What are Polycyclic Aromatic Hydrocarbons?
Polycyclic aromatic hydrocarbons are also known as polycyclic aromatic
compounds, polyaromatic hydrocarbons or as polynuclear aromatics. PAHs
are a group of over 100 different chemicals consisting of carbon and
hydrogen in fused-ring structures. Small amounts of PAHs are common in the
environment (Minnesota Pollution Control Agency, 1998a). Most are harmless
or mildly toxic, but some PAHs are carcinogenic (Minnesota Pollution Control
Agency, 1998a). Polycyclic aromatic hydrocarbons are found in coal and
petroleum, but they are also products of incomplete combustion, of either
natural or anthropogenic origin. Anthropogenic sources to the environment
are more abundant than natural sources and include burning of wood, coal,
oil and gas, garbage, or other organic substances like tobacco or charbroiled
meat. The most important natural sources are forest fires and volcanoes
(National Research Council, 1983).
Figure 1. Chemical Structure of a few PAHs
Human Exposure of PAHs
For U.S. residents, the greatest PAH exposure is through the ingestion of
food, but this can vary depending on lifestyle (Agency for Toxic Substance
and Disease Registry, 1995). Other important exposure pathways are
through active and passive inhalation of the compounds in tobacco smoke,
wood smoke and contaminated air, as well as dermal contact with coal tar in
cosmetics and shampoos (Agency for Toxic Substance and Disease Registry,
1995). Human daily exposure is estimated to be 0.207 ìg from air, 0.027 ìg
from water, and 0.16 to 1.6 ìg from food. These estimates may be as much
as twice as high in smokers (Agency for Toxic Substance and Disease
Registry, 1995). PAHs are stored mostly in the kidneys, liver and fat
(Agency for Toxic Substance and Disease Registry, 1995). In long-term PAH
exposure scenarios, cancer, cataracts, kidney and liver damage, and jaundice
may develop. However, in humans most PAHs are released in feces and urine
within a few days (Agency for Toxic Substance and Disease Registry, 1995).
The U.S. Food and Drug Administration states that over-the-counter products
with coal tar concentrations between 0.5% and 5% are safe. The FDA
maintains that there is no evidence that coal tar products cause cancer
(National Psoriasis Foundation, 2005).
The Environment and PAHs
In the environment PAHs can volatilize, photolyze, oxidize, biodegrade, bind
to suspended particles, or accumulate in aquatic organisms (Agency for Toxic
Substance and Disease Registry, 1995). The bulk of PAHs in the environment
is tied to organic matter in soil.
The Air and PAHs
Most PAHs, whether the result of natural or anthropogenic processes, are
released into the air. Residential burning of wood is the largest anthropogenic
source, however on a local scale other emission sources can be dominant
(Peters et al., 1981; Ramdahl et al., 1982). Urban air contains as much as 5
times higher PAH concentrations (0.15-19.3 ng/m3) than rural air (0.02-1.2
ng/m3) (Agency for Toxic Substance and Disease Registry, 1995). Winter
concentrations are 5-10 times higher than summer concentrations due to the
difference in temperature, sunlight radiation, and an increase in heating
emissions (Agency for Toxic Substance and Disease Registry, 1995). An
estimated 10,000 to 30,000 tons (8,900- 26,800 metric tons) of PAHs are
emitted annually in the U.S. (Baek et al., 1991; EPA, 1998).
The Water and PAHs
The most important source of PAHs in surface water is from the deposition
of airborne PAHs (Jensen, 1984). Other sources include municipal waste
water discharge (Barrick, 1982), urban storm water runoff (MacKenzie
and Hunter, 1979), runoff from coal storage areas (Stahl et al., 1984;
Wachter and Blackwood, 1979), effluents from wood treatment plants
and other industries (DeLeon et al., 1986; Snider and Manning, 1979;
United States Department of Agriculture, 1978), oil spills, and petroleum
processing (Giger and Blumer, 1974). In a localized environment any of the
above sources can be dominant. Background levels of PAHs in drinking water
range from 4 to 24 ng/L (Sorrel et al., 1980).
The Soil and PAHs
Soil, like water, receives most PAHs from atmospheric deposition after local
and long-range transport (Agency for Toxic Substance and Disease Registry,
1995). Other sources include sludge disposal from public sewage treatment
plants, automotive exhaust, tire and asphalt wear, irrigation with coke oven
effluent, leachate from bituminous coal storage sites, and use of compostbased fertilizers (Perwak et al., 1982; Santodonato, 1981; Stahl et al., 1984;
White and Lee, 1980).