Health experts are warning people with lung and heart conditions to avoid strenuous exercise as a cloud of Saharan smog is forecast to arrive in Lincolnshire.
The Department for Environment, Food and Rural Affairs (Defra) has said there is a risk of high levels of air pollution in the county on Thursday.
A level seven red warning is in place for the region, with potentially toxic fog and dust threatening to cover the area.
Individuals experiencing discomfort, such as sore eyes, a cough or sore throat, are advised to consider reducing physical activity.
Asthma sufferers may also have to use their inhaler more frequently and are being asked to stay indoors.
In a pollution forecast, Defra said: “There is a risk of moderate or high levels of air pollution across parts of England and Wales on Thursday due to Saharan dust, although large uncertainties remain with the timing and amount of dust.”
Air pollution levels are expected to return to low on Friday.
Saharan dust is a mixture of sand and dust from the vast desert area that covers most of North Africa.
The dust is normally carried west into the Atlantic Ocean but southerly winds can occasionally cause it to drift towards the UK.
Asthma UK Director of Advice, Andrew Proctor, said: “People with asthma need to take extra care.
“We are facing a perfect storm of asthma triggers, with air pollution and weather changes increasing the risk of potentially fatal asthma attacks for millions of people with the condition.”