In the Brussels Region a 50km/h speed limit is being introduced.
A lack of wind has resulted in a concentration of pollution in the air. Concentrations of particulate matter and nitrogen oxide have shot up.
On the motorway the smog alert will be indicated using special signposts . The alert will last for at least two days.
Brussels police have announced that speed checks will be carried out. Vehicles are at their least polluting when driven at speeds between 50 and 60km/h.
The public authorities are asking the people of Belgium to help to limit polluting emissions. People are being asked to cut down on journeys if at all possible and work from home. Public transport or even a walk should be favoured. People who are forced to use their car should be flexible and limit their speed.