When evaluating possible effects from exposure to chemical contaminants one must carefully evaluate doses likely to be received by individuals throughout their lifetime or at critical periods within their life cycle. Based on toxicity test results one can estimate dose-response relationships for the species being evaluated. It is important to remember that concentration and dose are not synonymous. Concentration refers to the amount of contaminant in a given amount of media (e.g., a surface water concentration for lead could be 10µg/l). Dose refers to the amount of contaminant that an organism is exposed to, usually in amount of contaminant per unit body weight of the organism (e.g., a dose of lead to a bird might be expressed as 5 mg lead/kg body weight). Sometimes, dose is expressed as a dose per unit time (e.g., 5 mg lead/kg body weight/day).
Depending upon whether a particular toxicity test is designed to evaluate the effects of media concentration or the dose received by an organism, two statistics are commonly used to describe the degree of lethality: the median lethal concentration (LC50) or median lethal dose (LD50). These statistics refer to the concentration or dose that kills 50 percent of the exposed test organisms. Similarly, sublethal effects can be statistically described as the median effect concentration (EC50) or median effect dose (ED50). These metrics identify the concentration or dose at which 50% of an exposed population is affected. When comparing the toxicity of contaminants using these statistics, lower values indicate greater toxicity. Figure 2.1 and Figure 2.2 show hypothetical dose responses for sheepshead minnows exposed to contaminants at different concentrations under different exposure durations.
Researchers conducting toxicity tests may also identify chemical concentration (NOAEC) or dose (NOAEL) levels below which there are no adverse effects on the test organisms. The lowest level at which an adverse effect is observed is referred to as the lowest observed adverse effect concentration (LOAEC) or level (LOAEL for doses). Together, a NOAEL and a LOAEL (or NOAEC and LOAEC) describe a range for which effects are unknown. The assumption is that effects begin to occur somewhere within this range. Figure 2.3 depicts a hypothetical NOAEC and LOAEC observed during toxicity testing of sheepshead minnows exposed to different concentrations of a contaminant. Depending on how large the range between the NOAEC and LOAEC is, it may be desirable to conduct additional toxicity tests for an ERA. For example, the uncertainty range for a contaminant with a NOAEC of 10 ppm and a LOAEC of 200 ppm may be unacceptable to the risk assessment team and thus require additional exposure tests to more narrowly identify where effects begin to occur.
Susceptibility to chemical contaminants may differ among various life stages of an organism. Table 2.2 shows the variation in LOAECs for different life stages of fathead minnows exposed to a hypothetical contaminant.