Polycyclic Aromatic Hydrocarbons

What it is

Polycyclic aromatic hydrocarbons (PAHs) are a family of more than 100 chemicals made up of fused benzene rings. They form whenever organic material (wood, coal, oil, gas, tobacco, food) burns incompletely. EPA tracks 16 PAHs as priority pollutants, including naphthalene, fluorene, phenanthrene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, and the most studied, benzo[a]pyrene (B[a]P).

Lighter PAHs (2 to 3 rings) tend to be in the gas phase. Heavier PAHs (4 to 6 rings, including B[a]P) attach to fine particles, especially PM2.5 and black carbon, and travel with them.

Where it comes from

Outdoor PAHs in the U.S. come mainly from vehicle exhaust (especially diesel), residential wood and coal burning, wildfires, asphalt paving, aluminum production, coke and steel manufacturing, and oil and gas operations. Indoors, the largest sources are tobacco smoke, wood stoves and fireplaces, candles, incense, and high-temperature cooking (especially frying, grilling, or charring food on gas or wood). Tobacco smoke is the single largest indoor PAH source in homes with smokers; benzo[a]pyrene in a heavily smoked room can reach 22 nanograms per cubic meter, compared with typical outdoor urban background of 0.5 to 2.

PAHs in food (smoked, grilled, or charred meats) are a major ingestion source, but the inhalation pathway is the focus of air quality work.

Health effects

IARC classifies benzo[a]pyrene as Group 1, carcinogenic to humans, and several other PAHs (including dibenz[a,h]anthracene, benzo[a]anthracene, benzo[b]fluoranthene) as Group 2A or 2B (probable or possible carcinogens). PAH mixtures cause lung cancer in occupational settings (coke ovens, aluminum smelting, coal tar pitch work) and have been linked to skin and bladder cancer.

Beyond cancer, prenatal PAH exposure is associated with lower birth weight, reduced head circumference, and impaired cognitive development in children. A series of studies from the Columbia Center for Children's Environmental Health found lower IQ scores at age 5 in children whose mothers were exposed to higher airborne PAHs during pregnancy. PAHs also contribute to oxidative stress, inflammation, and cardiovascular disease.

How it's measured

There is no NAAQS for PAHs as a class. The WHO Air Quality Guidelines treat benzo[a]pyrene as a marker for the carcinogenic PAH mixture and estimate a unit risk of about 8.7 cases of lung cancer per 100,000 people per lifetime exposure to 1 nanogram per cubic meter of B[a]P. The European Union sets a target of 1 ng/m3 annual mean for B[a]P. OSHA regulates coal tar pitch volatiles (a PAH mixture) with a PEL of 0.2 mg/m3 over an 8-hour workday.

Typical outdoor B[a]P concentrations: 0.1 to 1 ng/m3 in rural areas, 1 to 10 ng/m3 in cities, much higher near coke ovens, aluminum smelters, or heavy wood-burning communities.

What you can do

Most PAH exposure reduction overlaps with reducing PM2.5 and combustion smoke. Indoors: keep your home smoke-free, vent gas stoves with a range hood that exhausts outdoors, avoid burning candles and incense, and choose cooking methods that produce less smoke (boiling, steaming, slow roasting beat searing, charring, and high-heat frying).

Avoid wood-burning fireplaces and old wood stoves; upgrade to EPA-certified stoves and use only dry seasoned wood if you must burn. Run a HEPA air purifier during wood-smoke or wildfire events, since PAHs ride on PM2.5.

Outdoors: minimize time near busy diesel corridors at peak traffic hours, and avoid jogging or biking along truck routes when possible.

Sources

This article is for educational purposes only. Canairy does not provide medical advice, diagnosis, or treatment. Talk to a qualified health professional about your specific situation.