Those with asthma are told they may find they need to use their inhaler more often and older people are also advised to reduce physical exertion.
When pollution levels reach 10, the general population is told to “reduce physical exertion, particularly outdoors, especially if you experience symptoms such as sore eyes, cough or sore throat”.
Defra said the elevated pollution levels have been caused by a combination of light south-easterly winds, the continental air flow and dust which has blown up from the Sahara desert.
North-west Norfolk should experience the highest level of air pollution on Tuesday, while East Anglia will experience "high" levels and parts of south-east England and the Humber region will experience "moderate" pollution.
Experts are anticipating "high" or "very high" air pollution levels across much of England and Wales on Wednesday.
The high levels of pollution are expected to continue across East Anglia and the Midlands on Thursday, but the air pollution is expected to ebb away by Friday.
Last weekend, some people found their cars to be covered in a light coating of red dust. The Met Office said that a large amount of sand and dust was swept up by storm winds in the Sahara Desert.
Experts said that the airborne particles of dust were blown north to the UK where they combined with our warm air and were deposited during showers.