VI.  OVERVIEW OF THE HEALTH RISK ASSESSMENT

A.  Introduction

As part of the investigation into the potential adverse health effects on Gulf War veterans from exposures to PM, DHSD analysts completed a health risk assessment (HRA) to estimate the potential for long-term effects. Subject matter experts from the private and public sectors and from academia peer-reviewed the HRA, which examines the risks associated with PM exposures, specifically the silica and soot fractions. Based on available air quality data and US personnel unit location information, analysts provided estimates of total exposure to PM and compared these estimates to widely accepted US exposure guidelines. Such a comparison provides an indication of the potential health risks[54] faced by Gulf War veterans from exposure to PM.

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

The literature review searched existing major scientific reference sites including the occupational and environmental health effects literature from OSHA and the Environmental Protection Agency (EPA), National Institute for Occupational Safety and Health reports on silica health effects, and the exposure and health effects studies conducted on Gulf War veterans by the DoD. 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 PM exposure. In addition to this review, the authors of the HRA interviewed members of USAEHA and other groups that authored papers on, or had specific knowledge of, PM exposure.

The exposure assessment uses a methodology described in the National Academy of Sciences Report, "Human Exposure Assessment for Airborne Pollutants/Advances and Opportunities." Federal agencies such as the EPA and OSHA use this methodology 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 per year) and the total dose that the individual accumulates over the period of exposure.

The total dose is calculated by multiplying the cumulative exposure by the daily inhalation rate. The HRA uses an inhalation rate of 24 cubic meters per 24-hour day.[55] This is slightly higher than the 20 cubic meters per day that is used in the literature,[56] but the higher respiration rate would result in an increase in the dose received and more closely reflect several physical factors exhibited by US servicemembers (e.g., higher metabolic rate, increased activity, and higher respiration rate).[57] The USACHPPM uses an even higher inhalation rate in their HRA studies. In their technical guidance document, "Long-term Chemical Exposure Guidelines for Deployed Military Personnel,"[58] the USACHPPM recommends an inhalation rate of 29.2 cubic meters per day when conducting exposure and HRAs involving US servicemembers. This rate is based on studies conducted by the US Army Research Institute of Environmental Medicine.[59] In their report, the institute 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 routine activities conducted on a daily basis, the associated inhalation rate for each of these activities, and an estimate of the time spent on each activity. The inhalation rate is the weighted-average of the rates for the individual activities.[60] The impact of this higher rate is discussed in the section on Cumulative Exposure.

B.  Discussion

The HRA focuses on two types of exposure to assess the potential long-term or chronic effects from inhalation of PM — the cumulative exposure and total dose. It is necessary to consider both types when characterizing the potential chronic effects from silica and soot exposure.

1.  Cumulative Exposure

The cumulative exposure is 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 in milligrams per cubic meter of the contaminant in the ambient air by the length of time in fractions of a year that an individual was exposed. 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 HRA are 1 mg/m3 for silica and 87.5 mg/m3 for soot.[61] These are the acceptable exposure levels below which there would be no observed adverse health effects (NOAEL) expected. The levels are calculated based on the results of human and animal studies. For example, 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.[62]

Contaminant concentration levels are based on monitoring results obtained during the USAEHA monitoring at seven locations in Kuwait and Saudi Arabia in the May-December 1991 time period. The maximum-recorded values at each of these locations are multiplied by an occupationally derived time equivalent of 1.76 years[63] to obtain an estimated cumulative exposure estimate at each of the seven locations. The estimated cumulative exposures for silica at the seven monitoring locations were between 1/50th to 1/10th of the respirable silica acceptable NOAEL cumulative exposure. For soot, the estimated cumulative exposures at the seven monitoring locations ranged between about 1/1000th to 1/250th of the respirable soot acceptable NOAEL cumulative exposure.[64] The specific cumulative exposure estimates for silica and soot and their respective standards are presented in Table 2.

Table 2. Estimated Exposures vs. Health Standards[65]

Contaminant

KTO Estimated Cumulative Exposure (mg/m3)

Acceptable (NOAEL) Cumulative Exposure (mg/m3)

KTO Estimated Total Dose (mg)[66]

Acceptable (NOAEL) Total Dose (mg)

(inhalation rate of 24 cubic meters per day) (inhalation rate of  29.2 cubic meters per day)

Silica

0.02 to 0.10

1

49 to 208

60 to 254

3,066

Soot

0.08 to 0.35

87.5

184 to 735

224 to 897

766,500

Note: 1) The estimated cumulative exposure values for silica and soot are based on maximum recorded values at each location, assume an occupationally equivalent exposure of 1.76 years that is based on an average period of deployment of 153 days[67] , and assume an average inhalation rate of 24 m3/day. 2) The acceptable cumulative exposure and total doses assumed exposure to respirable, crystalline silica. This dose may be overestimated because it is assumed that all silica is in the more toxic crystalline form as opposed to the benign amorphous form. The comparable values for soot are based on total soot from all sources without regard for whether it originated from industrial combustion sources, oil well fires, or transportation sources.

2.  Total Dose

The total dose is the units of mass (mg) of respirable matter that is taken into the body. The HRA assumes that the body absorbs 100 percent of the inhaled PM10. In Table 2 the estimated total dose for inhaled respirable particulates is a function of the estimated cumulative exposure value and an inhalation rate of 24 cubic meters per day for the period of exposure. The acceptable NOAEL total dose is also presented and is a function of the acceptable NOAEL cumulative exposure for silica and soot and 24 cubic meters per day inhalation rate.[68] At total doses below the acceptable NOAEL, one would not expect to see increased chances for the onset of latent or chronic disease from the intake of respirable contaminants. The estimated total dose for silica and soot at each of the seven monitoring locations was about 1/60th to 1/14th the acceptable NOAEL total dose for silica and about 1/4000th to 1/1000th the acceptable NOAEL total dose for soot.[69] These dosages are based on 95 percent upper confidence level data and suggest that the maximum total dose of silica and soot internalized by deployed US personnel is significantly less than the level at which one would expect to see the onset of adverse health effects.

If USACHPPM’s suggested inhalation rate of 29.2 cubic meters per day were used, the total dose received by US servicemembers would increase by about 22 percent. Table 2 presents the estimated total doses for silica and soot based on inhalation rates of both 24 and 29.2 cubic meters per day. Even at the elevated inhalation rate, the total doses for silica and soot remain significantly lower than the acceptable NOAEL total dose and therefore, no latent or chronic health effects would be expected to occur as a result of these PM10 exposures especially in a healthy population. However, acute effects may be expected.

C.  Findings

Although high levels of PM were observed in monitoring activities conducted after the Gulf War, these concentrations fell within a range consistent with background levels observed in Kuwait where the average level of PM10 is nearly 600 mg/m3, the highest in the world. Average PM10 concentrations measured by the US Army Environmental Hygiene Agency during a nine-month period in 1991 ranged from 265 to over 670 mg/m3. This range is about two to five times greater than the US standard of 150 mg/m3. The chemical composition of the samples indicated that roughly 75 percent of the airborne PM consisted of clays (primarily calcium and silica) that originated from the sand; 10 to 23 percent consisted of carbon (soot) that originated from a combination of the oil well fires and various industrial sources; and less than 10 percent came frommiscellaneous sources.

Several studies report on the acute health effects from exposure to PM. Typical symptoms reported in the literature were similar to those experienced by US personnel and include cough, runny nose, eye and throat irritation, and shortness of breath. These symptoms also have been associated with common viral and bacterial infections in Gulf War veterans and are brought about in some situations by the communal living lifestyle of deployed US personnel. However, medical records for both out-patient and hospitalization cases do not, as a rule, specify the cause of the symptoms, making it difficult to determine the relative percent of adverse respiratory health effects caused by PM.

A review of the data from air particulate sampling conducted from May-December 1991 indicates that ambient levels of PM10 were significantly high and that concentrations often exceeded the levels considered safe for the protection of human health. Particles in this size range have the potential for entering the lower respiratory tract where they are deposited. Both of these factors suggest that some personnel with pre-existing respiratory problems may have experienced aggravated symptoms. For example, the inhalation of ambient levels of PM10 could have caused acute symptoms and could have aggravated asthmatic conditions in some personnel.

The medical literature was reviewed for adverse health effects from long-term exposure to silica and soot. The literature notes that, when PM is found at high concentrations in an occupational environment and under conditions of extended exposure, changes in lung function, damage to lung tissue, and altered respiratory defense mechanisms (an impaired ability to naturally eject foreign matter via exhalation) have resulted. However, health professionals do not consider occupational exposures to be similar to the exposures experienced by US forces during the Gulf War. In occupational studies, exposure estimates are a function of the duration of the exposure, which is typically expressed as 8 hours per day, 40 hours per week, 52 weeks per year. In contrast, Gulf War veterans were exposed 24 hours per day for 153 days (average length of deployment), not the working lifetime used in occupational exposure assessments. Therefore, one to one comparisons of occupational and Gulf War-type exposure levels may not be appropriate, especially when attempting to compare total dose or body burdens.

The HRA calculated the estimated cumulative exposures and estimated total dosages of respirable silica and soot and compared them to US exposure guidelines. The guidelines are expressed as a NOAEL that represents the concentration below which no adverse effects have been observed during human and animal laboratory and clinical studies. The report concludes that ambient levels of PM10 often exceeded levels established to protect health in the US. However, due to the relatively low duration of exposure (1.76-year equivalent compared to a working lifetime) the estimated cumulative exposures and estimated total dosages to which US personnel were exposed were below the levels known to cause adverse health effects. Therefore, long-term health effects would not be expected to occur.[70] However, reversible, short-term or acute effects may occur. These may include runny nose; eye, nose, and throat irritation; cough; and shortness of breath.

The HRA also addressed dermal exposures to PM. Silica dusts in an occupational setting have been associated with specific types of dermatitis or skin inflammation. The HRA notes, however, that these irritations are not expected to produce chronic skin disorders in Gulf War veterans, as longer exposure periods (typically greater than three years) are normally required to cause these symptoms.[71]

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