Benzene (also known as benzol) is a colourless and highly flammable liquid with a sweet odour. It evaporates into the air very quickly and partly dissolves in water. Most people can begin to smell benzene in air at approximately 60 ppm of air and recognize it as benzene at 100 ppm. It was first discovered and extracted from coal tar in the 1980s and subsequently from petroleum due to vast increase in its demand. Since then, various industries use benzene to make other chemicals, such as styrene, cumene and cyclohexane. Benzene is also an important ingredient for the manufacturing of some types of rubbers, lubricants, dyes, detergents, drugs and pesticide. Hence, individuals employed in these industries may be exposed to benzene in air far greater than the levels normally encountered by the general population. 
Concerns on potential health effects resulting from benzene exposure such as bone marrow depression leading to anaemia, leukopenia and thrombocytopenia, as well as decreases in circulating blood cells have been reported. In addition, laboratory studies on humans and animals indicated that benzene exerts its effect by damaging the genetic material of cells. Hence, benzene is being classified as confirmed human carcinogen. 
In Singapore, every employer has the responsibility to take reasonably practicable steps in ensuring no one at work is exposed to benzene above the permissible exposure level (PEL) specified in regulations. The prescribed permissible exposure limit (long term) for benzene is 1 ppm over an 8-hour working day and a 40-hour workweek.  These exposure standards are usually set based on risk evaluation using data obtained from human and/or animal health effects studies. 
Thus, this paper will discuss the effects of benzene exposure as well as the adequacy of local PEL in ensuring that workers are protected from its toxicity.
2. EXPOSURE CONCENTRATION
Inhalation exposure is the major route exposure to benzene, although ingestion and dermal routes are also significant. Exposure to high concentrations of benzene of about 20,000 ppm in air within 5 to 10 minutes can result in death. Lethality in humans has been attributed to asphyxiation, respiratory arrest, central nervous system depression, or suspected cardiac collapse. However, in the absence of further exposure, people will stop feeling these effects because 80% of the benzene will be eliminated by chemical breakdown in the body or by metabolic excretion in the urine within about two days.
Excessive exposure to benzene increases the chance for infection and lowering the body’s defence against cancer. Long-term exposure to benzene can cause cancer of the blood-forming organs termed as acute myeloid leukaemia (AML). [1, 5]
In animals, acute inhalation exposure to high concentrations of benzene has caused death. An inhalation LC50 value for rats was calculated as 13.700 ppm following a 4-hour exposure. Early deaths of rats and mice have...