HEALTH EFFECTS :

·        General Exposure and Daily Intake :

Benzene is ubiquitous in the environment, resulting in the exposure of most humans to trace levels (or more) of this chemical.  Exposure in the general population is primarily to air borne benzene and derives from active and passive tobacco smoke, industrial activities, and use of the automobile (gasoline fumes from refilling, etc. and exhaust emissions).  Estimates of the daily amounts of benzene consumed in drinking – water and food-stuffs vary considerably and are of the order of µg/day.  Depending upon the assumptions made with respect to levels of benzene from tobacco products and foodstuffs, estimates for the exposure of the general smoking population in industrialized countries range from 2000 to 3500 µg benzene/day.  Adult (70kg) non-smokers are considered to be exposed to about 200 to 1700 µg benzene/ day (about 3 to 25 µg/kg body weight per day).  It would be helpful to have more information on total human exposure, particularly in developing countries.

Table 8 :  Estimated Daily Intake of Benzene* From Different Sources

·        Sources : Ambient air – Department of the Environment; ( Based on rural daily mean of 0.5 ppb and urban maximum daily mean of 12.2 ppb at Exhibition Road, London.  Intake calculated using the World Health Organisation method)

About 50% of inhaled Benzene in air is absorbed.  Benzene intake based on 24 hour exposure volume of 20 m³ at  rest will be 10 mg/day for each 1 mg/m³ benzene in air.  The daily adult intake at a typical benzene level of 16 µg/ m³ will, therefore, be about 160 µg.  Together with other pollutants, benzene also participate in photochemical process which result in formation of oxidants and smog.  Exposure to high level of benzene causes neurotoxic symptoms.  Persistent exposure to high level of toxic level of benzene may cause injury to human bone-marrow.  Early manifestation of toxicity are anaemia, leucocytopania or thromo-cytopania.  Benzene is a known human carcinogen (IARC Group).

Typical in vehicle and refueling exposure in US in 1987-1991 are reported to be 40 ug/m³ (12ppb) and 288µg/m³  (89ppb), respectively

·        Occupational Exposure :

The major factors controlling industrial exposure to benzene are process technology, worker practices and the efficiency and sophistication of engineering controls.  When appropriate engineering controls are in place, available monitoring data indicate that exposures of workers involved in the production, handling and use of benzene and benzene- containing materials which vary from non-detectable levels to approximately 15 mg/m³ (8-h TWA), in addition to the amounts estimated for the general population.  In developing countries the exposure can be several times higher.  Due to the nature of the processes involved, a small percentage of workers may be exposed to more than 320 mg benzene/shift.  In some developing countries, benzene exposure may be sufficiently high to cause acute toxicity.  Dermal exposure to benzene has generally not been included in these estimates.

As regards to permissible level of benzene concentration for occupational exposure, American Conference of Government Industrial Hygienists prescribes a threshold limit value (TLV) of 0.5 ppm (1622 µg/m³) (TWA). OSHA regulations also call for human exposure limit of less than 0.5 ppm.  National Institute of Occupational Safety and Health (NIOSH) suggests Recommended Exposure Limit (REL) of 320 µg/m³ (o.1ppm)

The results of a Benzene exposure survey study carried out in USA in different Industries & occupations are given in the following table-9.

Table 9 : Percentage of Employees in the USA Potentially Exposed to

                Benzene

Thus, a non- smoker living in an unpolluted rural area may be exposed to as little as 120 µg benzene daily, while a 20 cigarettes per day smoker living in a city may be exposed to as much as 1,250 µg daily

·        Toxicological & Carcinogenic Effects :

Long term exposure to benzene in air causes leukaemia in human beings.  In animal studies, leukaemias, lymphomas and other types of tumours are observed.  Exposure to benzene is linked to genetic changes, increased proliferation of bone marrow cells and occurrence of certain chromosomal aberrations in humans and animals.  US EPA has classified benzene as Group A human carcinogen.  The International Agency for Research on Cancer (IARC), Lyon (France) has list ed benzene as carcinogenic to humans.  In addition a number of noncancer health effects are associated with benzene exposure such as disorders of blood, harmful effects on bone marrow, anaemia and reduced abilities of blood to clot, damage to immune system and a reproductive and developmental toxicant .  WHO estimates a 4 in 1 million risk of leukaemia on exposure to benzene to a concentration of 1 ug/m³  (0.31 ppb)                 

High levels of benzene exposure produce haematotoxic effects like leucopenia, lymphopenia & anaemia in laboratory animals.  Exposure to high levels of benzene causes neurotoxic symptoms.        

Substances that can induce benzene –metabolizing enzymes are likely to modify the haematotoxicity of benzene. It has been found that benzene itself, Phenobarbital, Toluene & ethanol can modify the metabolism & haematotoxicity of benzene if animals an pretreated with these substance. Toluene has been found to inhibit the metabolism of benzene & decrease its haemototoxicity. Ethanol inhances the haemototoxicity of benzene in mice.

Acute lethal doses of benzene in experimental animals cause narcosis,   ventricular tachycardia and respiratory failure. 

At benzene concentration above 32 ug/m³ in air, there is a co-relation between phenol excretion in urine and the level of exposure.                                                        

Table 10 : Estimated Percentage of Worker Populations that might develop Bone Marrow  Depression or Aplastic Anaemia after Chronic Exposure to Benzene

This estimation is an interpretation of the literature and is based on the experience of the Task Group.  The speculative nature of this table precludes its  use in occasional cases.

The BTX (Benzene Toluene and Xylene) compounds are harmful toxic pollutants causing exposure related health effects in human beings.  This  group of volatile organic compounds (VOCs) are evidently carcinogenic in human beings.  The exposure of Benzene may cause respiratory disorders, narcosis, changes in blood pattern, anemia, leucopoenia and leukemia.

Toluene in comparison to Benzene is less toxic may cause drowsiness, impaired coordination etc..  High dose exposure of toluene can produce kidney and liver damage and hyperplasia of brne marrow, anaemia, depression in central nervous system which may lead to impairment of coordination and slowed reaction time.

Acute Xylene exposure may be marked by dizziness, weakness, headache, nausea, vomiting, breathing difficulty and loss of coordination.  In severe exposure, there are visuallurring, tremors, heart beat irregulaties, paralysis and loss of consciousness.