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Polycyclic Aromatic Hydrocarbon Structure Index


What are polycyclic aromatic hydrocarbons?

Polycyclic aromatic hydrocarbons (PAHs) are a group of over 100 different chemicals that are formed during the incomplete burning of coal, oil and gas, garbage, or other organic substances like tobacco or charbroiled meat. PAHs are usually found as a mixture containing two or more of these compounds, such as soot.

Some PAHs are manufactured. These pure PAHs usually exist as colorless, white, or pale yellow-green solids. PAHs are found in coal tar, crude oil, creosote, and roofing tar, but a few are used in medicines or to make dyes, plastics, and pesticides.

Macrophage Activation by Polycyclic Aromatic Hydrocarbons: Evidence for the Involvement of Stress-Activated Protein Kinases, Activator Protein-1, and Antioxidant Response Elements1

Chemical Characterization and Bioactivity of Polycyclic Aromatic Hydrocarbons from Non-Oxidative Thermal Treatment of Pyrene-Contaminated Soil at 250-1,000C

Polynuclear Aromatic Hydrocarbons (PNAs)

Biodegradation of Polynuclear Aromatic Hydrocarbons (PAHs)

The main chemicals of concern are polycyclic aromatic hydrocarbons (PAHs) which are also known as polynuclear aromatic hydrocarbons (PNAs). Possible health hazards associated with PAH exposure include cancer, skin problems, immunodeficiency, and reproductive difficulties for both the exposed and their offspring.

More than 100 different chemicals are compiled into the general category of polycyclic aromatic hydrocarbons. Each of these chemicals may affect the body in different ways. Therefore, identifying chemicals of concern in CTPVs is important. These PAHs include benzo[a]pyrene and dibenz[a]anthracene, which are known to cause cancer, and pyrene, acridine, chrysine, phenanthrene, and anthracene.

Links to more information about these chemicals.

Reasonably Anticipated to be a Human Carcinogen Eighth Report on Carcinogens

TITLE: Chemical Contaminants in House Dust: 

Occurrence and Sources AUTHOR(S) John W. Roberts1, William T. Budd2, Jane Chuang3, and Robert G. Lewis4 

PERFORMING ORGANIZATION NAME AND ADDRESS 3Battelle, Columbus, OH 2Envirometrics, Inc. - Seattle, WA 1Engineering Plus, Inc., 1425 E. Prospect #3, Seattle, WA 98112 and 4U.S. EPA, AREAL, RTP, NC 27711 

SPONSORING AGENCY NAME AND ADDRESS U.S. Environmental Protection Agency Atmospheric Research and Exposure Assessment Laboratory Research Triangle Park, NC 27711


 Pesticides, polynuclear aromatic hydrocarbons (PAHs), lead (Pb), and other metals accumulate in home soil and house dust. Exposure of infants and toddlers to Pb by dust may be greater than other pathways. Many pollutants found in carpet dust are protected from degradation and accumulate over time. Greater numbers of pesticides are found in house dust than in indoor air. Pb and PAH concentrations appear to be magnified in house dust when compared with midyard and walkway soil samples. Studies using methods developed by the U.S. EPA show evidence of track-in and/or carry-in of Pb, PAHs and pesticides on clothes. Preliminary results of an eight-home study report PAHs in house dust at 1 to 100 ppm and suggest track-in as a major source of PAHs in house dust. FOR ADDITIONAL INFORMATION, CONTACT GLORIA J. KOCH, EPA, (919) 541-4109 OR FTS 629-4109

Introduction to Aromatic Hydrocarbons

When a double bond replaces one of the single carbon-carbon bonds in cyclohexane, the double bond behaves just as a single bond in hexene or any other alkene. Introduction of a second double bond on a non-adjacent carbon gives cyclohexadiene; the two double bonds behave much like double bonds when only one is present in the molecule. However, introduction of a third double bond in the ring structure to give cyclohexatriene produces a radical change in the electron distribution in the ring. Halogens and hydrogen no longer are capable of adding to it easily, nor is it easily oxidized. All carbon-carbon bonds are found to be identical in length and reactivity. All of the carbon-hydrogen bonds are identical as well. The carbon atoms now lie in a plane, where in cyclohexane and cyclohexene they do not. Thus the structural representations appropriate for cyclohexatriene do not accurately describe the nature of the cyclic C6H6 molecule.

As a consequence, the ring structure is called benzene rather than cyclohexatriene and is often represented by a circle rather than alternating bonds.

Benzene is the simplest of the aromatic hydrocarbons, which are those hydrocarbons in which one or more ring has aromatic character due to the rearrangement of the pi bonding electrons in what would otherwise be alternating single and double bonds. Aromatic character can arise whenever two equivalent canonical structures involving alternate single and double bonds can be drawn for an organic molecule, even if it is not a ring. The actual structure is sometimes called a resonance hybrid of the two equivalent canonical structures. Rings with aromatic character are remarkably stable to chemical attack.

Substituted benzenes can be formed by reagents such as chlorine. Chlorine adds directly across the double bond of cyclohexene to give 1,2-dichlorohexane, but chlorine reacts only under more extreme conditions with benzene and when it does so the products are chlorobenzene and hydrogen chloride.

While substituted benzenes can be named by the usual IUPAC substitution method, many of them are commonly known by trivial names. Methylbenzene, for example, is known as toluene, and the three possible dimethylbenzenes are known as the xylenes.

The denoting of adjacent groups on a ring as ortho, those of one carbon apart as meta, and those on opposite sides of the ring as para is a common, though non-systematic, method of designating positions of substituent groups on aromatic rings in more complex cyclic compounds. Other examples of trivial names for substituted benzenes are phenol (hydroxybenzene), aniline (aminobenzene or, more commonly, benzylamine), anisole (methoxybenzene), and phenetole (ethoxybenzene).

More complex cyclic hydrocarbons may involve many rings, some aromatic and some non-aromatic or aliphatic. In most cases the entire ring structure is given a particular non-systematic name. Two of the simplest are the two-ring and three-ring aromatic structures, which are called napthalene and anthracene. Multiple-ring structures are often called polynuclear aromatic hydrocarbons.

Substitution upon polynuclear hydrocarbons whether aromatic or aliphatic is described in the usual way, but the numbering of the carbons is not obvious in complex structures. Reference works giving the ring structure and indicating carbon number have been compiled and should be consulted in order to name the structures.

Copyright 1997 James R. Fromm (

Copyright 1996 James A. Plambeck (



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