The commission wrote to the government last week asking it to give evidence of measures that it is taking to reduce excess emissions. Road traffic is a major contributor to emissions of harmful particulate matter.
Discussions are in progress in Europe to enforce more stringent laws. Limits for nitrogen dioxide emissions will come into force in 2010. Figures indicate that the UK will miss its planned target.
A Department for Transport spokeswoman pointed out that emissions from road transport have fallen by 50 per cent since 1990, despite traffic increasing by a fifth.