LONDON (AP) -- London's mayor has issued a smog alert for the British capital, advising people with heart and lung conditions to avoid outdoor exercise.

Mayor Sadiq Khan says Londoners with asthma or heart problems "are advised to consider reducing strenuous physical activity, particularly outdoors."

The London Air Quality Network, run by King's College London, said there would be moderate-to-high air pollution Thursday.

Under a system introduced by the mayor last year, air quality alerts are displayed at bus stops and subway station entrances during times of elevated pollution. Road signs instruct drivers to switch their engines off when stopped to reduce emissions.

Khan has vowed to improve London's worsening air quality. Some of the pollution comes from diesel vehicles, although Thursday's alert is partly due to industrial emissions from continental Europe.