Estimating Chemical Exposure

Theron Blickenstaff
Occupational and Environmental
Medicine Consulting
Kingsport, Tennessee

Risk is a function of hazard and exposure. For a very large number of chemicals, we know a great deal about hazard. Millions upon millions of dollars have been spent on animal testing and other types of studies to elucidate the harm that might be done. The exposure part of the equation, however, is much less well understood and is widely accepted to be the Achilles' heel of risk assessment.

What is meant, toxicologically speaking, by "exposure?" In order to do harm, a substance must come into direct contact with living tissue in such a way that it can exert an effect. Merely being in the proximity of a chemical does not constitute exposure, as long as it is properly contained.

There are four main ways to be exposed to something: you can swallow it, breathe it, get it on your skin, or have it injected. These correspond to the toxicological concepts of ingestion, inhalation, and dermal and parenteral exposures. There can be a direct effect on the surface, such as a chemical burn, or the substance can be absorbed and cause damage to the absorbing tissue. It may also travel by way of blood or the lymphatic system to other parts of the body. If something has no direct toxic effect, and it is not absorbed, it can cause no harm except mechanically. An example might be a piece of plastic that is swallowed and passes through the digestive system intact.

What if you smell something? Doesn't that mean you are exposed? Yes, but this brings up the most basic concept of toxicology: dose. Nearly all harmful effects are dose-dependent. Our sense of smell is capable of detecting some chemicals that are present in the air at concentrations of parts per billion, far too little to cause harm. Other chemicals, some of which are quite toxic, can't be detected at all even at extremely high concentrations - for example, carbon monoxide. The olfactory nerve connects directly to the limbic portion of the brain, where emotions reside. Perhaps this is why it is very difficult for the average person to accept that there is a total lack of correlation between odor and risk.

Determining what dose of a substance someone is exposed to usually involves making a series of assumptions such as what is the concentration, how much of the medium comes into contact with the person, and how much is actually absorbed. In the classical risk assessment, the worst case is assumed at each step, then all these worst case assumptions, plus some safety factors, are multiplied to come up with a total. Many believe that this results in greatly exaggerated risk estimates.

Three examples might help put our ability to accurately estimate exposure into context: medication, the workplace, and ambient pollution.

1. Perhaps the most accurate exposure estimates come from the taking of medication. If someone is administered a known amount of a drug, using knowledge of pharmacokinetics derived from studies of groups of individuals, the amount reaching a target organ can be estimated. Sometimes we can even take a blood sample and get a more precise estimate. Measuring levels within tissues would be even more exact, but is not often done in humans. Without direct measurements, we make estimates that may or may not be very accurate in any given person because of individual variability.
2. Intermediate in degree of difficulty might be a workplace situation where the level of contamination in the air has been measured by industrial hygiene methods. Detected levels can be compared to occupational exposure limits such as permissible exposure limits (PELs) or threshold limit values (TLVs). The ambient air in a work area can be analyzed, or a more precise measurement can be obtained by attaching the collecting device directly to the employee. A worker's blood, urine or expired air can also be analyzed and the results compared to biological exposure indices (BEIs). Levels below these standards are assumed to be safe for the majority of workers.
3. The least accurate estimates of exposure in our series of examples come from the case of ambient levels of contaminants in the environment. Direct measurements for an individual are very rarely done. Usually monitoring data are used, and these will be less accurate the farther away the person is from the monitoring site, and the less time the individual spends outdoors. For a soil contaminant, for example, it is assumed that there is a certain concentration in the soil, that a certain amount of soil gets ingested, another amount may become airborne and breathed, and a different amount may make its way into the water and get imbibed. Inevitably, there is some error in each of the estimates, so the total error tends to increase as the estimates are summed.

Recently the Centers for Disease Control and Prevention (CDC) has undertaken a Biomonitoring Program that promises to increase our knowledge of the amounts of environmental chemicals in the human body by analyzing blood, urine, serum, saliva or tissues. Definitive answers to the multitude of questions in this area are years away. Meanwhile we must deal with public concern over the mere detection of any amount of a contaminant, regardless of what it may or may not actually mean.

Theron Blickenstaff is owner of Occupational and Environmental Medicine Consulting in Kingsport, Tennessee. Dr. Blickenstaff provides litigation support in Workers' Compensation, toxic tort and product liability cases, and consults in epidemiology and chemical product hazard communications. He has a medical degree from the University of Cincinnati, an undergraduate degree in biology from Manchester College, a masters of public health degree from the University of North Carolina at Chapel Hill, and is board certified in occupational and environmental medicine.