. They depend for their existence on contributions from those who are fearful of such large corporations. 3. People fear the unfamiliar. Air fresheners, drinking water, and cleaning solvents are such ordinary items that they are unable to get exercised over their potential risks. Thus, as many analysts have pointed out, spending for environmental problems reflects the layperson’s ranking of risks and not the expert’s. Several studies of the cancer risks of VOCs all rank three chemicals as the highest risks: benzene, chloroform, and p-DCB (106–108). Upper-bound risk estimates for each are in the neighborhood of several hundred cases annually. Upper-bound estimates for all VOCs combined are in the neighborhood of several thousand cases annually. For purposes of comparison, estimates for radon are more firmly based on human epidemiological studies of uranium miners and are in the neighborhood

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of 13,000 lung cancer cases annually. However, about 10,000 of these are due to the synergism between smoking and radon (smoking allows radon daughters to be transported to the lung on the smoke particles), so the radon impact on nonsmokers may be roughly similar to the impact of VOCs. It is interesting to note that in none of these cases is the major source chemical companies, hazardous waste sites, urban air pollution, or the other usual suspects. Instead, for benzene it is smoking, for chloroform it is water chlorination, and for p-DCB it is use of moth cakes or room air fresheners. Obviously, in the latter case, exposures can easily be eliminated simply by not using moth cakes or air fresheners. In the case of benzene, main sources of exposure for nonsmokers are ETS, driving, filling fuel tanks, and parking hot cars in attached garages, there to quietly emit gasoline vapors that enter the house. Although not driving is not an option for most of us, avoidance of ETS and letting cars cool before parking them in attached garages are easy and effective ways to reduce exposure. Finally, for chloroform, filtering tap water and taking cooler, shorter showers while using the bathroom exhaust fan can reduce exposures. These simple, nearly cost-free measures can reduce exposure by significant amounts in a far more cost-effective manner than regulation of major sources that are in fact only minor contributors to exposure. For example, a complete ban on emissions of p-DCB to the atmosphere (obviously an impossible task) would reduce exposure by only 2%, whereas a decision not to use moth cakes or room air fresheners in the home would eliminate nearly all exposure to this chemical. (Since there are no requirements to list ingredients on air fresheners, it is not easy for persons using such products to avoid those containing p-DCB.) However, several other popular air freshener ingredients, such as limonene (lime scent) and alpha-pinene (pine scent), are also either animal carcinogens or mutagens (109), so it would seem that the safer course would be to eschew use of air fresheners; improved hygiene would no doubt result. Awareness is growing that most exposure comes from these small nearby sources (110–112). In California, Proposition 65 focuses on consumer products, requiring makers to list carcinogenic ingredients. The EPA carried out a shelf survey of solvents containing only six VOCs and found some thousands of consumer products containing the target chemicals (113). ETS was declared a known human carcinogen by the EPA in 1991; smoking has been banned from many public places and many private workplaces during the past few years. However, an unintended result of increased consumer awareness of VOC emissions from building materials may be the replacement of some volatile and odorous chemicals with less volatile but longer-lasting chemicals of unknown toxicological properties. For example, a study of 51 renovated homes in Germany with complaints included a number in which the complaints had only begun 2 years after renovation (114). An investigation found a number of new VOCs, including longifolene, phenoxyethanol, and butyldiglycolacetate. These may represent a class of less-traditional compounds that have been added to building materials to replace the bad actors identified by toxicological and carcinogenic studies; however, these

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compounds may themselves have toxic properties that will emerge following new studies. They have the property that instead of being emitted in large quantities shortly following application of the surface coating, they are emitted in smaller quantities at first but tend to keep a steady emission rate for much longer periods of time. Our knowledge of chemical carcinogenicity is heavily reliant on the 2-year rat and mouse studies carried out by the National Toxicology Program. Since each test costs in the neighborhood of $500,000, only a small fraction of the thousands of chemicals in commerce can be tested. Selection of the chemicals is based on likelihood of causing cancer based on short-term mutagenicity tests, similarity of chemical structure to known carcinogens, volume of the chemical in commerce, etc. Animals are first provided large doses until the dose producing 50% mortality is determined. Then the dose is lowered (often to about half the 50% lethal dose, with a second dose halving that value again) for long-term administration. During the first 10 years of this program, roughly half of all chemicals tested proved carcinogenic. Although this might be expected based on the selection criteria, some scientists have brought up the consideration that perhaps the highly toxic dose levels, which may be expected to kill off cells and stimulate increased cell production, are themselves responsible for much of the observed carcinogenicity (115). Short-term mutagenicity tests, which can be carried out at a fraction of the cost of the carcinogenicity tests, have been carried out on a much larger group of chemicals. Chemicals common to both testing programs show roughly 80% agreement—most carcinogens are mutagens and vice versa. The number of natural chemicals tested for mutagenicity is much larger than the number tested for carcinogenicity. Since many of these natural chemicals, produced by plants and animals, are mutagens, it is expected that they may also be carcinogens. On the basis of these mutagenic tests, it has been estimated that a very large fraction of chemical-caused cancer may be due to these natural carcinogens (115).

SICK BUILDING SYNDROME, MULTIPLE CHEMICAL SENSITIVITY, AND IRRITATION POTENTIAL Up to now, we have been focusing on the carcinogenic aspects of selected chemicals. However, because the estimation of risk is so uncertain, it is difficult to assess the overall impact, economic and otherwise, due to chemically-induced cancer. It is easier to evaluate some of the acute effects that have been attributed to VOCs. In the early 1970s, Sweden experienced a large number of cases of eye irritation, stuffy nose, fatigue, and other conditions related to preschool buildings. The cause was eventually traced to a self-leveling cement used in some hundreds of buildings. A protein (casein) used in the material, and emitted under certain conditions of temperature and humidity, apparently caused immune system

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reactions. Although in this particular case the cause was eventually recognized, a growing number of such building-related symptoms began to be reported in many countries. The collection of symptoms became known as sick building syndrome (SBS). The most common symptoms are eye irritation, congested or runny nose, headache, fatigue, difficulty concentrating, and dry skin. In Canada in the mid-1990s, it was estimated that 10,000 buildings might have been afflicted with this syndrome in the course of a single year. In a study of the headquarters of the US EPA, about 5000 headaches per week were documented in an agency-wide survey (116–121). (There being about 5000 employees, this corresponded to an average of one headache per week per employee). Although seemingly minor, the estimated cost in reduced productivity of one headache is between $2 and $8 (122). Using the geometric mean of $4, we can calculate the cost of the 250,000 headaches per year experienced by EPA employees to be about $1,000,000. One conclusion of the study was that perceived dust exposure was associated with the largest number of symptom groupings. This was also the conclusion of the Danish Town Hall study (123–125). Many common VOCs have well-documented health effects, often neurobehavioral, at high (occupational) concentrations. Acute effects at lower environmental concentrations are often difficult to observe under controlled conditions, although Mølhave and coworkers (126–129) observed some subjective effects, such as a reported headache from a mixture of 22 common VOCs at a total concentration of 5 mg/m3, which is high but is sometimes encountered in new or renovated buildings. The EPA later confirmed these findings (130). Recently, following up on the observed relationship of dust to reported symptoms, Molhave’s group collected dust from seven Danish office buildings, analyzed it thoroughly (131), and exposed 24 healthy, nonsensitive volunteers to the resuspended dust at two concentrations. Increased eye irritation was noted at both levels (132), along with some influences on test performance, leading the investigators to conclude that the threshold level for acute and subacute (next day) effects must be below the lowest concentration tested (140 µg/m3). Despite the difficulty of observing effects under controlled conditions, a very common worldwide phenomenon is the sickening of large numbers of workers following occupation of a new or renovated building (133–136). Since such new buildings have very high levels of VOCs for a period of 6 months or so after completion, SBS has been thought to be a possible effect of VOC exposure. However, only one study has succeeded in linking a VOC-based metric to SBS symptoms (137). In this study of 12 buildings in California without perceived health problems among the workers, seven different VOC-based metrics, such as total VOC, irritancy-weighted VOC, etc., were tested using multivariate regression analysis for their effect on reported symptoms. Only one of the seven metrics showed relatively strong significant relationships with multiple reported symptoms, including eye, nose, throat, and skin irritation: a principal-component’s

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vector identified as associated with carpet emissions (with styrene as the VOC with the highest loading) and with cleaning products (2-butoxyethanol and 2-propanol having the highest loadings). It has also been suggested that a similar, more serious syndrome, multiple chemical sensitivity (MCS) (138), is a result of VOC or pesticide exposure, either chronic or following a single massive dose. People suffering from MCS often trace their condition to a single massive exposure to a particular chemical or chemical mixture. A tendency toward increasingly extreme reactions to minute trace quantities of chemicals in perfumes, air fresheners, etc., is often displayed. An EPA study of 31 fragrance products confirmed that a large number of polar organics are emitted from these products (139; for a comprehensive recent review of MCS, see Ashford & Miller 140). A less serious but possibly very costly result of VOC exposure may be reduced productivity resulting from minor ailments, such as headache and eye irritation. The total annual cost of poor indoor-air quality has been estimated to be in the neighborhood of 100 billion dollars (141). Although many large studies have been undertaken to investigate SBS, none has arrived at a satisfactory explanation. Most find a multitude of factors, ranging from dust and glare, to poor ventilation, to poor labor relations. VOCs are prime suspects, but most studies are unable to find relationships between VOC exposure and illness. In fact, some studies have found an inverse relationship, leading to speculation that perhaps the missing VOCs have undergone reactions that create highly irritating products that are not collected by normal sampling methods (136). Recently more support for this view has been developed by Weschler & Shields (142, 143), who have documented a large increase in ultrafine particles due to reactions of indoor-air terpenes (e.g. limonene) with ozone intruding from outdoors. These particles could affect eye irritation by direct contact or nose and chest congestion by being inhaled into the lungs and stimulating mucous secretion. In such a case, the health effect would be associated with a reduction in the observed levels of both ozone and limonene, even though ozone and limonene were the proximate cause. Animal studies supporting the irritative effects of ozone-terpene reactions have been reported recently by Wolkoff et al. (144–147), others, with definite effects on animals, remain unknown. Finally, much interest in pollutants that act on the endocrine system has arisen recently, partly stimulated by a popular book (149). The pollutants of interest include mostly semivolatile organics, including chlorinated pesticides such as DDT, the polychlorinated biphenyls, dioxins, and furans. Although effects have been documented mainly in animals and birds, a more recent popular work (150) includes many studies of effects in humans, such as the continuing, apparently lifelong effects observed in children of mothers receiving diethylstilbestrol when pregnant. Although a topic of great interest, insufficient measurements of human exposure have been made to allow any further treatment in this review.

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VOCS OF RECENT INTEREST


MTBE Concern has recently arisen as an unanticipated result of attempts to reduce the carbon monoxide and certain volatile organics emitted from incomplete combustion in automobiles. To improve combustion, oxidizers are required to be added to gasoline in those areas of the United States not meeting the standards set by the 1990 Clean Air Act. The most popular of these is methyl-tert-butyl ether (MTBE), added to gasoline in amounts as high as 17%. MTBE has been associated with symptoms such as headaches, and complaints have been received from residents of some (but not all) of the areas where it has been added to gasoline. Additionally, enough time has passed for it to have become one of the most common contaminants of ground water (151). Several studies have documented the human exposure resulting from refueling autos (25, 70). The widespread contamination of groundwater supplies by leaking underground storage tanks and the possible toxic potential of MTBE led to a ban, announced by the EPA in 2000 to take effect in later years, on its use as a gasoline additive. However, the potential for exposure remains for people using contaminated ground water; therefore research is continuing on such aspects of exposure as inhalation and dermal absorption during showers and baths, and the pharmacokinetics of MTBE.

Disinfection By-Products In areas that chlorinate their drinking water, chlorination by-products such as chloroform and other trihalomethanes (THMs) contaminate the finished water leaving the treatment plant (152–154). The discovery of chloroform in the blood of New Orleans residents (155) led to the Safe Drinking Water Act of 1974, which set a limit of 100 µg/liter for total THMs in finished water supplies. A series of nationwide surveys of THM levels in water supplies have been carried out in the United States since the passage of the Safe Drinking Water Act. The National Organics Reconnaissance Survey of 80 treatment plants (156) indicated that about 20% exceeded the THM standard of 100 µg/liter. A more recent survey of 727 utilities (157) representing more than half of consumers indicated that only about 3.6% of water supplies surveyed continued to exceed the standard. In succeeding years, it has become more evident that THM exposure occurs in ways other than drinking chlorinated water. Since treated water is normally used for all other household purposes, volatilization from showers, baths, and washing clothes and dishes is an important route of exposure. Studies of experimental laboratory showers by Andelman (158, 159) resulted in the estimate that chloroform exposure from volatilization during a typical 8-min shower could range from 0.1 to 6 times the exposure from drinking water from the same supply, depending on the

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amount of water ingested per day. Other studies of full-scale showers corroborate these conclusions, with estimates of exposure equivalent to ingestion of 1.3–3.7 liters of tapwater per day at the same concentrations as the water in the shower (90, 91, 160–162). Inhalation of airborne chloroform during the rest of the day was also found to be comparable to ingestion, based on measurements of 800 people in their homes in the TEAM study. Backer et al. (163) exposed volunteers to tap water chemicals through drinking, showering, and bathing and measured blood levels before and after exposure. Showering and bathing resulted in sharply increased levels of three THMs, but drinking 1 liter of water resulted in only a small increase. Food and beverages made from treated water (ice cream, soft drinks) have been found to contain chloroform and other THMs (164). A comprehensive review (165) of the scientific literature on human exposure to chloroform through all routes (inhalation, ingestion, dermal absorption) concluded that all routes contributed substantially to human exposure, with drinking of tap water estimated at only about 20% of total exposure. In 1998, the EPA accepted the concept of a 20% contribution from drinking water (as opposed to its previous adoption of an 80% estimate) in setting proposed new limits on chloroform in drinking water. However, because of the possible carcinogenicity of chloroform, in December 1998 the EPA set a maximum contaminant level goal (MCLG) of zero. MCLGs of zero are often set for carcinogens whose mechanism of action is on DNA; since one molecule could lead to a mutation in the cell, leaving it damaged but free to reproduce, no threshold level exists—the dose-response curve is considered to be linear. At the same time, however, the EPA stated that it now believed that chloroform acts to kill cells, not to damage them at the nuclear level. This cytotoxicity promotes cell regeneration, leading to higher rates of mutation and, ultimately, cancer. Since cytotoxicity has a threshold, and the dose-response curve is therefore nonlinear, this undercut the EPA’s establishment of a zero-level MCLG. Following a suit brought by the Chlorine Chemistry Council and the Chemical Manufacturer’s Association, on March 31, 2000, the agency’s actions were found by the US Court of Appeals for the Washington, DC, Circuit to be arbitrary and capricious and exceeding legislative authority; the agency’s zero-level MCLG was thus vacated (166). The EPA is now considering an MCLG of between 70 and 300 ppb.

SUMMARY AND CONCLUSIONS Methods of measuring personal exposure to VOCs are well advanced. Sorbents such as Tenax and improved carbon-based materials amenable to thermal desorption have driven sensitivity of measurement to levels of everyday environmental concentrations for scores of compounds. However, there is still a need for development of methods capable of measuring the more reactive compounds. Even greater advances have been made in analyzing exhaled breath for VOCs. Beginning with sampling in 20-liter bags, the amount required for adequate

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analysis has steadily decreased, down to 2-liter canisters and finally to singlebreath (750-ml) canisters. The recent development of a method to sample exhaled breath continuously frees us of dependence on expensive discrete samples and should allow a far more detailed investigation of the distribution and residence times in the body of a number of important VOCs. These advances in measurement methods have made possible a series of largescale studies of human exposure, which in turn have resulted in a deep understanding of the major sources of exposure. In many cases, these are consumer products, building materials, and personal activities, such as driving, smoking, and showering. Indoor sources are normally several times more important than such accepted “major” outdoor sources as vehicle exhaust, manufacturing facilities, urban areas, and hazardous waste sites. This conclusion has been reached by a number of task forces over the past decade. Nonetheless, because of political and social factors, an adequate response to these findings has not emerged. We hope that continued scientific studies coupled with effective communication of findings to the public will ultimately result in improved public health. DISCLAIMER This article was written in the author’s private capacity on his own time and does not reflect the policy of any government agency. Visit the Annual Reviews home page at www.AnnualReviews.org

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Summary Analysis. Vol. 1. EPA 600/687/002a, NTIS PB88-100060. Washington, DC: US EPA 7. Akland G, Hartwell TD, Johnson TR, Whitmore R. 1985. Measuring human exposure to carbon monoxide in Washington, D.C., and Denver, Colorado, during the winter of 1982–83. Environ. Sci. Technol. 19:911–18 8. Immerman FW, Schaum JL. 1990. Nonoccupational Pesticide Exposure Study: Final Rep. EPA/600-3-90-003. Washington, DC: US EPA ¨ 9. Ozkaynak H, Xue J, Spengler JD, Wallace LA, Pellizzari ED, Jenkins P. 1996. Personal exposure to airborne particles and metals: results from the Particle TEAM Study in Riverside, CA. J. Expo. Anal. Environ. Epidemiol. 6:57–78

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CONTENTS PREFATORY BIOGRAPHY Frontispiece—Robert W. Kates

xiv

Queries on the Human Use of the Earth, Robert W. Kates Frontispiece—Harvey Brooks

1 28

Autonomous Science and Socially Responsive Science: A Search for Resolution, Harvey Brooks

29

ENERGY END-USE AND CONVERSION Indicators of Energy Use and Carbon Emissions: Explaining the Energy Economy Link, Lee Schipper, Fridtjof Unander, Scott Murtishaw, and Mike Ting Energy Conservation in Chinese Residential Buildings: Progress and Opportunities in Design and Policy, Leon R. Glicksman, Leslie K. Norford, and Lara V. Greden Policy Modeling for Energy Efficiency Improvement in US Industry, Ernst Worrell, Lynn Price, and Michael Ruth

49

83 117

RESOURCES AND TECHNOLOGIES Storage of Fossil Fuel-Derived Carbon Dioxide Beneath the Surface of the Earth, Sam Holloway

145

Historical and Future Trends in Aircraft Performance, Cost, and Emissions, Joosung J. Lee, Stephen P. Lukachko, Ian A. Waitz, and Andreas Schafer

167

RISKS AND IMPACTS Interim Storage of Spent Fuel in the United States, Allison Macfarlane

201

Protecting Agricultural Crops from the Effects of Tropospheric Ozone Exposure: Reconciling Science and Standard Setting in the United States, Europe, and Asia, Denise L. Mauzerall and Xiaoping Wang Human Exposure to Volatile Organic Pollutants: Implications for Indoor Air Studies, Lance A. Wallace

237 269

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Carbon Dioxide Emissions from the Global Cement Industry, Ernst Worrell, Lynn Price, Nathan Martin, Chris Hendriks, and Leticia Ozawa Meida

303

ECONOMICS


Global Electric Power Reform, Privatization, and Liberalization of the Electric Power Industry in Developing Countries, R. W. Bacon and J. Besant-Jones

331

POLICY Federal Fossil Fuel Subsidies and Greenhouse Gas Emissions: A Case Study of Increasing Transparency for Fiscal Policy, Doug Koplow and John Dernbach The PCAST Energy Studies: Toward a National Consensus on Energy Research, Development, Demonstration, and Deployment Policy, John P. Holdren and Samuel F. Baldwin

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391

ENVIRONMENTAL SCIENCE Carbon Sinks in Temperate Forests, Philippe H. Martin, Gert-Jan Nabuurs, Marc Aubinet, Timo Karjalainen, Edward L. Vine, John Kinsman, and Linda S. Heath

435

INDEXES Subject Index Cumulative Index of Contributing Authors, Volumes 1–26 Cumulative Index of Chapter Titles, Volumes 1–26

ERRATA An online log of corrections to Annual Review of Energy and the Environment chapters (if any, 1997 to the present) may be found at http://energy.AnnualReviews.org

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