Sources of Exposure to Benzene

5. There are no well-defined natural sources of benzene and all the benzene observed at ground level in the northern hemisphere is likely to have resulted from human activities, in particular the use of petrol and oil. Benzene is present in petrol and can escape into the air, for example, at filling stations. The proportion of benzene currently in both leaded and unleaded petrol is about 2% by volume in the United Kingdom. This is below the maximum of 5% allowed by European Community legislation. There has been no increase in the amount of benzene in leaded petrol over the last decade, in spite of a threefold reduction in its lead content.
6. While people working with petrol, in its manufacture and distribution, might be expected to derive their main exposure from this source, a major source of exposure for the general population is from vehicle exhausts, and here benzene is mostly produced by chemical reactions occurring during combustion of petrol in the engine. The importance of this source is illustrated in Table 1, which shows that no less than 78% of atmospheric benzene in the United Kingdom is derived from petrol engine exhausts. From this it is clear that for air quality standards to be achieved and bettered, control of this source is essential.

   Table 1 Sources of benzene emissions in the UK in 1991

   	tonnes/year*	%
   petrol engine exhausts	39,250	78
   diesel engine exhausts	4,550	9
   petrol evaporation from vehicles	3,350	7
   petrol refining and distribution	1,350	3
   combustion of oil, wood, etc	950	2
   gas leakage	400	<1
   other industrial processes	16-350	<1

   *Figures rounded to the nearest 50 tonnes

7. While petrol exhaust is the major source of benzene in the air we breathe outdoors, especially in urban areas, it is important to realise that there are other sources of human exposure, some of which contribute a significant proportion of an individual's total intake of the chemical. Cigarette smoke contains benzene, and may be the main source of exposure for a heavy smoker; passive smoking may make a small contribution to benzene intake. Benzene may also be present in low concentrations in some foods and in drinking water, and diet may be the main source of benzene for non-smokers living in unpolluted, rural areas. Table 2 gives some estimated figures for the different amounts of benzene (measured by weight in millionths of a gram - µg) to which we may be exposed daily from these sources.

   Table 2 Estimated daily intake of benzene*

   		µg
   Ambient air	- rural	15
   	- urban	400
   Cigarette smoke	- 10 per day	300
   	- 20 per day	600
   Food		100-250
   Water		1-5

   *Sources.

   ambient air - Department of the Environment; (Based on rural daily mean of 0.5 ppb1 and urban maximum daily mean of 12.2 ppb at Exhibition Road, London. Intake calculated using the World Health Organisation method) smoke, food and water - World Health Organisation

8. 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.