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Occupational Exposures to Air Contaminants at the World Trade Center Disaster Site --- New York, September--October, 2001

Amid concerns about the fires and suspected presence of toxic materials in the rubble pile following the collapse of the World Trade Center (WTC) buildings on September 11, 2001, the New York City Department of Health (NYCDOH) asked CDC for assistance in evaluating occupational exposures at the site. CDC's National Institute for Occupational Safety and Health (NIOSH) collected general area (GA) and personal breathing zone (PBZ) air samples for numerous potential air contaminants. This report summarizes the results of the assessment, which indicate that most exposures, including asbestos, did not exceed NIOSH recommended exposure limits (RELs) or Occupational Safety and Health Administration (OSHA) permissible exposure limits (PELs) (1,2). One torch cutter was overexposed to cadmium; another worker was overexposed to carbon monoxide (CO) while cutting metal beams with an oxyacetylene torch or a gasoline-powered saw, and two more were possibly overexposed to CO. NIOSH recommended that workers ensure adequate on-site ventilation when using gas-powered equipment and use rechargeable, battery-powered equipment when possible.

Toxic substances of concern included asbestos (from insulation and fireproofing materials), concrete (made from Portland cement and used in the buildings' construction) and the crystalline silica it contained, CO (from fires and engine exhaust), diesel exhaust (from vehicles and equipment), mercury (from fluorescent lights), chlorodifluoromethane (Freon-22, from air conditioning systems), heavy metals (from building materials), hydrogen sulfide (from sewers, anaerobically decomposing bodies, and spoiled food), inorganic acids, volatile organic compounds (VOCs), and polynuclear aromatic hydrocarbons (PAHs) (from fires and engine exhaust). Environmental sampling during September 18--October 4 focused on search-and-rescue personnel, heavy equipment operators, and workers cutting metal beams (Figure 1) but also included various other occupations. A total of 1,174 air samples was collected, including 804 for asbestos. NYCDOH contractors collected most of the asbestos samples; NIOSH personnel collected all other samples. In addition, NIOSH collected 33 bulk samples of dust, debris, and other materials. All samples were collected and analyzed according to the NIOSH Manual of Analytic Methods (3) with some modifications.

A total of 29 bulk samples of undisturbed settled material from various locations was analyzed for asbestos; 27 of these also were analyzed for crystalline silica and metals. Of the 29 samples, 26 (90%) had <1% asbestos (by mass); the three others had 1%--3%. All but one of 27 samples had crystalline silica; concentrations (by mass) ranged from 0--18%, with a median (for all 27 samples) of 3.2%. The most abundant metals in the samples were calcium, magnesium, aluminum, iron, and zinc. Lead, arsenic, cadmium, and beryllium concentrations (by mass) were <0.1%. Three bulk samples of fireproofing material on I-beams from the main debris pile were analyzed for asbestos; one was negative, and two had <1% asbestos. A sample of paint from a metal beam had 0.3% lead.

Phase contrast microscopy (PCM) revealed fibers in 358 (45%) of the 804 asbestos air samples. Excluding 30-minute samples, 25 samples had fiber concentrations that, if the fibers had been asbestos, would have exceeded the REL of 0.1 fibers per cubic centimeter of air (f/cc) (1). None of the 30-minute sample concentrations exceeded the OSHA short-term excursion limit of 1.0 f/cc (2). Of the 25 samples with fiber concentrations >0.1 f/cc (range: 0.1--0.5 f/cc) by PCM, 18 were analyzed by transmission electron microscopy (TEM), which can distinguish between asbestos and nonasbestos fibers. All had asbestos concentrations <0.1 f/cc. The seven samples not analyzed by TEM had fiber concentrations ranging from 0.1--0.2 f/cc. Differential analysis by polarized light microscopy of these same 25 air samples revealed most nonasbestos fibers to be fibrous glass, gypsum, and cellulose.

Air concentrations of total (36 samples) and respirable (18 samples) particulate ranged up to 2.3 milligrams per cubic meter (mg/m3) and 0.3 mg/m3, respectively, which are below the corresponding RELs of 10.0 mg/m3 and 5.0 mg/m3 for Portland cement (1). Respirable crystalline silica was not detected in any of 18 air samples. Of 45 air samples analyzed for various metals, one from a 6½-hour PBZ sample from a torch cutter had a cadmium concentration (8.6 microgram per cubic meter [µg/m3]) that would have exceeded the PEL (8-hour time-weighted average [TWA]) of 5.0 µg/m3 even without further exposure during the remainder of the 8-hour shift. None of the samples had concentrations of lead, arsenic, beryllium, or other metals that exceeded NIOSH or OSHA exposure limits.

Two instantaneous peak CO measurements (1,239 and 1,368 parts per million [ppm]) exceeded 1,200 ppm, the level NIOSH considers an immediate danger to life and health (1). One was from a torch cutter and the other from a gasoline- powered saw operator. In 99 air samples, concentrations of CO ranged from 0.2 to 242.0 ppm; the highest finding (in a 32½-minute PBZ sample from a saw operator) exceeded the NIOSH limit of 200 ppm and would have exceeded the PEL of 50 ppm (8-hour TWA) had it been sustained for 2 hours (1,2). CO concentrations of 41 ppm and 45 ppm in PBZ samples from torch cutters and 40 ppm in a GA sample near a saw operator, with sampling durations of ½, 5, and 2½ hours, respectively, would have exceeded the REL of 35 ppm had they represented full-shift exposures (1,2).

Hydrogen sulfide was present in seven of 10 samples, one or more inorganic acids in all 27 samples, mercury in five of 16 samples, and one or more VOCs in 14 of 76 samples; all concentrations were below applicable NIOSH and OSHA exposure limits except for two benzene concentrations (0.4 mg/m3 and 0.5 mg/m3) that exceeded the REL of 0.3 mg/m3 (1). Both were in GA samples from a smoke plume and did not represent any specific worker's exposure. The highest concentration of elemental carbon (an indicator of diesel exhaust) was 0.023 mg/m3. Neither NIOSH nor OSHA has a numerical exposure limit for diesel exhaust, but the American Conference of Governmental Hygienists has proposed a limit of 0.2 mg/m3 (measured as elemental carbon) (4). No Freon-22 was detected in any of five samples. Small amounts of various PAHs were present in all 12 samples, but not at concentrations that exceeded individually or collectively any applicable NIOSH or OSHA exposure limit.

Reported by: K McKinney, MPA, New York City Dept of Health; S Benson, New York City Office of Emergency Management; A Lempert, New York City School Construction Authority. M Singal, MD, Cincinnati, Ohio. K Wallingford, MS, E Snyder, MS, Div of Surveillance, Hazard Evaluations and Field Studies, National Institute for Occupational Safety and Health, CDC.

Editorial Note:

In addition to the physical hazards associated with work at the WTC site, hazardous exposures to toxic dusts and gases were suspected initially. Asbestos exposure, in particular, was an occupational and community health concern. The findings of this survey documented no occupational exposure to asbestos, at least after September 18, in excess of NIOSH or OSHA occupational exposure limits. The seven air samples that had fiber concentrations (determined by PCM) higher than the REL for asbestos probably would have had asbestos concentrations <0.1 f/cc if analyzed by TEM. In many other samples, asbestos concentrations determined by TEM tended to be lower than those determined by PCM. The NIOSH asbestos sampling did not provide data about occupational exposures before September 18 and was designed to assess occupational exposures, not community exposures, which probably were lower.

The absence of exposure to respirable crystalline silica, despite its presence in the bulk samples, indicates either that the crystalline silica in the dust at the site consisted of larger, nonrespirable particles or that work activities were not causing the dust to become airborne. In the absence of effective dust-control measures, the former explanation seems more likely. Although the air sampling indicated the presence of respirable airborne particulate, this material was apparently not crystalline silica. One torch cutter had cadmium overexposure, and excess CO was associated with workers using oxyacetylene torches and gasoline-powered saws. To reduce CO exposure, NIOSH recommended replacing gasoline-powered saws with rechargeable, battery-powered saws.

At the time of the NIOSH sampling, the ambient air did not appear to be contaminated with toxic substances from the buildings or their contents or with combustion products to an extent that posed an occupational health hazard. However, the presence of hazards related to specific work activities at the WTC disaster site underscores the importance of assessing suspected occupational exposures. In response to the WTC disaster, NIOSH has issued guidelines for addressing a variety of occupational safety and health hazards at disaster sites (5).


This report is based on data contributed by: New York City School Construction Authority. Data Chem Laboratories, Salt Lake City, Utah. B Bernard, MD, D Booher, G Burr, E Esswein, MSPH, R Hall, MS, J Harney, MS, D Hewett, MS, B King, MPH, S Lenhart, MSPH, B Lushniak, MD, R McCleery, MSPH, K Martinez, MSEE, D Mattorano, MS, A Weber, MS, Div of Surveillance, Hazard Evaluations, and Field Studies; K Linch, MS, P Middendorf, PhD, Div of Respiratory Disease Studies; S Earnest, PhD, A Echt, MPH, J Fernback, A Grote, C Neumeister, E Kennedy, PhD, T Zimmer, PhD, Div of Applied Research Technology, National Institute for Occupational Safety and Health, CDC.


  1. National Institute for Occupational Safety and Health. NIOSH pocket guide to chemical hazards. Cincinnati, Ohio: U.S. Department of Health and Human Services, CDC, National Institute for Occupational Safety and Health, 1997 (DHHS publication no. 97-140).
  2. Occupational Safety and Health Administration. Toxic and hazardous substances, 29 C.F.R. 1910 Subpart Z. U.S. Department of Labor, Occupational Safety and Health Administration. Available at
  3. National Institute for Occupational Safety and Health. NIOSH manual of analytical methods, 4th ed. Cincinnati, Ohio: U.S. Department of Health and Human Services, CDC, National Institute for Occupational Safety and Health, 1994 (DHHS publication no. 94-113).
  4. American Conference of Government Industrial Hygienists. Threshold limit values for chemical substances and physical agents & biological exposure indices. Cincinnati, Ohio: American Conference of Governmental Industrial Hygienists, 2001.
  5. National Institute for Occupational Safety and Health. Suggested guidance for supervisors at disaster rescue sites. Cincinnati, Ohio: U.S. Department of Health and Human Services, CDC, National Institute for Occupational Safety and Health. Available at

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