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Risk Assessment is another type of causation: predicting whether an exposure, usually in a population rather than a single individual, will increase the risk of developing some adverse health effect, without necessarily determining whether such health effect actually occurs. Risk means that all such exposed individuals are more likely to develop an adverse effect, but it does not mean that any particular individual will or even that the majority of individuals will. Smoking, for example, increases risk of lung cancer from approximately one in 100 to 1 in 10--a significant increase. Nevertheless, ninety percent of smokers do not develop lung cancer, even though they are all at increased risk. Risk Assessment is used by regulatory agencies, such as the EPA, to determine how much of a chemical can be released into the environment without causing an unacceptable increase in risk of an adverse effect. 'Unacceptable' is more of a policy decision than a scientific one.
Specific causation analysis can be divided into the following three components:
(1) Hazard Assessment
(2) Exposure Assessment
(3) Health Assessment
(1) Hazard Assessment: what harm can the chemical cause, based on intrinsic toxicity and circumstances of exposure, form of chemical (gas, liquid, solid) and susceptibility of individual.
(2) Exposure Assessment how much of the chemical is in a media (air, water, food, soil) available to be taken into the body? Is this a one-time or multiple exposure? Does it occur over a short period of time (acute) or over many months or years (chronic)? How does the length of exposure affect the toxicity of the chemical? How much of the chemical gets absorbed into the body (dose), and where in the body does it end up (distribution and target organ/tissue)?
(3) Health Assessment: what type of health effect develops (or gets exacerbated), and does this occur immediately or after a delay (lag time)? Is this a new effect in the individual or is there a history of this type of problem, made worse by the chemical exposure? Is the individual in a high risk group (in utero, infant, elderly, reduced immune function)? Are there other (alternative) known causes for this problem and were these causes present ?
A recent study in the Bronx by researchers at New York University School of Medicine looked at the relationship of air pollution to asthma sufferers at four Bronx schools. What was unique about this study was that individuals were outfitted with portable monitors so that air pollution levels could be monitored directly to the individual and compared with pulmonary function testing, as well as with diary recordings of subjective symptoms. The four schools in the Bronx are in proximity to a number of major highway complexes--Cross Bronx, Major Deegen, Bruckner and the Bronx River Parkway, from which very heavy car and truck traffic constantly spews exhaust containing particulate matter, sulfur oxides and nitrogen oxides, as compared to those without asthma.
The particulate matter, especially that less than 2.5 microns in diameter (fine particulate) has been identified as a major culprit of this effect, as well as other respiratory problems and heart and circulatory abnormalities. This is because these particles, sometimes incorrectly referred to as soot (which is only one type of particle involved), are able to penetrate deep into the lungs, into the respiratory region which includes the narrow bronchioles and alveolar sacs--where oxygen exchange take splace with the circulatory system. It is at this level that damage and inflammation can occur to excerbate breathing problems. It is important to note that the sulfur dioxides and nitrogen dioxides can be quite irritating to lung tissue and can adsorb onto the particulate matter and be carried into the lungs. These chemicals can cause reversible or irreversible damage to the lung tissue and may likely be involved in this effect that is observed.
The study found that on the days with the highest polluiton levels as recorded by the individual monitors asthma symptoms were twice as bad among the same individiuals. It is estimated that students at these schools are within 500 feet of one of the major highways, throughout their grade school through high school education. Bronx students display one of the highest averages of asthma--20 to 25 percent of students--in the nation, about three times the national average.
It was found that about ten percent of soot pollution was contributed by truck traffic but that this appears to account for the greatest exacerbation of symptoms. Again, because of other toxics in the air, including nitrogen and sulfur oxides, and other chemicals from a dozen nearby waste transfer stations, the effect is likely to be some interaction between these compounds.
Asthma and Air Pollution
By Thomas F. Schrager, Ph.D.