Gulf War report confirms link between lung cancer and combustion products

Dec 21 2004

The available evidence is too sparse or of insufficient quality to determine whether the majority of health problems that may be experienced by Gulf War veterans could be associated with exposures to fuels for military vehicles, propellents in Scud missiles, or substances given off by combustion sources such as oil-well fires, exhausts, and tent heaters, according to the latest report on the Gulf War and health from the Institute of Medicine of the National Academies.

However, data from studies of occupational and environmental exposures to air pollution, vehicle exhaust, and other combustion products led the committee that wrote the report to conclude that exposure to such substances is associated with an increased risk of lung cancer.

"Studies of people exposed to air pollution, vehicle exhaust, and burning of coal or other heating and cooking fuels consistently show that such exposures are linked to an increased risk for developing lung cancer," said committee chair Lynn Goldman, professor, Bloomberg School of Public Health, Johns Hopkins University, Baltimore. "This provides sufficient evidence that exposure to combustion products during the Gulf War could be associated with lung cancer in some veterans." Military personnel may have encountered combustion products from diesel-fueled heaters in poorly ventilated tents, cooking stoves, vehicle exhaust systems, and oil-well fires. "It should be emphasized that smoking is the major culprit for lung cancer, accounting for 80 percent of all cases, according to the American Cancer Society," Goldman added.

The committee also found some evidence that exposure to combustion products is linked to asthma and cancers of the nose, mouth, throat, and bladder, as well as to low birth weight and premature births in women exposed while pregnant; the data were weaker in these cases, however. The data on whether the majority of cancers, neurological problems, and other health problems are associated with exposure to fuels, propellants, or combustion products were inadequate to draw conclusions. "While we would like to have more definitive answers to questions about the specific diseases that may be associated with these substances, in most cases the evidence simply is not strong enough or does not exist," Goldman said.

Because scant information exists on actual exposure levels experienced by individual service members -- a critical factor when assessing health effects -- the committee could not draw specific conclusions about Gulf War veterans' chances of developing lung cancer or any other health problems as a result of exposures. No systematic monitoring of air contamination from oil-well fires was conducted in the Persian Gulf region until May 1991, and this monitoring did not measure levels of contamination produced by other combustion sources, such as heaters or engines. Moreover, no data are available that would allow comparisons between levels of exposure to air contaminants during the Gulf War and exposures to similar contaminants in civilian occupational and environmental settings.

Veterans who have experienced chronic health problems following their service in the Persian Gulf region are asking whether exposure to various chemical, biological, or environmental agents might be responsible. This IOM report is the third in a series that responds to requests from the U.S. Department of Veterans Affairs and Congress to examine the health effects of potentially harmful agents to which Gulf War veterans might have been exposed. The first report focused on potential health effects from depleted uranium, pyridostigmine bromide, sarin, and vaccines; the second centered on insecticides and solvents. These reports did not directly assess whether health effects could occur as a result of service in the Gulf War.

For the current report, the committee evaluated the published, peer-reviewed research on exposure to unburned fuels, combustion products, and hydrazines and nitric acid -- components of the propellant used for Scud and other missiles -- for any evidence of links to specific cancers, neurological effects, or other health problems that persist after exposure. More than 600 oil-well fires were ignited in Kuwait by retreating Iraqi troops during the Gulf War conflict, sending up large plumes of smoke that occasionally remained low to the ground. Troops also may have been exposed to combustion products through vehicle exhaust, heaters in poorly ventilated tents, and cooking stoves. Military personnel may have had contact with hydrazines and nitric acid when they disarmed or disposed of Scud missiles or were downwind of a missile explosion. They also may have come into contact with fuels when refueling ground vehicles, aircraft, and equipment.

Of the approximately 800 studies reviewed in detail for this report, most involved individuals who were exposed to these agents in occupational settings over long periods of time. Only a small number actually studied veterans who may have been exposed while serving in the Persian Gulf. The committee carefully assessed the quality, limitations, and relevance of each epidemiologic study, and used five categories to describe the strength of the evidence.

Sufficient evidence of a causal relationship, the strongest level of evidence, means that many studies have established a clear link between exposure to an agent and a health outcome. Among the other requirements, there must be a plausible biological explanation for the relationship. None of the compounds evaluated in this report met these criteria.

Evidence that establishes a link between exposures and a health outcome with reasonable certainty, but fails to meet the higher standard of proof needed for causality, is characterized as sufficient evidence of an association. The evidence for an association between lung cancer and combustion products falls into this category.

When a limited number of studies suggest that a link exists, but without reasonable certainty, the evidence is said to be limited or suggestive of an association. This category describes the evidence for links between combustion products and nasal, oral, laryngeal, and bladder cancers; asthma; and low birth weight and preterm births by women exposed while pregnant. Likewise, the evidence for an association between hydrazine exposure and lung cancer fits this definition.

If several studies of adequate quality consistently fail to show a positive association at any level of exposure, the evidence is described as limited or suggestive of no association. And evidence that lacks sufficient quality, consistency, or statistical power to draw any conclusion is judged to be inadequate or insufficient to determine whether an association exists. The majority of the evidence on fuels, combustion products, and propellants falls into this final category.