The mayor of London, Sadiq Khan, has triggered the capital’s emergency air quality alert as polluted air from the continent combines with toxic air in London to create dangerous levels of pollution.

The alerts will see warnings displayed at bus stops, road signs and on the underground. Khan has also asked TV and radio stations across the capital to warn their viewers and listeners in news bulletins.

Anyone with lung or heart problems is advised to reduce strenuous exercise, especially outside. The young and elderly are particularly vulnerable.

Today’s alert has been triggered by “high” levels of air pollution. It is the seventh time in 13 months that the mayor has used the alert system. One instance was because the level of pollution was deemed “very high”, and on six occasions because it was “high”.

Khan said: “The shocking and illegal state of London’s filthy air means once again I am triggering a high air pollution alert today under my new comprehensive alert system.”

The government’s committee on the medical effects of air pollutants advises adults and children with lung problems, and adults with heart problems, to reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms. People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion.

Khan is implementing a range of measures to try to tackle the air pollution crisis in the capital, and he called on the government to do more.

“I am doing everything with the powers I have at City Hall and it’s now time for the government to step up by introducing a national diesel scrappage fund to rid our streets of dirty diesels, and to give me the powers I need to tackle non-transport sources of pollution.”

Responding to the news that the capital’s air quality emergency alert had been triggered, Prof Jonathan Grigg from Doctors Against Diesel said “dirty air is seriously damaging Londoners’ health and wellbeing.”

Grigg called on Khan to do more to tackle it. “Vulnerable people shouldn’t have to restrict their activities to stay safe,” he said. “Sadiq Khan must bring London’s air pollution down to legal levels as soon as possible, and commit to phase out diesel vehicles by 2025 to protect Londoners’ lungs.”

The government has come under increasing pressure over the UK’s air quality. It has suffered a string of humiliating defeats in the courts over its failure to clean up the nation’s air. Its latest proposal, released in July, was met with widespread criticism from clean air campaigners and regional politicians.

The latest episode of dangerously polluted air has been caused by mist, low cloud, fog and slow wind speed in London that has lead to a build up of pollution. This has combined with air arriving from the continent that has travelled slowly over industrial polluted areas giving it time to pick up emissions on the way. Experts say this is likely to produce high levels of PM2.5 and moderate levels of PM10 particulate pollution across areas of London and the south-east.