The East of England has been forecast very high air pollution levels.
The Department for Environment, Food and Rural Affairs (DEFRA) has forecast a 10 on its 10-point scale in Norfolk and Suffolk.
A spokesman from Weatherquest, a Norwich-based forecaster, said: "It's probably caused by a combination of things.
"We've had light winds recently, meaning air's not moving around much and pollution is building up.
"We've also had a fair bit of high pressure which creates inversion and traps pollution in a lower part of the atmosphere.
"Put together these mean that the pollution is being produced but can't disperse. With winds set to pick up this weekend, the levels will likely drop."
Tomorrow there will also be very high levels forecast for parts of eastern and southeast England.
There is currently health advice in place for at-risk individuals.
Adults and children with lung problems, adults with heart problems and older people should avoid strenuous physical activity.
People with asthma may find they need to use their inhalers more often.
For the general population, DEFRA recommends to reduce physical exertion, particularly outdoors, especially if you experience symptoms such as cough or sore throat.
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