Paris authorities have requested emergency traffic-limiting measures to be put in place on Friday, as the City of Light and much of northern France suffers from a choking smog.

City Mayor Anne Hidalgo asked national authorities to stop one in every two cars entering the capital and make all public transport free for a day in a bid to drive down pollution.

These emergency measures were last implemented almost exactly a year ago - on March 17 - during a particularly bad spike in pollution levels.

On Wednesday, the concentration of potentially dangerous particulates in the air went over the recommended maximum, pressing authorities into action.

In Paris, authorities measure the concentration of particulates with a diameter of less than 10 microns - so-called PM10-in the air to determine pollution levels.

These particulates are created by vehicles, heating and heavy industry, and include the most dangerous particles, which measure less than 2.5 microns in diameter and which can penetrate deep into the lungs and the blood system and cause cancer.

The safe limit for PM10 is set at 80 micrograms per cubic meter.

According to a 2011 World Health Organization report, the planet's most polluted city was Ahvaz in Iran with an average of 372 mcg/m3.