Paris and much of northern France have seen dangerously high pollution levels this week, forcing authorities to offer free public transportation to Parisians for the next few days. On Thursday, the French capital and several other regions were on high alert for the third consecutive day, as unseasonably warm yet windless weather has left small, dangerous particles lingering in the yellowed Parisian air.

Smog levels are similarly elevated in London, where air quality index readings on Friday were higher than the smog-plagued Chinese cities of Shanghai and Beijing, according to the nonprofit group Clean Air in London. On Friday, Paris' Air Quality Index (AQI) readings hovered around 185, putting it on par with Beijing, one of the world's most polluted cities. The UK Department for the Environment, Food and Rural Affairs (DEFRA) forecast high pollution levels for southern England today, though it has yet to issue an air pollution alert. Neighboring countries like Belgium have also seen spikes in pollution this week.

French authorities have taken steps to mitigate the problem, which the European Environmental Agency (EEA) described as the worst pollution spell since 2007. Yesterday, the head of Paris' transit authority announced that all forms of public transportation will be free this weekend, as part of an effort to reduce the pollution caused by cars. The city had earlier announced that its public bike and car sharing services will also be free while pollution levels remain high, though environmental groups have urged the government to do more.

Mayoral candidate and environmentalist Christophe Najdovski criticized Paris authorities for showing "irresponsible inaction" yesterday, saying they should have taken steps to reduce traffic levels much sooner. He and his party have called for new traffic regulations, including the creation of a new route that would allow heavy trucks to bypass Paris and a system whereby drivers could only use their cars on alternate days, depending on whether their license plate ends in an even or odd number.

After pollution levels spiked in December, the city lowered the speed limit for some cars and banned some large trucks from entering Paris. The government is now considering stronger restrictions on the use of high-polluting vehicles, according to Jean-Paul Huchon, the socialist chairman of Paris' regional council.

Experts blame the spike on the abnormally warm days and cold nights that Paris has seen in recent days. When the ground cools at night and suddenly heats up under the sun, particles from cars and heating systems can become trapped under warm air, making it difficult for them to rise into the atmosphere or be carried away with the wind.

"In the morning, there is a layer of very cold air which stays and stagnates close to the ground, whereas usually there is a natural lift of air from the ground up into to the atmosphere," says Jérôme Clave, managing director of Airparif, an organization that monitors air quality in the Paris region. This temperature inversion, he says, is "keeping all pollutants close to the ground."