PRINCIPLES OF TOXICOLOGY

18.1 RISK ASSESSMENT BASICS 441
For example, a risk assessment for a contaminated site might include consideration of workers at the
site, occasional trespassers or visitors to the site, or individuals who live at the site if the land is (or
might become) used for residential purposes. If residential land use is contemplated, risks are often
calculated separately for children and adults, since they may be exposed to different extents and
therefore have different risks. Depending on the goals of the risk assessment, risks may be calculated
for one or several populations of interest.
Many chemicals move readily in the environment, from one medium to another. Thus, a chemical
spilled on the ground can volatilize into the air, migrate to groundwater and contaminate a drinking
water supply, or be carried with surface water runoff to a nearby stream or lake. Risk assessments have
to be cognizant of environmental movement of chemicals, and the fact that an individual can be exposed
to chemicals by a variety of pathways. In formulating the risk problem, the risk assessor must determine
which of many possible pathways are complete; that is, which pathways will result in movement of
chemicals to a point where contact with an individual will occur. Each complete pathway provides the
opportunity for the individual to receive a dose of the chemical, and should be considered in some
fashion in the risk assessment. Incomplete exposure pathways—those that do not result in an individual
coming in contact with contaminated environmental media (e.g., air, water, soil)—can be ignored,
since they offer no possibility of receiving a dose of chemical and therefore pose no risk.
Risk assessments can vary considerably in the extent to which information on environmental fate
of contaminants is included in the analysis. Some risk assessments, for example, have attempted to
address risks posed by chemicals released to the air in incinerator emissions, and subsequently
deposited on the ground where they are taken up by forage crops that are consumed by dairy cattle.
Consumption of meat or milk from these cattle was regarded as a complete exposure pathway from
the incinerator to a human receptor. As the thoroughness of the risk assessment increases, so does the
complexity. As a practical matter, complete exposure pathways that are thought to be minor contributors
to total exposure and risk are often acknowledged but not included in the calculation of risk to make
the analysis more manageable.
Often, exposure can lead to uptake of a chemical by more than one route. For example, contaminants in
soil can enter the body through dermal absorption, accidental ingestion of small amounts of soil, or inhalation
of contaminants volatilized from soil or adherent to small dust particles. Consequently, the manner of
anticipated exposure is important to consider, as it will dictate the routes of exposure (i.e., inhalation,
dermal contact, or ingestion) that need to be included in the risk assessment for each exposure scenario.
As discussed in the following section contrasting human health and ecological risk assessment,
problem formulation is more challenging when conducting ecological risk assessments. Instead of one
species, there are several to consider. Also, the exposure pathway analysis is more complicated, at least
in part because some of the species of interest consume other species of interest, thereby acquiring
their body burden of chemical. Unlike human health risk assessments, where protection of individuals
against any serious health impact is nearly always the objective, goals for ecological risk assessments
are often at the population, or even ecosystem, level rather than focusing on individual plants and
animals. Consequently, development of assessment and measurement endpoints consistent with the
goals of the ecological risk assessment is essential in problem formulation for these kinds of analyses.
Human Health versus Ecological Risk Assessments: Fundamental Differences
Ecological risk assessments are defined as those that address species other than humans, namely, plant
and wildlife populations. Historically, the risk assessment process has focused primarily on addressing
potential adverse effects to exposed human populations, and the development of well-defined methods
for human health risk assessment preceded those for ecological risk assessment. However, increasing
concern for ecological impacts of chemical contamination has led to a “catching up” in risk assessment
methodology. While detailed methods for both human health and ecological risk assessment are now
in place, they aren’t identical. The conceptual basis may be similar, including some form of hazard
identification, exposure assessment, dose–response assessment, and risk characterization. However,