Benzidine is a synthetic chemical that does not occur naturally. It is a crystalline (sandy or sugar-like) solid that may be grayish-yellow, white, or reddish-gray in color. It will evaporate only slowly, especially from water and soil. Its flammability, smell, and taste have not been described. Benzidine also has other names, such as 4,4'-diphenylenediamine or Fast Corinth Base Br. In the environment, benzidine is found in either its "free" state (as an organic base), or as a salt (for example, benzidine dihydrochloride or benzidine sulfate). In air, it is found most often attached to suspended particles.

In the past, industry used large amounts of benzidine to produce dyes for cloth, paper, and leather. However, it has not been manufactured for sale in the United States since the mid-1970s. Major U.S. dye companies no longer make benzidine-based dyes. Benzidine is no longer used in medical laboratories or in the rubber and plastics industries. However, small amounts of benzidine may still be manufactured or imported for scientific research in laboratories or for other specialized uses. Some benzidine-based dyes (or products dyed with them) may also still be imported.

Structural diagram: National Institutes of Health

Fate & Transport

In the past, benzidine entered the environment largely during its manufacture, processing, or use to produce dyes. Industry released it to waterways in the form of liquids and sludges and transported benzidine-containing solids to storage or waste sites. Benzidine was sometimes accidently spilled and it was released to the air as dust or fumes. For the most part, companies no longer make or use benzidine and the government strictly regulates these activities. Today, most benzidine still entering the environment probably comes from waste sites where it had been disposed of. Some may also come from the physical, chemical, or biological breakdown of benzidine-based dyes, or from other dyes where it may exist as an impurity.

Only very small amounts of free benzidine will dissolve in water at moderate environmental temperatures. When discharged to waterways, it will sink and become part of the bottom sludge. Benzidine salts are more soluble in water than free benzidine. Only a very small portion of dissolved benzidine will pass into the air. Benzidine exists in the air as very small particles, which may be brought back to the earth's surface by rain or gravity. In soil, most benzidine is likely to be strongly attached to soil particles, so it does not easily pass into underground water.

Certain other chemicals, light, and some microorganisms (for example, bacteria) can slowly destroy benzidine. Certain fish, snails, algae, and other forms of water life may take up and store very small amounts of benzidine, but accumulation in the food chain is unlikely.

Exposure Pathways

The general population is not likely to be exposed to benzidine through contaminated air, water, soil, or food. Benzidine is a synthetic chemical that does not occur naturally in the environment. Today, U.S. industry makes and uses very little (if any) benzidine, and no releases to air, water, or soil are reported on the Toxic Release Inventory (TRI). Only rarely has benzidine been detected in areas other than waste sites and it has not been found in food.

If you live near a hazardous waste site, you could be exposed to benzidine by drinking contaminated water or by breathing or swallowing contaminated dust or soil. Benzidine can also enter the body by passing through the skin.

Some quantities of dyes made from benzidine may still be imported for use in the United States. These may contain small amounts of benzidine as a contaminant, or may be broken down in the body to benzidine. If you use such dyes to dye paper, cloth, leather, or other materials, you may be exposed through breathing or swallowing dust, or through skin contact with dust. You may be exposed in a similar way if you work at or near hazardous waste sites.


Benzidine can enter your body if you breathe air that has small particles of benzidine or dust to which benzidine is attached, if you drink water or eat food that has become contaminated with benzidine, or if the chemical touches your skin. Generally, it will take only a few hours for most of the benzidine to pass into your body through the lungs and intestines. It may take several days for most of the benzidine to pass through your skin. Breathing, eating or drinking, or skin contact with benzidine-based dyes may also indirectly expose you to benzidine. Your body contains intestinal bacteria that can break down the dyes into benzidine.

Once in your body, only a small portion of benzidine will leave as waste in your urine and feces. Your body will change most of it into many different chemical forms that dissolve readily in your body fluids and are easy for your body to remove. Some of these changed forms of benzidine appear to cause many of the chemical's harmful effects. Studies show that after benzidine has entered your body, most of it (and its changed forms) will be removed within a week.

Health Effects

Very little information is available on the noncancer health effects that may be caused by exposure to benzidine. Benzidine contact with your skin could possibly cause an allergic contact dermatitis (a skin allergy reaction to benzidine). Except for the cancer discussed below, benzidine has not been definitely shown to harm any other organ system in humans.

Benzidine can cause cancer. This has been shown by studies of workers who were exposed for years to levels much higher than those experienced by the general population. It is important to note that most of the workers did not develop cancer, even after such high exposures. When cancer does occur, most often it is cancer of the urinary bladder. Some evidence suggests that other organs such as the stomach, kidney, brain, mouth, esophagus, liver, gallbladder, bile duct, and pancreas may also be targets. Experiments with animals have also shown benzidine to be a carcinogen. The Department of Health and Human Services (DHHS), the International Agency for Research on Cancer (IARC), and the EPA have determined that benzidine is a human carcinogen. In addition, dyes made from benzidine, such as Direct Blue 6, Direct Black 38, and Direct Brown 95, have been shown to be carcinogenic in animals, and there is some evidence that they may cause bladder cancer in humans. The DHHS has determined that Direct Black 38 and Direct Blue 6 are carcinogenic to animals, and IARC has also determined that Direct Black 38, Direct Blue 6, and Direct Brown 95 are carcinogenic to animals.

Information excerpted from:

Toxicological Profile for Benzidine August 1995

Agency for Toxic Substances and Disease Registry
U.S. Dept. of Health and Human Services