Introduction - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 7

The risk alluded to is the incremental lifetime cancer risk (ILCR), and is given by

ILCR = SF × CDI, (1.2)

where SF is the slope factor for the chemical and CDI is the chronic daily intake
of the chemical by the defined exposure route. A listing of the SFs can be found
in Appendix 9. The CDI requires the use of a multimedia fate model to obtain the
concentration value that goes into its determination. Thus, knowledge of the SF and
the CDI allows the determination of the lifetime cancer risk.
If the excess cancer risk from the inhalation route is to be estimated, we can also
write the following equation:

Risk air = IUR × C air , (1.3)

g/m 3 ) (see Appendix 9) and C air is the
average exposure concentration in air. For example, for formaldehyde the unit risk
is 1.3

μ

where IUR is the inhalation unit risk (in per

g/m 3
over a lifetime (70 years), the lifetime risk is 1 in 10 5 . For noncancer risk assessment
in air, we use a hazard quotient (HQ):

10 − 5 g − 1 /m 3 , and if someone is exposed to a concentration of 0.77

× μ μ

C air
RfC ,

HQ = (1.4)

where RfC is the reference concentration (see Appendix 9 for definitions). If HQ is
< 1, the risk is acceptable.
Within the regulatory framework, it is now mandatory to assess the potential harm-
ful effects on humans and the environment from the use of new chemicals and from
the continued use of existing ones. Examples are the Toxic Substances Control Act
(TSCA) in the United States, the Canadian Environmental Protection Act (CEPA) in
Canada, and the 7th amendment in the European Union (EU). Once risk is estab-
lished, the next step will be to isolate the pollutant from the ecosystem to minimize
the risk.
In the above context, the following specific questions will need to be addressed:

(i) What is the final equilibrium state of the pollutant in the environment, that is,
which of the environmental compartments is the most favorable, how much
resides in each compartment at equilibrium, and what chemical properties
are important in determining the distribution?
(ii) How fast does the pollutant move from one compartment to another, what
is the residence time in each compartment, and how fast does it react within
each compartment or at the boundary between compartments?

The answer to the first question employs the tools of chemical thermodynamics
and the second question requires the applications of concepts from chemical kinetics.
Multimedia fate and transport (F&T) models are recognized as a necessary com-
ponent for risk assessment, chemical ranking, management of hazardous waste sites,