Polycyclic Aromatic Hydrocarbons

Overview

Polycyclic aromatic hydrocarbons (PAHs), also referred to as polynuclear aromatic hydrocarbons, are a class of chemicals comprising thousands of organic compounds, each consisting of three or more aromatic (benzene structure) rings (EPA, undated). PAHs can also contain atoms other than carbon and hydrogen, such as nitrogen. They are uncharged, non-polar, and planar, and most are colorless. PAHs generally have high melting and boiling points, low vapor pressures, and low solubility in water (ITRC, 2017).

PAHs are formed by the incomplete combustion of coal, oil, gasoline, garbage, residential wood burning, and other organic materials. They can be found in coal tar, creosote (at wood treater sites), roofing tar, mothballs, medicines, dyes, plastics, and pesticides (ITRC, 2017). Although PAHs occur naturally in substances like coal, crude oil, and creosote, some are manufactured (ASTDR, 1995). For example, naphthalene is manufactured to make other chemicals and mothballs (CDC, 2022).

PAHs are found throughout the environment in air, water, and soil (ATSDR, 1995). PAHs tend to persist in the environment due to their chemical and physical properties (e.g., chemical stability, low solubility, high sorption capacity) (Van Hamme, J.D., et al, 2003). Because of their relatively low solubility, PAHs are primarily associated with particulate matter, and thus can be transported by air and water and settle out in either soil or sediments, which can then serve as contaminant sinks (ITRC, 2017; Hussar, E., et al, 2012). Over time, PAHs in soil degrade, volatilize, or slowly diffuse into more sorptive and inaccessible phases (ITRC, 2017).

PAHs are one of the most common contaminants of concern (COC) addressed at Superfund sites, particularly in soil. Forty percent of the source remedies at Superfund sites for fiscal years 2015, 2016, and 2017 addressed PAHs (EPA, 2020). PAHs have been found in at least 600 of 1,430 National Priorities List (NPL) sites (ATSDR, 2014).

Common PAHs include acenaphthene, anthracene, fluoranthene, naphthalene, phenanthrene, fluorene, pyrene, and chrysene. However, there are hundreds of known PAHs, of which 16 have been designated priority pollutants1 by EPA due to their potential toxicity in humans and other organisms and their prevalence and persistence in the environment (EPA, 2014).

16 PAHs Designated as Priority Pollutants by EPA:

• Acenaphthene
• Acenaphthylene
• Anthracene
• Benzo(a)anthracene
• Benzo(a)pyrene
• Benzo(b)fluoranthene
• Benzo(g,h,i)perylene
• Benzo(k)fluoranthene
• Chrysene
• Dibenz(a,h)anthracene
• Fluoranthene
• Fluorene
• Indeno(1,2,3-c,d)pyrene
• Naphthalene
• Phenanthrene
• Pyrene

Information and supplemental references on the chemistry and behavior, occurrence, toxicology, detection/characterization, and treatment of creosote and coal tar substances that contain high concentrations of PAHs are available on the CLU-IN Contaminant Area for Dense Nonaqueous Phase Liquids (DNAPLs). EPA's Technology Innovation News Survey (TINS) contains abstracts of documents that address the assessment and cleanup of contaminants, including PAHs.

---

References:

Agency for Toxic Substances and Disease Registry (ATSDR), 1995. Toxicological Profile for Polycyclic Aromatic Hydrocarbons. U.S. Department of Health and Human Services, August, 487 pp.

Centers for Disease Control and Prevention (CDC), 2022. Polycyclic Aromatic Hydrocarbons (PAHs) Factsheet. Page last reviewed March 2, 2022.

Hussar, E., et al, 2012. Human Health Risk Assessment of 16 Priority Polycyclic Aromatic Hydrocarbons in Soils of Chattanooga, Tennessee, USA. Water Air Soil Pollution. Nov 1:233(9):5535-5548.

Interstate Technology and Regulatory Council (ITRC), 2017. Bioavailability of Contaminants in Soil: Considerations for Human Health Risk Assessment. Chapter 8: Polycyclic Aromatic Hydrocarbons (PAHs). Bioavailability in Contaminated Soil Team.

U.S. EPA, undated. Polycyclic Aromatic Compounds. Webpage consulted November 22, 2022.

U.S. EPA, 2020. Superfund Remedy Report 16th Edition. Office of Land and Emergency Management, July, 85 pp.

U.S. EPA, 2018. Method 8270E Semivolatile Organic Compounds By Gas Chromatography/Mass Spectrometry, June, 64 pp.

U.S. EPA, 2014. Priority Pollutant List, December, 2 pp.

U.S. EPA, 1986a. Method 8100 Polynuclear Aromatic Hydrocarbons, September, 10 pp.

U.S. EPA, 1986b. Method 8310 Polynuclear Aromatic Hydrocarbons, September, 13 pp.

Van Hamme, J.D., et al, 2003. Recent Advances in Petroleum Microbiology. Microbiology and Molecular Biology Reviews 67(4), 503-549.