Pollution levels have fallen across England and Wales after several days of smog caused in part by Saharan dust, but health officials warned people to check local air quality before taking part in outdoor exercise.

London remained in the grip of moderate levels of air pollution and about 40 other areas around the country were suffering moderate to high levels, according to the Department for Environment, Food and Rural Affairs (Defra).

Forecasters say that a mass of fresh air moving across the UK from the west over the weekend would lead to higher air quality, with Defra saying conditions should improve into next week. But experts warned that the UK's underlying pollution needed urgent action.

An environmental lawyer at charity Client Earth, Alan Andrews, said: "The smog might have lifted, but the problem hasn't gone away. We're still left with illegal levels of traffic fumes in towns and cities throughout the country and a government that refuses to admit that there is a problem."

David Cameron came under withering attack from opposition politicians, experts and the European commission earlier this week, who accused him of failing to get to grips with air pollution levels after he labelled the smog a "naturally occurring weather phenomenon".

The European commission's environment spokesman, Joe Hennon, described the prime minister's comments as "more than disappointing" and of showing a "clear misunderstanding" of the air pollution crisis facing the UK.

On Friday Public Health England urged people to consult Defra's air quality maps before planning strenuous outdoor activity. The London Ambulance Service recorded a 14% jump in emergency calls for help with breathing difficulties, asthma and heart problems at the peak of the crisis this week when London experienced "very high" levels of pollution – the highest on Defra's official scale.