Why is the air quality bad today?
While many of us are taking advantage of the UK heatwave and rushing to sit outside and bask in the sun, this can lead to higher levels of pollution.
Defra data showed high levels of air pollution across much of south-east England, particularly Essex and Norfolk.
Other places such as Leeds, Sheffield, Kent and Sunderland also face high levels of pollution.
The high pressure brings polluting particles to the UK, but the calm air means they are no longer carried away.
When there is little breeze or wind to remove air pollutants, it means the air quality is deteriorating.
In densely populated areas with lots of traffic and little greenery, such as London, the air quality can quickly deteriorate.
People were urged to walk, cycle or use public transport to reduce air pollution.
London Mayor Sadiq Khan said it was particularly bad right now because we are “importing pollution from the continent” as well as local emissions piling up.
He said: “Toxic air is extremely dangerous for Londoners – and I’m doing everything in my power to address it.
“It’s particularly important to protect those who are more vulnerable to heavy pollution.”