POLYNUCLEAR HYDROCARBONS
                     POLYNUCLEAR 
HYDROCARBONS
INTRODUCTION
A polynuclear aromatic hydrocarbon is a 
hydrocarbon made up of fused aromatic ring 
molecules. These rings share one or more sides 
and contain delocalized electrons. Another way 
to consider PAHs is molecules made by fusing 
two or more benzene rings.
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Examples
There are numerous examples of polynuclear 
aromatic hydrocarbons. Typically, several 
different PAHs are found together. Examples of 
these molecules include:
• anthracene
• phenanthrene
• tetracene
• chrysene
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Properties
Polynuclear aromatic hydrocarbons are lipophilic, 
nonpolar molecules. They tend to persist in the 
environment because PAHs are not very soluble 
in water.
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Properties Cont.
While 2- and 3-ring PAHs are somewhat soluble in 
aqueous solution, the solubility decreases nearly 
logarithmically as molecular mass increases. 2-, 
3-, and 4-ring PAHs are sufficiently volatile to 
exist in the gas phases, while larger molecules 
exist as solids. Pure solid PAHs may be colorless, 
white, pale yellow, or pale green.
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Sources
PAHs are organic molecules that form from a 
variety of natural and anthropogenic reactions. 
Natural PAHs form from forest fires and volcanic 
eruptions. The compounds are numerous in fossil 
fuels, such as coal and petroleum.
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Sources Cont.
Man contributes PAHs by burning wood and by 
the incomplete combustion of fossil fuels. The 
compounds occur as a natural consequence of 
cooking food, particularly when food is cooked at 
a high temperature, grilled, or smoked. The 
chemicals are released in cigarette smoke and 
from burning waste.
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Health Effects
Polynuclear aromatic hydrocarbons are 
extremely important because they are 
associated with genetic damage and diseases. 
Also, the compounds persist in the environment, 
leading to increased problems over time.
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Health Effects 
Cont.
PAHs are toxic to aquatic life. In addition to 
toxicity, these compounds are often mutagenic, 
carcinogenic, and teratogenic. Prenatal exposure 
to these chemicals is associated with lowered IQ 
and childhood asthma.
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PAHs Classified 
as Carcinogens
The Environmental Protection Agency has 
identified seven polynuclear aromatic 
hydrocarbons as likely human carcinogens, or 
cancer-causing agents:
• benzo[a]anthracene
• benzo[a]pyrene
• benzo[b]fluoranthene
• benzo[k]fluoranthene
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