Contaminant of the Month: Benzene

What it is:

• Benzene is a hydrocarbon manufactured from petroleum and also a natural product in some foods.
• It is a six-member carbon ring with a hydrogen attached to each carbon.
• Benzene is the base product among the family of BTEX (benzene, toluene, ethylbenzene, xylene) hydrocarbons produced in very high volume. The other BTEX have a methyl group (CH₃^–), or ethyl (C₂H₅^–) or two methyls attached to the benzene ring.
• Its boiling point is 80.1^o C (176.2^o F), and water solubility is about 2 grams per liter. Its molecular formula is C₆H₆, and its molecular weight is 78.11 daltons. It is called an aromatic hydrocarbon but that is more so a name of the family of chemical structures, rather than an odor indication.
• It is a VOC and lighter than water with a specific gravity of about 0.9 g/ml.
• The odor detection threshold is about 5 mg/m³.

What benzene is used for:

• It is a solvent and building block for many chemicals, including BTEX, styrene, detergents and cumene that is converted to phenol and acetone.
• U.S. production is at least 12 billion lbs. per year.
• The largest use is in gasoline as an octane enhancer, especially since the elimination of lead. Gasoline contains about one or two percent benzene and other aromatics.

Human exposure:

• Exposure occurs from occupations, airborne inhalation, traces in foods and alcoholic beverages and minimal amounts in some drinking waters.
• Outdoor airborne levels average about 1 ppb and contribute around 7.5 µg/day.
• Indoor air is greater with contributions from secondhand cigarette smoke, gas cooking, wood burning stoves and fireplaces, as well as releases from furnishings and attached garages, with levels as high as 31 µg/m³.
• Cigarette smoking is the single, greatest human exposure source. A cigarette smoker can inhale about 1.8 mg (1800 µg) per day directly from 32 cigarettes.
• Drinking water is a negligible source for the vast majority of people.

Occurrence in water:

• Most surface and groundwaters contain no detectable benzene.
• Well water contaminated with gasoline from leaking underground storage tanks or a nearby hazardous waste site often contains some benzene.

Health:

• As with most volatile solvents, benzene can cause drowsiness and headaches at high inhalation levels.
• Benzene is a known human carcinogen — EPA Group A, based upon occupational epidemiology.
• Leukemias are the principal cancer concern.
• EPA’s lifetime risk calculation for inhalation is about one in 100,000 to one in one million for exposure at 1 µg/m³.
• EPA’s calculated risk of one in one million for ingestion through drinking water is between 10 and 100 ppb.
• The other BTEX hydrocarbons have much less chronic risk than benzene.

Analytical methods:

• Analyses are by purge and trap gas chromatography and related procedures.

Water treatment:

• Granular activated carbon and aeration are available to water treatment plants. Reverse osmosis is not effective because an organic solvent, such as benzene, can dissolve in the membrane and migrate to the treated water.
• POU and POE using activated carbon are effective, however, they must be replaced before exhaustion.

Regulation:

• The drinking water Maximum Contaminant Level Goal (MCLG) is zero, and the Maximum Contaminant Level (MCL) is 5 ppb. Some states, including California and Florida, have MCLs of 1 ppb. The World Health Organization (WHO) drinking water guideline is 10 ppb. All of these are very conservative values.

Dr. Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He is a former director of the U.S. EPA Drinking Water Standards Division.