Thomas, et al (2000) has completed a peer-reviewed study on Gulf War veteran exposure to particulate matter. The heart of the study is an exposure assessment that examines particulate matter concentrations, specifically the silica and soot fractions, to which US personnel were exposed during the Gulf War. Based on available air quality data and US personnel unit location information, estimates were made of total exposure to particulate matter and compared to widely accepted US exposure guidelines. Such a comparison provides some sense of the potential health risks^[48] faced by Gulf War veterans from exposure to particulate matter.

The main elements of the Thomas report are: 1) the data obtained during a comprehensive air quality monitoring study completed in 1991 by USAEHA (see Section III); 2) a scientific review of the literature on the environmental and occupational health effects of exposure to particulate matter; and 3) a standard methodology to assess the effect of exposure to particulates. Information on the physical properties of particulate matter and its sources is also discussed in the report.

The literature review searched existing major scientific databases; these included: the occupational and environmental health effects literature from the Occupational Safety and Health Administration (OSHA) and the US Environmental Protection Agency (EPA), National Institute for Occupational Safety and Health (NIOSH) reports on silica health effects, and the exposure and health effects studies conducted on Gulf War veterans by the Department of Defense. The review also included relevant exposure and health effects studies of human populations in the Middle East. In all, the review covered 154 articles related to particulate matter exposure. In addition to this review, the authors of the Thomas report also interviewed members of the US Army Center for Health Promotion and Preventive Medicine (USACHPPM), formerly known as USAEHA, and other groups which authored papers on, or had specific knowledge of, particulate matter exposure.

The exposure assessment uses a methodology described in the National Academy of Sciences (NAS) Report, "Human Exposure Assessment for Airborne Pollutants/Advances and Opportunities." This methodology is used by federal agencies like EPA and OSHA to develop total human exposure scenarios (24-hours a day, seven days a week). The methodology estimates the cumulative exposure to which an individual is exposed (measured in units of concentration versus time, i.e., milligrams per cubic meter times year) and the total dose that the individual accumulates over the period of exposure.

The total dose (discussed in section VI.B.2) is calculated by multiplying the cumulative exposure by the daily inhalation rate. The Thomas report uses an inhalation rate of 24 cubic meters per day.^[49] This is slightly higher than the 20 cubic meters per day that is used in the literature;^[50] but, as the authors indicate, adds a conservative level to the analyses due to several physical factors exhibited by US troops (i.e., higher metabolic rate, increased activity, and higher respiration rate).^[51] The USACHPPM has developed an even more conservative inhalation rate value for use in their health risk assessment studies. In their technical guidance document, "Long-term Chemical Exposure Guidelines for Deployed Military Personnel,"^[52] the CHPPM recommends an inhalation rate of 29.2 cubic meters per day when conducting exposure and health risk assessments involving US troops. This rate is based on studies conducted by the US Army Research Institute of Environmental Medicine (USARIEM).^[53] In their report the USARIEM estimates the metabolic rate as an indicator of heat stress for various physical tasks routinely performed in a military environment, and provides information on the inhalation rates associated with each of these activities. The inhalation rate of 29.2 cubic meters per day is a function of the activities that are conducted (i.e., those that are considered to be ‘routine’ or representative) on a daily basis, the associated inhalation rate for each of these activities, and an estimate of the amount of time spent on each activity. The inhalation rate, therefore, is the weighted average of the rates for the individual activities.^[54] The impact of this higher inhalation rate is discussed below.

The Thomas report focuses on two types of exposure to assess the potential long-term or chronic effects from the inhalation of particulate matter. These are the cumulative exposure and total dose. It is necessary to consider both types of exposure when characterizing the potential chronic effects from silica and soot exposure.

This is a measure of the amount of a contaminant to which an individual is exposed over a specified time period. Its value is calculated by multiplying the measured concentration (expressed in milligrams per cubic meter) of the contaminant in the ambient air by the length of time (expressed in fractions of a year) that an individual was exposed to that concentration. Cumulative exposure is significant because it provides an indication of when the level of a contaminant in the air may approach levels of concern when compared to an established air quality standard. The standards for comparison used in the Thomas report are 1 mg/m³ x year for silica and 87.5 mg/m³ x year for soot. These are the levels below which there are no observed adverse health effects (NOAEL). The levels are calculated based on the results of human and animal studies. For example, the Thomas report notes that the risk of chronic health effects from silica exposure over a 5 to 45 year time frame begins to occur at cumulative exposure levels above 1 mg/m³ x year.^[55]

Contaminant concentration levels were determined based on monitoring results obtained during the USAEHA monitoring at seven locations in Kuwait and Saudi Arabia. The maximum-recorded values at each of these locations were multiplied by an occupationally derived time equivalent of 1.76 years^[56] to obtain a worst-case cumulative exposure estimate at each of the seven locations. When compared against the cumulative exposure NOAEL, the estimated cumulative exposures for silica at the seven monitoring locations were between 1/50^th to 1/10^th of the respirable silica acceptable cumulative exposure NOAEL. When compared against the cumulative exposure NOAEL for soot the estimated cumulative exposures for soot at the seven monitoring locations ranged between about 1/1000^th to 1/250^th the respirable soot acceptable cumulative exposure NOAEL.^[57] The specific cumulative exposure estimates for silica and soot and their respective standards are presented in Table 2. Since the estimated cumulative exposure levels for silica and soot are well below their respective NOAEL, it is unlikely that the concentrations to which US personnel were exposed while in the KTO would result in the onset of adverse health effects.

This is a measure of the amount of respirable matter that is actually absorbed by the body. The total dose for inhaled respirable particulates is estimated by multiplying the cumulative exposure value by an inhalation rate of 24 cubic meters per day. Therefore, the NOAEL for total dose is obtained by multiplying the cumulative exposure NOAEL for silica and soot by the inhalation rate of 24 cubic meters per day. At total doses below this level one would not expect to see increased chances for the onset of disease from the intake of respirable contaminants. The estimated total dose for silica and soot at each of the seven monitoring locations was about 1/60^th to about 1/14^th the total dose NOAEL for silica and 1/4000^th to about 1/1000^th the total dose NOAEL for soot.^[58] In other words the maximum total dose of silica and soot received by US personnel is significantly less than the level at which one would expect to see the onset of adverse health effects. These values and their relationship to the applicable NOAEL are also presented in Table 2.

If an inhalation rate of 29.2 cubic meters per day were used as suggested by the CHPPM the total dose received by US troops would increase by about 22%. Table 2 presents the total doses for silica and soot based on an inhalation rate of 29.2 cubic meters per day for comparison against the acceptable total dose NOAEL. From the table it can be seen that even at the elevated inhalation rate the total doses for silica and soot remain significantly lower than the acceptable total dose NOAEL.