People with health problems are being advised to avoid strenuous activity on Thursday as high air pollution levels are expected in Northamptonshire.
Defra issued the warning earlier this week as it recorded high pollution levels on Tuesday.
The pollution is said to be a mixture of local and European emissions as well as dust from the Sahara.
The elderly and those with lung or heart disease are among those warned against exercising outside.
A ten-point scale is used to measure air quality, with level one signifying a low risk of air pollution and level ten meaning the air has very high levels.
The concentration in Northamptonshire tomorrow will be about level eight, according to maps released by Defra.
The figures are arrived at by measuring the concentration of five pollutants including NO2, sulphur dioxide (SO2) and ozone.
For more information, visit uk-air.defra.gov.uk.