The government has issued another warning after some areas have reached the highest levels on the air quality scale.
London and the south east of England had reached the maximum level 10 - meaning very high, on Thursday morning. Here, skyscrapers in London's Canary Wharf are surrounded by the smog.
High levels of air pollution are usually reached about five times a year. This current smog is expected to clear on Friday.
The government says parts of eastern and north-west England, the Midlands, north Wales and south-west Scotland would be the worst affected on Thursday. Here, Dover Castle in Kent, is surrounded by the haze.
People with breathing conditions such as asthma, have been told to take care and not to do any heavy exercise outdoors in the affected areas.
It is quite hard to make out but this is the O2 arena in London surrounded by the smog.
A combination of dust blown over from the Sahara and pollution, has made this one of the worst smogs we've seen in a while. Clouds of dust particles linger over these houses in Birmingham.
It has been a smoggy few days in some parts of the UK and the government has issued another warning after some areas reached the highest levels on the air quality scale.