Paris authorities said today they would make public transport free for three days to encourage drivers to leave their vehicles behind due to severe pollution.

Jean-Paul Huchon, the head of the organisation that oversees transport in Paris and neighbouring areas, said transport would be free from Friday morning until Sunday evening.

He said this was due to the "significant risks to the health of residents" posed by the pollution.

The French capital has been under maximum pollution alert for several days, as have many other regions across the country.

A strong prevalence of particles with a diameter of less than 10 microns has been reported by several monitoring centres. These are the most monitored as the smaller and lighter a particle is, the longer it stays in the atmosphere and can remain suspended for weeks.

Particles less than 2.5 microns in diameter are also the most dangerous to health as they can penetrate deep into the lungs and the blood system. They can also lead to asthma, allergies and respiratory ailments.