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Description
People are exposed to air pollutants in both “direct” and “indirect”
ways. Inhalation is the most obvious and direct pathway of exposure;
indirect exposure pathways involve contaminant deposition from air to
soil, surface waters, sediments, and locally produced feed and food. These
indirect pathways predominate for many important chemicals or mixtures,
such as polychlorinated dibenzo(p)dioxins and furans (PCDD/PCDFs), that
persist and bioaccumulate in the environment. Cambridge Environmental
staff have been among the pioneers of multi-pathway assessments since the
mid-1980’s, when we conducted multi-pathway risk assessments of emissions
from waste-to-energy facilities and other stationary sources. Over the
years our scientists and engineers have developed and applied models, and
analyzed relevant data, to generate best estimates and upper-bound
estimates of risk from many pathways, including, for example, risks to
nursing infants (from contaminants transferred in mother’s milk), and, in
one case, risks to consumers from ingestion of locally harvested
alligators! We also analyze, and emphasize, key uncertainties inherent in
these assessments.
Sample Projects
Links are provided to several
reports that highlight multi-pathway risk assessment issues:
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“Evaluating the Local Impacts of Mercury Emissions
from a Point Source,” presented at the A&WMA Specialty Conference on
Mercury Emissions: Fate, Effects and Control August 21-23, 2001,
Chicago, Illinois — which emphasizes the importance of using
site-specific data in assessing potential exposure to mercury from
combustor emissions.
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“HWC Risk Assessments: Ruled by Uncertainty,”
presented at the Hazardous Waste Combustors Specialty Conference and
Exhibition, April 1-2, 2003, Charleston, South Carolina — which
describes specific areas of the U.S. EPA’s Human Health Risk
Assessment Protocol guidance that can introduce significant errors and
uncertainties.
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“Risk Assessment Perspectives on Air
Dispersion Modeling,” presented at the Guideline on Air Quality
Models: Applications and Flag Developments A&WMA Specialty Conference,
April 26-28, 2006, Denver, Colorado — which discusses features and
shortcomings of pollutant deposition algorithms incorporated within
the U.S EPA’s AERMOD system.
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“Recent Observations on Risk Assessments of
Combustor Stack Emissions,” presented at the International Thermal
Treatment Technologies A&WMA Specialty Conference, May 12-16, 2008,
Montreal, Quebec — which compares deposition modeling predictions of
the AERMOD and ISCST3 models and describes modeling extensions to
evaluate topics such as the promotion of invasive plant species due to nitrogen deposition, and the potential for acid fog formation
to cause subsequent
damage to vegetation.
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A human health and ecological risk
assessment was conducted for a Portland cement manufacturing facility
located in Greencastle, Indiana that uses hazardous waste as an
alternative fuel. The multi-pathway assessments helped the U.S. EPA
evaluate potential risks to human health and the environment due to
the facility’s stack and fugitive emissions. Detailed procedures were
developed to model atmospheric deposition that were useful in
assessing and comparing mercury impacts due to both the measured and
the proposed MACT-based mercury emission rates.
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Harrisburg, Pennsylvania renovated its
waste-to-energy facility to meet stringent federal MACT standards.
This multi-pathway health risk assessment finds very small incremental
risk levels due to the facility’s emissions of cadmium, lead, mercury,
polychlorinated dibenzo(p)dioxins/furans, and various other
pollutants. One interesting aspect of the assessment involves the
modeled relationship between mercury concentrations in surface water
and fish. Sampling data in Pennsylvania (collected by others) support
a high bioaccumulation factor for methyl mercury similar to the factor
endorsed by the U.S. EPA. However, because measured fractions of
methylated mercury in Pennsylvania surface waters were lower than the
default fraction recommended by the EPA, a significantly lower overall
mercury bioaccumulation factor was used.
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