Hot weather in July caused the highest levels of ozone pollution in London and the south east since 2003's heatwave, research has found.
High levels of ozone can irritate the lungs and eyes and cause migraine
Ozone pollution exceeded recommended EU levels on up to 20 occasions in July, compared to 10 times in a normal year.
There is "mounting evidence" to support links between air pollution and adverse health effects, said research manager Gary Fuller of King's College London.
Mayor Ken Livingstone said a cleaner environment was a "high priority".
"London suffers the worst air quality in the UK and amongst the worst in Europe," Mr Livingstone said.
"This is why I see measures to clean up the environment as such a high priority for my administration."
Researchers recorded high levels of ground level ozone, known as "summertime smog episodes", on 15 to 20 occasions in London and the south east in July.
The government and EU recommends that such levels are only reached up to 10 times per city per year.
High levels of ozone can irritate and inflame the lungs, cause eye irritation, migraine and coughing.
The research and other air pollution information has been made available on the London Air Quality Network website.