measurements of lead in the atmosphere have been made in the central reserve of the motorway from 1973 to 1979. annual average concentrations of lead are available for the period 1974 to 1977 when levels fell from 14.1 to 9.3 micrograms per cubic metre. during this period the maximum permissible level of lead in petrol was reduced from 0.64 g/l to 0.50 g/l with a consequential change in the average concentration of lead in fuel sold from 0.52 g/l to 0.45 g/l. the flow of petrol-engine vehicles increased by about 4 per cent during this period. four-monthly average levels for june to september are available for the period 1973 to 1979 and these are shown to be similar to the annual averages. the lead concentrations changed from 23.8 to 9.7 micrograms per cubic metre during this 7 year period. traffic flows at the beginning and the end of the period were similar. there appears to have been little change in the pollution levels for the period 1977 to 1979. a simple relationship between traffic flow and speed, lead content of fuel and wind speed has been developed which can be used to estimate lead concentrations at similar sites.(a)

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