Dust clouds from the Saharan desert are expected to sweep over some parts of London in the coming days, forecasters have warned.
The government's Department for Environment, Food and Rural Affairs (DEFRA) said parts of the capital could experience moderate levels of air pollution on Thursday and Friday (April 16 and 17) as a result, while southern parts of the country risk seeing high levels.
DEFRA is warning those with lung and heart problems to avoid strenuous exercise, particularly outdoors, and says asthmatics may need to use their inhalers more often.
Anyone experiencing discomfort, such as sore eyes, coughing or a sore throat, should consider reducing activity, the department warned.
The warning comes as Met Office analysts are predicting that London will experience temperatures of 23C on Wednesday, before dropping to 15C on Thursday and 14C on Friday.
Spokeswoman Laura Young said: "Over the next 48 hours there is a good air quality in London.
"There's a degree of uncertainty as to the air quality in London over Friday.
"At the moment we are looking at a cooler air mass and cooler and fresher air moving southwards throughout the course of Thursday and Friday, which is indicating that the quality of air will remain good throughout the rest of the week.
"There is some degree of uncertainty as to the timings for Friday, so there is a small possibility of Saharan dust affecting some southern parts, which may lead to locally moderate or high levels of air pollution.
"If this did happen on Friday, it would be very short-lived."