A high air pollution warning has been issued in London for Thursday amid the ongoing heatwave.
Mayor Sadiq Khan said high temperatures combined with the capital's "toxic air, a lack of cloud cover and emissions travelling from the continent" meant pollution would hit "high levels".
Alerts will be displayed on various signs and vulnerable people are advised to avoid strenuous physical exercise.
It is the second time a high warning has been triggered this year.
The last recorded high pollution in London was on 7 May.
Temperatures are set to rise on Thursday and Friday with the Met Office warning the UK's all-time record may be broken.
City Hall said the weather conditions along with a southerly air feed bringing pollution from Europe "combined with London's own emissions will likely produce high levels".
Adults and children with lung or heart problems and older people are advised to "reduce strenuous physical exertion, particularly outdoors".
People with asthma have also been warned they may need to use their reliever inhaler more regularly.
Messages about the alert will be shown on bus countdown signs, roadside message boards and at the entrances to all 270 Tube stations.
It is the ninth time an air quality alert has been issued since Mr Khan became mayor.