Air pollution in southwest London has reached high levels in the past 24 hours and is expected to remain high until tomorrow.
Vulnerable people with health problems have been warned to take particular care because of the smog - which has been caused by local emissions and dust flown in from the Sahara.
The elderly and people with lung or heart disease are among those warned against exercising outside in the affected areas.
Defra (the Department for Environment, Food & Rural Affairs) recorded air pollution in London as in the "high" category - seven out of 10.
Kay Boycott, chief executive of Asthma UK, said people with asthma who find that air pollution makes their condition worse could be at an increased risk of an attack.
High pollution levels are expected to be recorded again today but then clear tomorrow (Friday) as winds change direction.