Journal of Occupational and Environmental Hygiene
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Respirable dust and silica exposure among World Trade Center cleanup workers Brian T. Pavilonis & Franklin E. Mirer To cite this article: Brian T. Pavilonis & Franklin E. Mirer (2017) Respirable dust and silica exposure among World Trade Center cleanup workers, Journal of Occupational and Environmental Hygiene, 14:3, 187-194, DOI: 10.1080/15459624.2016.1237773 To link to this article: http://dx.doi.org/10.1080/15459624.2016.1237773
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Date: 25 May 2017, At: 09:08
JOURNAL OF OCCUPATIONAL AND ENVIRONMENTAL HYGIENE , VOL. , NO. , – http://dx.doi.org/./..
Respirable dust and silica exposure among World Trade Center cleanup workers Brian T. Pavilonis and Franklin E. Mirer Department of Environmental, Occupational, and Geospatial Health Sciences, CUNY Graduate School of Public Health and Health Policy, New York, New York
ABSTRACT
KEYWORDS
The cleanup effort following the destruction of the World Trade Center (WTC) was unprecedented and involved removal of 1.8 million tons of rubble over a nine-month period. Work at the site occurred 24 hr a day, 7 days a week and involved thousands of workers during the process. The Occupational Safety and Health Administration (OSHA) conducted personal and area exposure sampling during the cleanup of the site. Secondary data analysis was performed on OSHA air sampling data for respirable dust and silica from September 2001 to June 2002 at the WTC recovery site to characterize workers’ exposure. Results for silica and respirable particulate were stratified by area and personal samples as well as job task for analysis. Of 1108 samples included in the analysis, 693 were personal and 415 were area. The mean result for personal silica samples was 42 μg/m3 (Range: 4.2–1800 μg/m3 ). Workers identified as drillers had the highest mean silica exposure (72 μg/m3 ; range: 5.8–800 μg/m3 ) followed by workers identified as dock builders (67 μg/m3 ; range: 5.8–670 μg/m3 ). The mean result for personal samples for respirable particulate was 0.44 mg/m3 (range: 0.00010–13 mg/m3 ). There were no discernable trends in personal respirable dust and silica concentrations with date.
Respirable dust exposure; silica exposure; World Trade Center workers
Introduction Within three weeks of the destruction of the World Trade Center (WTC) work began to remediate the site.[1] The cleanup involved removal of approximately 1.8 million tons of debris over a 9 month period.[2] More the 1,000 workers were involved in the remediation of the 16 acre site.[3] The silica content of concrete, consisting of sand and other aggregate, is in the range of 25–70%.[4] However, the “crystalline” silica content of the bulk material is not readily available, and the emission of respirable crystalline silica would depend on the mechanical force applied to the bulk material and the process by which it is applied.[4] Due to the large amount of concrete rubble that was disturbed, there was a potential for worker exposure to respirable silica. An extensive respirable dust and silica sampling data set was collected by the Occupational Safety and Health Administration (OSHA) during the cleanup period at the site and made publically available.[5] Previously reported exposure assessments of cleanup workers either did not sample for silica or did not find levels of concern.[1,3] Crystalline silica has been classified by IARC to be Group 1 carcinogen; OSHA has recently reduced the
Permissible Exposure Limit (PEL) for silica, one of the few reductions in recent years.[6,7] The OSHA PEL’s for silica in place in 2001–2002 were adopted in 1971; the limit in general industry was essentially 100 μg/m3 , the limit in construction was based on a particle count formula considered by OSHA to be equivalent to 250 μg/m3 . Both limits apply to 8-hr time weighted average. Effective June, 2016 OSHA reduced the silica PEL to 50 μg/m3 for construction and general industry.[7] NIOSH in 1974 recommended an exposure limit of 50 μg/m3 , combined with medical surveillance and exposure monitoring triggered by exposures at an action level of 25 μg/m3 .[8] In 1998 the American Conference of Government Industrial Hygienists (ACGIH) declared that silica be considered a “suspected” human carcinogen, and in 1999 proposed a reduction in the TLV to 50 μg/m3 , later adopted in 2000. In 2006, the TLV was further reduced to 25 ug/m3 .[9] Epidemiological studies have consistently observed an association between chronic crystalline silica exposure and silicosis, lung cancer, tuberculosis, and chronic obstructive pulmonary disease (COPD).[10–13] Prolonged exposure to respirable silica can lead to silicosis, an irreversible fibrotic lung disease which is confirmed via a
CONTACT Brian T. Pavilonis
[email protected] Department of Environmental, Occupational, and Geospatial Health Sciences, CUNY Graduate School of Public Health and Health Policy, New York, NY, . Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/uoeh. © JOEH, LLC
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radiological exam.[13] Studies have also observed significant decrements in lung function independent of silicosis and not visible on a chest exam.[10,14,15] Respirator compliance in the immediate aftermath of the collapse of the WTC was reported to be poor.[16–18] Notably, Antao et al. surveyed self-reported respirator use among workers involved in rescue and recovery at the WTC site.[16] Of the 9,296 workers surveyed, which included first responders and construction workers, 45% reported no respiratory protection use for some time period. The authors also reported fewer respiratory complaints among workers that wore a respirator compared to those that did not or used inadequate respiratory protection.[16] The motivation for this study was the continued need for exposure information concerning WTC workers that could be used for health surveillance of this cohort. This study examines personal and area respirable silica and particulate data collected by OSHA during the remediation of the WTC site. Samples were collected from September 17, 2001 to June 21, 2002. The goal of this secondary data analysis was to characterize workers’ exposure to silica and respirable dust and identify high exposure jobs relative to various occupational exposure limits. Additionally, data from this study could be used in future catastrophic building collapses to ensure workers are properly protected from airborne exposures.
Methods Air sampling results for respirable particulate and silica were downloaded from the OSHA website and entered into an Excel spreadsheet.[5] Results were stratified into area and personal samples for analysis. Relevant fields were date, sector of the worksite, operation (job task), air sample duration (minutes), sample type (personal or area), respirable particulate concentration (mg/m3 ), and silica fraction (percentage). Of 1,108 samples included in the analysis, 415 were categorized “area” and 693 were categorized as “personal” by OSHA. Ten samples labeled “personal” included “area” in the operation column and were reclassified as area samples. Area samples were collected by OSHA in locations that were deemed not feasible for personal sampling and were taken as close to an operation as possible. The Occupational Safety and Health Admiration deployed industrial hygienists based on information about planned activities from operational meetings, combined with concurrent observations by senior industrial hygienists. Sampling was concentrated in tasks expected to generate dust and to take into account main activities. While samples were collected for approximately 8 hr, the operations
continued 24 hr a day — two 12-hr shifts — and 7 days a week. Additional contaminants such as asbestos, metals and organic vapors were also sampled for, but are not included in this analysis. The OSHA sampling method for respirable silica specifies a size-specific cyclone be used with a personal sampling pump operated at 1.7 L per minute. This sampling method has a 50% collection efficiency for particles 3.5 μm in aerodynamic diameter. The method cites a lower analytical limit of 10 μg, translating to a limit of detection of approximately 12.25 μg/m3 for a full 8-hr shift, and proportionally higher for shorter sampling periods.[19] For this analysis samples were excluded from analysis if the sampling duration was less than 180 min. The only form of silica detected at the WTC site was quartz and all other forms such as cristobalite were not detected. Silica data were reported as a percentage of respirable mass. Therefore, silica exposure for each sample was calculated by multiplying the respirable particulate level by the fraction silica reported. If silica concentration for the sample was below the limit of detection (LOD), the measurements was substituted with the LOD/2. The LOD was determined using Equation (1) which includes the lower analytical limit of 10 μg for silica, the sampling time, and flow rate. Since concentration is conditional on sampling duration, each censored value had a unique silica LOD concentration. Since the LOD for silica is on the same order of magnitude as relevant occupational guidance values, LOD/2 was chosen to reduce positively biasing the mean.[20] C=
10 μg , t∗ f
(1)
where C= t= f=
concentration in μg/m3 sample duration in minutes flow rate (0.0017 m3 /min)
The data set included descriptions of the tasks conducted in 362 of the 693 personal samples collected. However, the majority of personal samples (52%) had no description of the task performed and were classified as “unidentified.” Any sample that was classified as personal but the task was not listed was classified by the authors as “unidentified.” The dataset originally contained 68 tasks which were subsequently reduced to 12 unique tasks. Many of these tasks could be combined with little interpretation. For example, many tasks had spelling errors or spelled different and were subsequently combined into one task. For other jobs, if they were deemed similar in activity they were grouped together. The category “emergency personnel” was originally listed by OSHA as five separate descriptions: “NY FIRE DEPARTMENT”,
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Table . Area respirable dust and silica exposure monitoring location. Sample and Location Silica (μg/m ) Northwest Southeast Southwest West Respirable Dust (mg/m ) Northwest Southeast Southwest West
N
n < LOD (%)
Mean
th Percentile
Median
th Percentile
Range
() () () () () () () () () ()
. . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . .
.– .– .– .– .– .–. .–. .–. .–. .–.
“NY POLICE DEPARTMENT”, “NY POLICE”, “POLICE OFFICER”, and “OTHER RESCUER.” Statistical analysis Statistical analysis was performed using SAS version 9.4. Silica and respirable dust measurements were highly left skewed. Normality of the silica and respirable dust measurements were assessed using a Kolmogorov-Smirnov test. As a result of the high number of censored measurements, personal and area respirable silica and dust were not normally distributed even when log-transformed. As a result, non-parametric statistics were used. Descriptive statistics including arithmetic mean, 25th % and 75th % quartiles, and range were calculated on continuous exposure measurements. Wilcoxon signed-rank tests were used to determine whether personal and area respirable dust and silica samples were statistically different. Additionally, Wilcoxon signed-rank tests were also used to assess whether sampling time was significantly different between tasks that were unidentified vs. identified tasks. Spearman’s correlations (ρ) were calculated on personal and area measurements and date. Scatter plots of exposures by date were created and the LOESS option was used to determine a best fit line through the data.[21]
Results Personal and area air samples were collected over the majority of an 8-hr shift. The mean sample duration was 398 min for area samples and 391 min for personal samples. Thirty-one percent of all samples reported detectable silica with a higher proportion of personal samples with detectable levels of silica (39%) compared to area (19%). Mean calculated respirable silica concentration was 33 μg/m3 for all samples, with mean personal samples (42 μg/m3 ) significantly greater (p < 0.01) than mean area samples (17 μg/m3 ). Personal respirable dust concentrations (0.44 mg/m3 ) were also significantly larger (p < 0.01) compared to area samples (0.20 mg/m3 ). In general, there was a strong positive correlation between respirable dust and silica concentrations (ρ = 0.59; p < 0.01).
Area sampling data by site sector are presented in Table 1. The locations of the sectors are shown in Figure 1. A large number of area samples were censored (81%), with the West and Southwest corners having a greater proportion of samples below the LOD. Area silica exposure showed less variability compared to respirable dust. Overall, area silica concentrations spanned two orders of magnitude (range: 5.4–610 μg/m3 ) while area respirable dust spanned four orders of magnitude (range: 0.00010– 5.3 mg/m3 ). The largest mean silica concentration was at the southeast sector (20 μg/m3 ) where the majority of silica samples (62%) were collected. Task specific personal silica measurements are shown in Table 2. The majority of tasks were unidentified (48%) and below the LOD (61%). The mean duration of samples with tasks identified (mean = 379 min) was significantly less (p < 0.01) than that for unidentified tasks (mean = 418 min). Workers that were identified as drillers had the highest mean exposure (72 μg/m3 ) followed by workers identified as dock builders (67 μg/m3 ). The largest single measurement was 1800 μg/m3 was a personal sample recorded for a driller. The lowest mean exposures were observed for crane operators, excavators, emergency personnel, grapplers, safety/foremen, and torch cutters, all of which were ࣘ12 μg/m3 . Calculated mean silica exposure for personal samples with task unidentified tasks were 47 μg/m3 . Summary results for personal respirable particulate measurements are presented in Table 3. More variability in exposure was observed in respirable dust compared to silica. The largest mean exposures were to iron workers (0.95 mg/m3 ) and individuals working as machine operators (0.82 mg/m3 ). The largest measured respirable dust exposure was found on a general laborer working around the debris (13 mg/m3 ). Tasks such as dock builder and driller had above average exposures to both silica and respirable dust. The number and percentage of samples above the PEL, REL, and TLV are displayed in Table 4. For respirable dust the OSHA PEL is 5.0 mg/m3 [22] while the ACGIH TLV for Particles Not Otherwise Specified (PNOS) is 3.0 mg/m3 for the respirable fraction.[23] Among personal
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Figure . Diagram of World Trade Center Site indicating areas from: Occupational Safety and Health Administration (OSHA). OSHA silica sampling area map - World Trade Center.[]
respirable silica samples, 30% of samples were greater than 25 μg/m3 , 16% greater than 50 μg/m3 , 8% were greater than 100 μg/m3 , and 2% were greater than 250 μg/m3 . Forty-four percent of personal respirable samples exceeded 0.2 mg/m3 , 18% exceeded 0.5 mg/m3 , 7% exceeded 1.0 mg/m3 , 2% exceeded 3.0 mg/m3 , and
1% exceeded 5.0 mg/m3 . For tasks such as dock builder and driller, the majority of silica samples exceeded the TLV (55% and 59%, respectively). Personal samples categorized as unidentified had a high frequency of exposure above the TLV with 37% of samples above the reference value.
Table . Personal respirable silica exposure by task. Task All Tasks Front End Loader Crane Operator Dock Builder Driller Excavator Emergency Personnel Grappler Iron Worker Laborer Misc. Machine Operators Safety/Supervisor Torch Cutter Unidentified
n
n < LOD (%)
Mean (μg/m )
th Percentile (μg/m )
Median (μg/m )
th Percentile (μg/m )
Range (μg/m )
() () () () () () () () () () () () () ()
.
. . . . . . . . . . . .
. . . . . . . . . . .
. . .
.– .– .– .– .– .– .– .– .– .– .– .– .– .–
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Table . Personal respirable dusta exposure by task. n
Mean (mg/m )
th Percentile(mg/m )
Median(mg/m )
th Percentile(mg/m )
Range(mg/m )
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
. . . . . . . . . . . . . .
.– .–. .–. .–. .–. .–. .–. .–. .–. .– .–. .–. .–. .–.
Task All Tasks Front End Loader Crane Operator Dock Builder Driller Excavator Emergency Personnel Grappler Iron Worker Laborer Misc. Machine Operators Safety/Foreman Torch Cutter Unidentified
a All samples were above the LOD.
workers during the process.[2,3] Results from this study showed personal silica exposures were high, with some tasks regularly above the TLV. As noted previously, there was poor compliance with respirator use by workers during the cleanup; although compliance did improve somewhat over time. Nevertheless, more than a quarter of the workers surveyed did not use any respirator after 1/1/2002.[16] Given the results of this analysis and documented poor respirator compliance at the site, it is likely workers experienced considerable silica exposure. A previous exposure assessment study, conducted by NIOSH a week after the collapse of the WTC, found no detectable levels of airborne silica (n = 18). In addition, bulk samples (n = 27) of settled dust were collected and analyzed for silica, with only one sample yielding detectable concentrations.[1] This study was conducted in the initial stages of the cleanup (9/18/2001–10/4/2001), most likely before large scale removal of debris occurred. The data collected by OSHA represents nine months of sampling and is more representative of exposures
The correlation between exposure measurements and dates were determined. For personal samples, there was no significant correlation between silica (ρ = −0.024; p = 0.54) and respirable dust concentrations (ρ = 0.0065; p = 0.86) and date. For area samples, silica concentrations significantly decreased as the cleanup progressed (ρ = −0.27; p < 0.01). This inverse trend was also observed with area respirable dust concentrations, albeit non-significantly (ρ = −0.089; p = 0.070). Exposure measurements over the entire sampling period were plotted and shown in Figures 2 and 3. Large day-to-day fluctuations in concentrations can be observed for both personal and area samples.
Discussion The scope of the cleanup at ground zero was unprecedented and involved removal of 1.8 million tons of rubble over a nine-month period. Work at the site occurred 24 hr a day, 7 days a week and involved thousands of
Table . Number and percentage of samples above various guidance values and standards. Respirable Dustc
Silica Task Area Samples Personal Samples Front End Loader Crane Operator Dock Builder Driller Excavator Emergency Personnel Grappler Iron Worker Laborer Misc. Machine Operators Safety/Foreman Torch Cutter Unidentified
n
n < LOD (%)
TLV n (%) > μg/m
REL n (%) > μg/m
PELa n (%) > μg/m
PELb n (%) > μg/m
TLV n (%) > mg/m
PEL n (%) > mg/m
() () () () () () () () () () () () () () ()
() () () () () () () () () () () () ()
() () () () () () () () () () ()
() () () () () () () () () ()
(
