Written or medically reviewed by a board-certified physician. See About.com's Medical Review Policy.
Updated July 10, 2015.
Benzene is a colorless, flammable liquid naturally present in coal tar, crude oil, and as a byproduct of volcanic eruptions and forest fires.  It has a sweet smell and evaporates quickly when exposed to the air.  It is classified by the International Agency for Research on Cancer as a Group 1 human carcinogen.
Benzene is among the 20 most widely used chemicals in production in the United States today.
Its primary function is in the manufacturing of other chemicals, including:
- nylon
- polystyrene
- pesticides
- pharmaceuticals
- detergents
- dyes
- explosives
- carpet glue, spray adhesive
- furniture wax
Its use as an industrial solvent and as an additive in gasoline have been reduced in recent years because of its carcinogenic qualities, but it's still present in both of these products.
Auto exhaust is responsible for the majority of benzene in outdoor air.  Diesel exhaust  and home heating oil also contain benzene.
Benzene is extremely volatile and primary exposure is through inhalation.
Benzene in Cigarette Smoke
Benzene is a by-product of the combustion of tobacco in cigarettes. Exposure to cigarette smoke accounts for roughly half of all human exposure to this toxin in the United States.Â
Cigarette smoke is also responsible for most of the benzene present in indoor environments.
Benzene Health Effects
Chronic exposure to benzene is associated with aplastic anemia due to its tendency to reduce the production of both red and white blood cells in the human body.
Additionally, benzene causes acute myeloid leukemia (acute non-lymphocytic leukemia), and there is concern that benzene may also contribute to acute and chronic lymphocytic leukemia, non-Hodgkin's lymphoma and multiple myeloma.Â
Other Effects of Exposure to Benzene
Short-term, high level exposure to benzene through inhalation or ingestion causes damage to the central nervous system and can lead to:
- paralysis
- coma
- convulsions
- dizziness
- sleepiness
- rapid heart rate
- tightness in the chest
- tremors
- rapid breathing
Long-term exposure to benzene is associated with:
- decreased red blood cells through damage to bone marrow
- anemia
- leukemia
- excessive bleeding
- genetic damage
- negative effects on the immune system
Benzene is just one of hundreds of  poisonous and carcinogenic chemical compounds found in air tainted with cigarette smoke. Breathing in ETS threatens life for smokers and nonsmokers alike.
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Sources:
World Health Organization. Exposure to Benzene: A Major Public Health Concern. http://www.who.int/ipcs/features/benzene.pdf. Accessed June, 2015.
U.S. National Library of Medicine. Benzene. http://toxtown.nlm.nih.gov/text_version/chemicals.php?id=5. Accessed June, 2015.
Department of Environmental Quality - State of Oregon. Air Toxics of Concern in Oregon. http://www.deq.state.or.us/aq/factsheets/05-AQ-003_AirToxics.pdf. Accessed June, 2015.
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