Pollution levels in London and southeast England were rated on Thursday at the maximum level of 10.
A freak combination of weather conditions has left parts of the country covered in a smog haze made up of high levels of particles, including dust from Africa's Sahara Desert.
Meteorologists say it's been compounded by local emissions and light winds.
There has been a rash of emergency calls from people with respiratory problems and people are being discouraged from exercising outside.
Forecasters say the smoggy conditions are likely to last into the weekend.
The BBC reports levels are determined by the concentration of five pollutants in the air - ozone, sulphur dioxide, nitrogen dioxide and two types of particulate matter.
High levels of air pollution are usually reached about five times a year.