Unseasonally high temperatures in parts of Europe have prompted some cities, including London and Paris, to issue pollution warnings.
Traffic, especially diesel cars, is one of the worst offenders when it comes to air pollution.
The map below shows the 20 worst places in Europe for vehicle-related pollution.
_(Source: European Environment Agency 2014)_
Euronews takes a look at how cities across the region are tackling the problem.
London’s congestion charge, which sees drivers pay to enter the city centre, was the first such scheme in a major European city when it was introduced in 2003.
Lorries, buses and coaches over a certain weight — and large vans — have had to meet certain emissions standards to be allowed into the city since 2012. This will be extended to all other vehicles by 2020.
One of the city’s main shopping areas, Oxford Street, has recorded some of the highest nitrogen dioxide levels in the world. Putney High Street, in southwest London, broke its annual emission limits just eight days into 2016.
The UK government’s Department for Environment was ordered to come up with a better set of proposals to tackle air pollution by July 2017 by the High Court.
The Spanish capital has introduced a series of measures that ban some cars from entering a specified area within the city in a push to curb greenhouse gas emissions.
The measure, introduced by Madrid's leftist mayor Manuela Carmena, has made an area of 472 hectares within the city centre off-limits to cars not belonging to residents or public transportation.
All other vehicles will only be able to enter this area if they carry an environmental label and are left in a public parking lot.
The initiative, backed by the European Commission, was introduced for public health reasons and aims to reduce pollution by 40% in the area — the limit required by the Commission’s Air Quality Directive.
All vehicles in Paris have to display a sticker attesting to how polluting they are, with some cars, such as those put on the roads before 1997, banned altogether.
Lorries and heavy goods vehicles are banned from entering the city at certain times, such as Monday mornings or Friday afternoons.
During some past pollution peaks, the city has tried to tackle the problem by only allowing vehicles with even-numbered registration plates one day, and odd the next.
The city also makes public transport free during these periods to encourage people to leave their vehicles at home.
Since 2017, diesel vehicles have had to meet certain emission standards (Euro 3) to be allowed on Paris’ streets.
The city regularly closes some of the worst-polluted streets. For example, the Champs Elysées is shut on the first Sunday of each month.
Paris is one of four cities worldwide to commit to banning diesel vehicles altogether by 2025.
The city is planning to ban private vehicles from the city centre this year and is heavily investing in improving public transport and increasing the number of cycle lanes.
There is already a charge for entering the city centre on Mondays to Fridays.
During periods of high pollution, the city can either temporarily ban the use of diesel vehicles that do not meet certain pollution standards, or, like Paris, allow vehicles with odd-numbered registration plates to circulate one day, but not the next, alternating with even-numbered vehicles.
The city has added at least 50 new toll stations in a bid to deter polluting traffic.
Stockholm and Gothenburg
Two Swedish cities, Stockholm and Gothenburg, also have a system of congestion taxes that apply to cars in and out of Sweden.
If the car is registered within Sweden, the payment is automatic, meaning it suffices to drive through a toll station and the receipt is sent to the owner of the car.
If the vehicle is from abroad, the Swedish Transport Agency identifies the owner of the vehicle and sends an invoice to that person.
Drivers in Milan have to pay a charge for entering the city centre, which is lower if you are a resident of the city.
On February 13, 2017, stricter limits were imposed on vehicles allowed into central Milan,
There is also a low emissions zone covering the province of Milan.
Munich, like scores of other German cities, has restrictions on what vehicles can drive on the city’s streets.
Vehicles have to meet certain standards (Euro 4, 5 and 6 for diesel vehicles in Munich) to be allowed to circulate. All vehicles have to put a sticker in the windscreen to indicate how polluting the vehicle is considered to be.
Trucks weighing more than 3.5 tonnes are banned from passing through the city centre.
There has long been debate about whether to introduce blue badges for vehicles, aimed at tackling nitrogen oxide levels.
Their introduction was rejected at a meeting of federal transport ministers in Stuttgart in October 2016, because it would ban older diesel cars from the roads.
However, ecologist Winfried Hermann, transport minister for the Baden-Württemberg region, said the idea was still on the political agenda.
In July 2016 the Bavarian Administrative Court ordered the state, which includes the city of Munich, to bring nitrogen oxide levels down within a year, or face a fine.
Malta’s capital, Valletta, also introduced a system of congestion charges to reduce traffic, parking problems, and pollution, particularly in the high tourism season.
What are other European cities doing to reduce traffic congestion?
Rome has various zones where traffic is limited but is thinking of introducing other measures to lower traffic congestion. It is also aiming to ban diesel vehicles by 2024.
Berlin has an "environmental zone" in which only low-emission vehicles are allowed in.
The cities mentioned in this article are just a selection of places with measures in place to help tackle air pollution._