Smog caused by Saharan dust hit south-east London and north Kent today.
Experts say "high to moderate" levels of pollution are expected in the area for the rest of the week, ebbing away on Friday.
During this period asthma sufferers and people with heart or lung problems have been warned not to take exercise outdoors.
The pollution has been caused by light south-easterly winds, the continental air flow and dust blown up from the Sahara desert.
It comes after some people found their cars covered in a light coating of red dust at the weekend.
The Met Office said a large amount of sand and dust had been swept up by storm winds in the Sahara Desert.
The airborne particles of dust were blown north to the UK where they combined with warm air and were deposited during showers.
A Defra spokeswoman said: "The high level of air pollution this week is due to a combination of local emissions, light winds, pollution from the continent and dust blown over from the Sahara.
"We want to keep improving air quality and have introduced a new five-day forecast service in addition to investing heavily in local and transport initiatives to tackle this issue head-on."
To view a map of pollution levels expected over the next few days, click here.
Has the smog affected you? Do you have any pictures? Call 01689 885 711 or send an e-mail.