Traffic fumes, dirty air blown in from Europe and Sarahan dust are combining to coat parts of Britain in smog.
The environment department says air pollution is "high" across much of the country today and could reach the top designation of "very high" in southeast England.
Officials say people with heart and lung problems or asthma should avoid exertion.
In London, haze clouded the view of landmarks including Tower Bridge and the Houses of Parliament from atop the Shard, Britain's tallest building.
Steven Godby, a geographer at Nottingham Trent University, said a contributing factor was North African dust that was "incorporated into a northerly airflow and can travel long distances before being deposited as far away as the UK."
Officials say fresher air from the Atlantic should ease pollution tomorrow.
