Toxicity testing is normally carried out under laboratory conditions to measure effects on target organisms exposed to a chemical in various concentrations during a specified time. It is not always possible or appropriate to conduct toxicity tests on species that actually occur at the site. A representative species is typically used as a surrogate for laboratory testing, especially in cases where an endangered species is the species of concern, where no data are available, or proven testing techniques are not known for the species of concern.
On the basis of the types of exposures to chemicals expected to occur at the contaminated site, field laboratory tests may be necessary to determine acute or chronic effects. In cases of acute toxicity exposures may lead to death or extensive impairment of physiological functions and behavior of individual organisms relatively soon (i.e., hours or days) after exposure to contaminants. Chronic toxicity occurs when organisms are exposed to low levels of contaminants over an extended time period (i.e., weeks, months, years).
In addition to considering whether the exposure is chronic or acute, the effects of contaminants can be broadly segregated into lethal and sublethal effects. Lethal effects, as the term implies, refers to direct mortality of exposed individuals. Sublethal refers to adverse effects other than death for exposed individuals. Examples of sublethal effects that may result from chemical exposure include reproductive impairment, reduced rates of growth or photosynthesis, behavioral changes, and increased susceptibility to disease or other environmental stresses (e.g., temperature changes). From an ecological standpoint, many sublethal effects may be as relevant as direct lethality from a contaminant. For example, if a contaminant significantly reduces photosynthesis, the change in plant production may lead to a reduction in organisms in all other trophic levels. Similarly, a contaminant exposure that results in reproductive failure for a given species will likely have drastic population-level effects within a single generation. Thus, an ERA will ultimately need to consider the ecological significance of any toxic effects.