Risk Characterization

The final stage of a four‐stage human health risk assessment is to estimate risks. Cancer is not the only undesired health consequence of pollution: exposure to ionizing radiation and coal dust, for example, can lead to radiation sickness and birth defects, as well as cancer. To illustrate risk characterization; however, we shall focus on the calculations that yield quantitative estimates of both the carcinogenic and noncarcinogenic risks to receptors for all exposure scenarios considered. Estimates are typically calculated for all three exposure routes and for the maximally exposed individual as well as the most probable exposed population. Such calculations are straightforward and yield quantitative estimates of risk. The challenge lies not in making the calculations but in interpreting the results such that they are applied properly in decision‐making. The overall effort is referred to as risk characterization. Risks from toxic chemicals, depending on the context, may be defined, described, and calculated in different ways.

Risk is normally defined as the probability for an individual to suffer an adverse effect from an event. What is the probability that certain types of cancer will develop in people exposed to aflatoxin in peanut products or benzene from gasoline? What is the likelihood that workers exposed to lead will develop nervous system disorders? In the context of this text, a chemical release is an example of an event. As with any relationship expressing or using probability, there is no defined way of expressing (mathematically or with scientific rigor) a single deterministic value of a phenomenon that is probabilistic. A conceptual way of expressing chemical risk is to write it as a function of hazard and exposure.

Hazard is the potential for a substance or situation to cause harm or to create adverse impacts on persons or the environment. The magnitude of the hazard reflects the potential adverse consequences, including mortality, shortened life‐span, impairment of bodily function, sensitization to chemicals in the environment, or diminished ability to reproduce. Exposure denotes the magnitude and the length of time the organism is in contact with an environmental contaminant, including chemical, radiation, or biological contaminants.

When risk is in terms of probability, it is expressed as a fraction, without units. It has values from 0.0 (absolute certainty that there is no risk) to 1.0 (absolute certainty that an adverse outcome will occur).

For chemicals the term hazard is typically associated with the toxic properties of a chemical specific to the type of exposure. Similar chemicals would have similar innate hazards. However, one must examine the exposure to that hazard to determine the risk. For example, let us say you have three pumps that are all transporting the same chemical (same hazard), but one pump has a seal leak. Which pump poses the greatest risk to the worker? The pump with the seal leak has the greatest potential for exposure, while the hazards are equal (same chemical), so the seal leak pump poses the greatest risk. To expand, let us say we have three pumps that are transporting different chemicals; which one poses the greatest risk to the worker? In this case, the engineer would need to examine the hazard, or innate inherent toxicity, of each of the chemicals, as well as the operation of the pumps to determine which poses the greatest risk.

Risk for Average and Maximum Exposures

In the exercises required for Superfund sites, both average and maximum exposure point concentrations are used to estimate risk. Performing a specific risk calculation using both values permits the estimation of a range of potential risks, which can frequently be useful in providing perspective regarding the potential hazards associated with a particular set of exposure conditions. The significance of either measurement depends on the amount of data and the associated confidence in that data. However, in general, calculation of potential risk using an average concentration permits a better estimate of risk associated with chronic exposures, since the average value represents a more likely estimate of the exposure point concentration to which a receptor would be exposed over time. Use of a maximum value is best in the estimation of shorter‐term, subchronic risks, although its use can provide a useful upper bound estimate of potential risk.

It should be noted that current risk assessment guidance emphasizes the use of a single, upper bound estimate of exposure point concentration in the calculation of potential risks. This value is often taken to be 95% CI, the 95th percentile upper confidence limit of the arithmetic mean. Use of this number generally provides a worst‐case estimate of risk, and can result in a significant overestimate of potential risk, especially when this value is used in combination with other worst‐case assumptions to define a reasonable maximum exposure.

Carcinogenic Risk

Carcinogenic risk may be defined as the chronic daily intake dose (developed in the exposure assessment) multiplied by the carcinogenic slope (selected by the toxicity assessment). The product is a real term: the probability of excess lifetime cancer from exposure to this chemical. The computation is as follows:

(5.4)

where

• CDI = chronic daily intake (mg/kg/day)
• SF = carcinogen slope factor (mg/day/mg)⁻¹

Characterization of carcinogenic risk first involves determining the intake for each chemical appropriate to the exposure route and pathway under study then multiplying each one by the proper exposure point concentration and slope factor. The classical methodology for risk assessment assumes additivity of risks from individual toxicants. For carcinogens this means that the total carcinogenic risk equals the sum by exposure route of carcinogenic risks from all individual substances. These calculations are repeated for each exposure scenario and exposed population. It must be emphasized that slope factors are specific to the exposure route (e.g. oral) and may be used only when the exposure data apply to the same route. With this caveat in mind, we can proceed to a consideration of assessing the risks associated with some familiar environmental pollutants.