A cloud of smog blowing over from the continent has put London and Britain under a pollution alert.
The pollution rating in London is expected to be around 5 or 6 compared to much higher readings elsewhere.
Experts are warning asthma sufferers, the elderly and anyone with respiratory conditions to be on their guard.
The advice in London is:
Adults and children with lung problems, and adults with heart problems, who experience symptoms, should consider reducing strenuous physical activity, particularly outdoors.
The Department for Environment, Food and Rural Affairs, Defra says:
Air pollution levels are expected to remain Moderate or High today across Wales, Northern Ireland and many parts of England under high pressure and light winds. In the far southeast of England, air pollution levels are forecast to be mainly Moderate under a northeasterly breeze.
For Scotland, Moderate levels are expected in many areas but a freshening southwesterly breeze will bring Low air pollution to the northwest.
Winds bringing in pollution from the continent, combined with locally generated pollution and still weather conditions has led to some high pollution measurements across the UK.
Levels are expected to return to low by Friday.
Kay Boycott, chief executive of Asthma UK, said:
When air pollution is high it’s vital people with respiratory conditions including asthma check air pollution forecasts, carry their reliever inhaler with them at all times, and ensure that they are taking their preventer inhaler every day because this will help build resilience to asthma triggers like air pollution.
The pollution risk has been heightened by weather conditions.
High pressure and lack of wind fails to disperse pollution clouds hanging over urban areas that are added to during rush hour periods with heavy traffic on the roads.