Germany has launched a scheme to retrofit its diesel public buses with exhaust-scrubbing systems, and introduce charging points to encourage drivers to switch to e-cars. Still, environmentalists say that's not enough. They want all diesel vehicles — including private cars — retrofitted, or taken off the road.

Milan, one of Italy's most polluted cities, has banned cars from its downtown area during certain hours. Other cities in Italy and abroad have experimented with similar schemes, for example permitting only cars with odd or even license plates on the road at given times in order to limit the amount of traffic.

The Macedonian capital of Skopje is battling with pollution levels up to 15 higher than permitted by the EU — though it's not yet a member state, so isn't facing fines. Macedonia's smog problem is largely down to burning coal and emissions from aging, inefficient industry and vehicles. To get people to leave their dirty old cars at home, the government has introduced free public transport.

One street in London exceeded the EU's annual nitrogen dioxide limit on January 30 — less than a month into 2018. Actually, this is an improvement — it's the first time in a decade the British capital has kept within the annual limit for more than six days. Mayor of London Sadiq Khan has announced he wants to alert the city's schools on days when pollution is particularly bad.

Cities all over the world are fighting against smog. A German court has ruled that cities are allowed to impose driving bans. Many German cities — including Stuttgart, shown here — have developed an air pollution problem and are debating how best to approach the problem.

A diesel ban is imposed in the Norwegian capital whenever air pollution levels rise above a prescribed limit. The ban went into effect for the first time on January 17, 2017. Ambulances and other public service vehicles running on diesel are exempt from the measure. The city plans to reduce even more cars by eliminating municipal parking spaces in the center starting in 2019.

Starting in 2024, the French capital will ban diesel vehicles; in 2030 it intends to expand the ban to gas-powered cars. Vehicles manufactured before 1997 are already prohibited in the city on weekdays. When air pollution levels exceed prescribed limits, Parisians have to follow a rotation system in which only cars with either even- or odd-numbered plates are allowed to be driven in the city.

If you want to drive into the center of London, a day's ride through the city will cost you 10 pounds ($13.80, €11.20). London introduced the congestion charge in 2003. Automatic number plate recognition is used to enforce the measures. Anyone who does not pay the fee faces a heavy fine of up to 240 pounds.

Copenhagen's mayor, Frank Jensen, wants to prevent new diesel cars from entering the city starting in 2019. Currently, over 300 kilometers of roads in the Danish capital can only be used by cyclists. With the new regulations, cycling will become easier, more convenient and cheaper than driving a car. About half of Copenhageners now cycle to work.

Car-free zones like the square in front of Madrid's Teatro Real are set to become a common sight in the city. Almost the entire center of the Spanish capital will be turned into pedestrian zones in the next five years. Madrid has high smog levels, due to being surrounded by mountains, which cause bad air to get trapped in the city.

Riding public transport will become even easier in the near future in Helsinki. In the next ten years, a mobility on demand system will be developed to include all forms of public transport in one app, including buses, self-driving cars and minibuses with flexible routes. The goal of the app is to be so good that no one will want to own a car.

Smog chokes Delhi and levels of air pollution regularly go off the scale. Electric rickshaws will hopefully alleviate the problem. By 2030, all new vehicles will be electrically powered and the city will phase out gas powered vehicles.

As the nations of the world discuss strategies to reduce emissions at COP23 in Bonn, on the other side of the world, thick smog has engulfed the Indian capital Delhi.

Smog is quite common in Delhi. In fact, the city on the Yamuna river is a sad record holder: it is the capital city with the worst air quality in the world.

While the advent of winter doesn't bring snow in Delhi, it habitually makes conditions worse in the megacity. That's because the cooler air traps the smog close to the ground.

An important contributor to smog is usually motorized traffic, which holds true for Delhi as well. Beyond that, farmers in nearby rural areas traditionally burn their fields after harvest, thereby making their own contribution to air pollution. This practice is technically banned but authorities tend to turn a blind eye.

This year, Diwali added to the pollution. The Hindu festival of light not only brought a lot of traffic to the city, it also involved burning large amounts of firecrackers. And it goes on for five days!

The smog in the city has gotten so thick that it has become difficult to breathe. It burns the eyes and throat and many people complain of headaches. It even obscures the sun.

People try to protect themselves against the fine particle pollution as best they can using scarves and face masks. But there is really no escaping it.

On Wednesday, pollution levels in the city reached almost 30 times those considered safe by the World Health Organization.

Even heads of state can't escape the thick air. The Belgian King Philippe, inspecting a military guard of honor during his state visit to Delhi is shrouded in smog just like everyone else.

In a first measure to reduce pollution, the city government has targeted motorized transportation. By lowering fares on public transport and raising the fees for parking, it hopes to keep people from driving.

The chocking smog has also led the city to shut all schools for the remainder of the week.

People in Delhi are fed up with the chronically bad air quality in their city. These women wearing face masks are on their way to a protest. So far, the city's response to the crisis has been half-hearted at best.