Aquatic environments Most PAHs are insoluble in water, which limits their mobility in the environment, although PAHs
sorb to fine-grained organic-rich
sediments. Aqueous solubility of PAHs decreases approximately
logarithmically as
molecular mass increases. Two-ringed PAHs, and to a lesser extent three-ringed PAHs, dissolve in water, making them more available for biological uptake and
degradation. Further, two- to four-ringed PAHs
volatilize sufficiently to appear in the atmosphere predominantly in gaseous form, although the physical state of four-ring PAHs can depend on temperature. In contrast, compounds with five or more rings have low solubility in water and low volatility; they are therefore predominantly in solid
state, bound to
particulate air pollution,
soils, or
sediments.
Human exposure Human exposure varies across the globe and depends on factors such as smoking rates, fuel types in cooking, and pollution controls on power plants, industrial processes, and vehicles. Developed countries with stricter air and water pollution controls, cleaner sources of cooking (i.e., gas and electricity vs. coal or biofuels), and prohibitions of public smoking tend to have lower levels of PAH exposure, while developing and undeveloped countries tend to have higher levels. .
Smoke from solid fuels like
wood is a large source of PAHs globally. Burning solid fuels such as
coal and
biofuels in the home for cooking and heating is a dominant global source of PAH emissions that in developing countries leads to high levels of exposure to
indoor particulate air pollution containing PAHs, particularly for women and children who spend more time in the home or cooking. in
Cairo. Particulate air pollution, including smog, is a substantial cause of human exposure to PAHs. Emissions from vehicles such as cars and trucks can be a substantial outdoor source of PAHs in particulate air pollution.
Catalytic converters are estimated to reduce PAH emissions from gasoline-fired vehicles by 25-fold. Industrial activity that can produce and distribute PAHs includes
aluminum,
iron, and
steel manufacturing;
coal gasification,
tar distillation,
shale oil extraction; production of
coke,
creosote,
carbon black, and
calcium carbide; road paving and
asphalt manufacturing;
rubber tire production; manufacturing or use of
metal working fluids; and activity of coal or
natural gas power stations. Exposure also occurs through drinking alcohol aged in charred barrels, flavored with peat smoke, or made with roasted grains. PAHs are typically at low concentrations in drinking water.
Soil and river
sediment near industrial sites such as creosote manufacturing facilities can be highly contaminated with PAHs. Two- and three-ringed PAHs can disperse widely while dissolved in water or as gases in the atmosphere, while PAHs with higher molecular weights can disperse locally or regionally adhered to particulate matter that is suspended in air or water until the particles land or settle out of the
water column. The study showed that parent (16 PAH) content ranged from 4 to 67 mg/kg (dry soil weight) and an average PAH concentration of 18 mg/kg (dry soil weight) whereas the total PAH content (33 PAH) ranged from 6 to 88 mg/kg and
fluoranthene and
pyrene were generally the most abundant PAHs. the normal background concentration of B
aP in the London urban sites was 6.9 mg/kg (dry soil weight). Burning of moorland vegetation such as
heather initially generates high amounts of two- and three-ringed PAHs relative to four- to six-ringed PAHs in surface sediments, however, this pattern is reversed as the lower
molecular weight PAHs are attenuated by biotic decay and
photodegradation. Consequently, the concentrations of
pollutants in estuaries tends to decrease at the river mouth. Understanding of sediment hosted PAHs in estuaries is important for the protection of commercial
fisheries (such as
mussels) and general environmental habitat conservation because PAHs can impact the health of suspension and sediment feeding organism. River-estuary surface sediments in the UK tend to have a lower PAH content than sediments buried 10–60 cm from the surface reflecting lower present day industrial activity combined with improvement in environmental legislation of PAH. In general estuarine sediments with a higher natural
total organic carbon content (TOC) tend to accumulate PAHs due to high
sorption capacity of organic matter. ==Human health==