Environmental Engineering Reference
In-Depth Information
Class D, “not classifi able” (USEPA, 2001a). All further
discussion will focus on approaches for applying noncancer
risk estimates to the management of environmental mer-
cury. Methylmercury is classifi ed as class C (USEPA, 1995).
This follows the general form of establishing a denomina-
tor value, either a reference dose (RfD from the EPA), a min-
imal risk level (MRL from the ATSDR), an allowable daily
intake (ADI from the Food and Dr ug Administration [FDA]),
or a provisional tolerable daily or weekly intake (PTDI or
PTWI from the World Health Organization (WHO) (FAO
[Food and Agriculture Organization]/WHO, 2003). If the
daily exposure is lower than the denominator value, the
hazard quotient (HQ) is less than 1. If the HQ exceeds 1,
then exposure is considered excessive.
food chain. In aquatic organisms, bio-accumulation is typi-
cally expressed using a ratio of chemical concentration in
the organism's tissue (often whole body) relative to chemi-
cal exposure concentrations in water or food organisms.
These bio-accumulation factors are highly variable among
organisms, and they usually vary inversely with exposure
concentration in the medium (DeForest et al., 2007).
In addition to directly examining levels of contaminants
such as mercury in organisms and their tissues, scientists
can use other organisms as bioindicators or they can place
sentinels (or surrogate species) in the contaminated environ-
ment and periodically measure contamination or effect (Van
der Schalie et al., 1999). In the former case, risk evaluators use
organisms at the same trophic level to assess exposure; top-
level predatory fi sh, birds, or mammals might indicate the
potential exposure of people to the same fi sh (see below). In
the latter case, other species are placed in the environment to
assess effects. For example, in urban environments cats can
be used as sentinels of exposure or effects of contaminants,
and in mines canaries were placed to assess the presence of
toxic gases. Earthworms of various species are often used as
sentinels for soil ecotoxicity evaluation (He et al., 2006).
PROBLEM FORMULATION
The problem-formulation stage for risk evaluation is criti-
cal because it determines the scope and extent of the prob-
lem in terms of the pollutants of concern (or mixtures of
contaminants), species of concern, and habitats of concern.
Considerable care is required for this phase, and it is at
this stage that the widest range of stakeholders should be
involved (PCCRARM, 1997). Further, stakeholders should
and can be included in other stages of the risk assessment,
especially if further data are required to evaluate pathways
and levels of exposure (Burger et al., 2007a,b).
RISK ASSESSMENT AND ENVIRONMENTAL JUSTICE
Environmental justice is the fair treatment and meaningful
involvement of all people, regardless of race, color, national
origin, or income with respect to the development, imple-
mentation, and enforcement of environmental laws, regu-
lations, and policies (USEPA, 2009a; USDOE [Department of
Energy], 2009). Executive Order 12898, signed by President
Clinton, states that communities should have the right to
have their opinions and perceptions heard and to be part
of the decision-making process on matters that affect their
well-being. Federal agencies were directed to take action to
address disparities leading to environmental injustice.
All phases of risk evaluation and risk assessment are man-
dated to take into account environmental justice communi-
ties, and the special exposures they face because of lifestyles,
proximity, or other factors (Corburn, 2002). Matrices devel-
oped to capture exposures should take into account Native
American, Alaskan Native, and minority and low-income pop-
ulations and the special exposures they face (Donatuto and
Harper, 2008; Burger et al., 2010; Burger and Gochfeld, 2011).
HAZARD ASSESSMENT AND SOURCES
One of the key features of risk assessment is the determi-
nation of the hazard and, where possible, the sources. A
hazard can be defi ned as a pollutant or activity that has a
disruptive effect on defi ned end points (e.g. extinction of
an endangered species, declines of populations, lowering
of reproductive success or survival). Obtaining data on con-
taminant levels in air, water, soil, and sediment is critical
to this process, and is referred to as “defi ning the source
term.” The source of mercury is usually either point-source
pollution (e.g. a chemical plant) or atmospheric deposition,
which can be regional or global.
EXPOSURE ASSESSMENT
Exposure assessment is critical for both HRAs and ERAs
because it is during this phase that the routes and pathways
of exposure and actual exposure of target species (or tar-
get organs) is established. Formal risk assessments require
information on all of these aspects, while other methods of
risk evaluation may require only some of this information.
For example, knowing the levels of a contaminant in fi sh
allows health professionals to evaluate the possible risk to
humans from consuming known quantities of fi sh.
Contaminants such as mercury can be at a steady state
(no new increase in tissues regardless of exposure) or may
bio-accumulate (accumulate over time in tissues). Bio-
accumulation results in bio-amplifi cation at each step in a
Relating Exposure to Possible Harm
to Humans and Ecoreceptors
Once concentrations in food or organisms (whole body or
tissues) are reported, the next step is to develop estimates
of daily exposure that can be compared with some bench-
marks, such as the RfDs, PTDIs, or MRLs for humans, the
HQs (for both), or a toxicity reference value for biota. Bench-
marks are numerical values used to guide risk assessors at var-
ious decision points in the risk process. Screening-level risk
evaluations, used to decide whether a full risk assessment is
necessary, often involve media-specifi c benchmarks, while
