ppb), the upper limit of the WHO guideline range.
Emissions from motor vehicles are the most significant source of carbon monoxide, nitrogen oxides and secondary pollutants such as ozone. These are expected to increase further due to the increased number of cars. Greater London had 2.7 million motor vehicles in 1988 (one car per three inhabitants) and together with the metropolitan area accounts for over 6 million cars (35 per cent of all UK cars). According to OECD estimates, road traffic in London has increased by 70 per cent during the last 20 years. A 7 per cent increase has occurred since 1989 alone.
Source: UNEP/WHO, 1992
Figure 10.3 - Annual mean concentration of SO2, smoke and NOx, London, 197389