Sunlight Multiplies Toxic PAH Air Pollutants from Fossil Fuels


Polycyclic Aromatic Hydrocarbons, or PAHs, a byproduct of coal-burning in power stations, causes air pollution. A new study has found that PAH toxins degrade in sunlight into child compounds and byproducts.

According to a Chemosphere report, the child compounds can become more toxic than the parent PAHs. The PAHs are also serious contaminants to rivers and dams and as they accumulate in larger numbers. The poisons coming out of coal-fired power stations are as damaging as smoke from cigarettes, exhaust pipes from cars, burning crop residues, aeroplanes and wildfire.

Increased Incidence of Cancer

PAHs are produced when fossil fuels or organic matter is not completely burnt up. These PAH signatures are distinct enough that scientists can tell what the likely sources of pollution are. They can do that by analyzing water and sediment samples from rivers and dams affected by pollution. Dr. Mathapelo Seopela, a researcher in the Department of Chemistry at the University of Johannesburg and the lead author of the study. She said:

“Burning processes create PAHs that vary in size from two to six fused benzene rings. The hotter the burning process, the bigger the compound that is formed, and the more harmful it is […] as an example, when coal is burned in a coal-fired power station for electricity, five and six-ring PAHs are likely to form. This is because the burning process is at a very high temperature, over a 1000 degrees Celsius.”

PAHs are a human health concern.  A number of studies show an increased incidence of cancer (lung, skin, and urinary cancers) in humans exposed to PAH mixtures. Many individual PAH compounds have been classified as probable or possible carcinogens by entities such as the National Toxicology Program and the Environmental Protection Agency (EPA).

Health effects of PAHs

The effects on human health will depend mainly on the extent of exposure (length of time, etc.), the amount one is exposed to (or concentration), the PAHs’ innate toxicity, and whether exposure occurs via inhalation, ingestion or skin contact. Various other factors can also affect health impacts from such exposure, including pre-existing health status and age.

The ability of PAHs to induce short-term health effects in humans is not clear. Occupational exposures to high levels of pollutant mixtures containing PAHs has resulted in symptoms such as eye irritation, nausea, vomiting, diarrhea and confusion. However, it is not known which of the mixture components were causing these effects.

Mixtures of PAHs are known to cause skin effects in animals and humans, such as irritation and inflammation. Anthracene, benzo(a), pyrene and naphthalene are direct skin irritants. In contrast, anthracene and benzo(a)pyrene are reported to be skin sensitizers, i.e. cause an allergic skin response in animals and humans.

Incomplete Combustion of Fossil Fuels

These large PAH compounds travel with the rest of the smoke from the power station’s cooling towers. Winds can then blow the compounds quite far away, to rivers, dams, agricultural land or the next city.

“When gasoline is burnt in a car engine, two to three-ring PAHs usually form. Similar PAHs are formed by airplanes, when farmers burn crop residues or grass, or with burning wood […] the PAHs end up in the atmosphere, in the air we breathe. Often, they can travel very long distances from the sources that produced them, such as power stations or wildfires.”

Many PAH compounds are harmful. They are formed from two or more fused benzene molecules, or rings, during the incomplete combustion of fossil fuels or organic matter. Benzene is a highly flammable, toxic liquid. It is partly responsible for the characteristic smell at a gasoline station.

Completely Different Byproducts

Seopela and the researchers from the Chesapeake Biological Laboratory of the University of Maryland Center for Environmental Science built a closed-circuit recirculation system for their laboratory study. They tested five PAHs listed by the U.S. EPA as priority pollutants. These were naphthalene, anthracene, benzo(a)anthracene, benzo(a)pyrene and benzo(ghi)perylene.

For each PAH, they tested pure samples of it in pure water as a control. Then they tested each pure PAH in pure water with a specific amount of natural organic matter (NOM) added to simulate river and dam conditions. They tested each PAH on its own and then mixed all of them together to see what happens. Prof Michael Gonsior from the Chesapeake Biological Laboratory at the University of Maryland says:

“We found that when sunlight falls on a parent PAH, it breaks down to smaller child PAHs, which we call degradation products. But at the same time, completely different byproducts are also formed.”