Benzene is a naturally occuring colourless liquid at room temperature (25^oC) and pressure (760 mm Hg). It has a characteristic aromatic odour, a relatively low boiling point (80.1^oC) and high vapour pressure, which causes it to evaporate rapidly at room temperature. Benzene is released into the environment from both natural and man-made sources, although the latter is the most significant source. Benzene in air exists predominantly in the vapour phase, with residence time varying between few hours to a few days depending on the environment and climatic conditions. Degradation of benzene in air occurs mainly by reaction with hydroxyl, alkoxy and peroxy radicals.

8.1 Benzene in the Environment

The presence of benzene in gasoline (petrol) and in industrial solvents can result in its widespread emissions to the environment. Outdoor atmospheric levels range from 0.2 µg/m³ in remote rural areas to as high as 350 µg/m³ in industrial centres with high density of automobile traffic. During refuelling of automobiles, levels upto 10 mg/m³ have been measured. Benzene measured in the ambient air of Delhi by Central Pollution Control Board during 1999 - 2000 by passive sampling show a mean concentration of 25 µg/m³ and maximum of 75 µg/m³.

Table 12 Benzene Levels in Major Cities of the World

Benzene is readily absorbed by the body during inhalation or ingestion and is rapidly distributed throughout the body, particularly in fatty tissues. Metabolism occurs primarily in liver and to less extent in the bone marrow, producing intermediates, which account for the toxicity of benzene. In human, half-life of benzene is 1 - 2 days. Accumulation is not expected for benzene or its metabolites. Benzene is primarily exhaled through the lungs unchanged or excreted as metabolites in the urine.

8.2 Health Effects of Benzene Exposure

8.2.1 Short Term Health Effects

Short-term exposure causes depression of the central nervous system marked by drowsiness, dizziness, headache, nausea, loss of coordination, confusion and unconsciousness. Nose and throat irritation has also been reported following short-term exposure. A period of feeling excited or giddiness may precede the onset of other symptoms. Benzene vapours can be irritating to the eyes.

8.2.2 Long Term Health Effects

Benzene causes a serious condition where the number of circulating red blood cells (erythrocytes), white blood cells (leukocytes) and clotting cells (thrombocytes) is reduced (pancytopenia). At this stage, effects are thought to be readily reversible. However, continued exposure of benzene can result in aplastic anemia or leukemia. Benzene also damages the bone marrow, where new blood cells are produced. Studies on benzene exposure have found changes in the immune system, which are at least partially related to the changes in the blood system. Benzene may cause effects on the peripheral nerves and/or spinal chord. Symptoms include an increased incidence of headache, fatigue, difficulty in sleeping and memory loss with significant exposures. The International Agency for Research on Cancer (IARC) has concluded that there is sufficient evidence for benzene carcinogenicity to humans.

Table 13 Gasoline Benzene Reduction Program In India

Source: Air quality Status and trends, CPCB Report NAAQMS/14/2000-2001