Is Paris Europe's smog capital?

Thick, polluted air clouds the picturesque of the Eiffel Tower in Paris. For several days in a row now, the air pollution has exceeded the maximum permitted levels for particulate matter.

On March 17, 2014, Paris officials introduced a driving ban in the city. Vehicles with an even number license plate are banned from entering the city on Monday; and on Tuesday cars with an odd number are banned. It is the first ban in over 15 years. Only electric and hybrid cars or those with more than three people are excluded from the ban.

Brussels, the capital of Belgium and the seat of the EU and NATO headquarters, is smaller than Paris, but competes equally in terms of air pollution. On Friday, March 14, 2014, even the Eiffel Tower could have disappeared in Brussels' smog. To improve air quality, an overall driving speed limit of 50 km/h was introduced in the city.

In Sarajevo, Bosnia, the concentration of particulate matter is the highest in Europe. The EU allows for an annual average of 40 micrograms per cubic meter. In Sarajevo it is 117 micrograms. The city's weather conditions are largely to blame. Without regular winds, smog and traffic exhaust accumulate over the city. High gas prices have also led residents to heat more with low-grade coal.

In China, pollution levels have been record high for several days this year already. In Beijing, notorious for its thick air, a smog alarm has been established. The levels for the particularly dangerous fine dust particles are about 14 times the limit recommended by the World Health Organization.

The city of Ahwaz ranks worst on the WHO's list when it comes to smog, making it officially the city with the dirtiest air in the world. The reason is the amount of heavy industry in the city, which uses oil, metals and natural gas in its production processes.

Ulan Bator is not only one of the coldest capitals on earth, it's also the city with the second worst air pollution worldwide. During the winter months, domestic fireplaces with coal and wood contribute up to 70 percent of the smog in the city.

Air pollution is one of Pakistan's main environmental concerns at the moment. The situation is particularly dramatic in the country's second largest city, Lahore. The smog is caused primarily by the high volume of road traffic, rubbish incineration and dust from the surrounding deserts.

In the nearly 10 million-strong city of New Delhi, the number of cars has increased from 180,000 to 3.5 million in the last 30 years. Still, it's the city's coal powered plants that are causing the biggest problem. They contribute to around 80 percent of the total air pollution in the city.

Sandstorms, like here in Riyadh, can contribute to smog forming because they increase the amount of particles in the air. In a place like Saudi Arabia, the intense ultra-violet rays also transform transport and industry emissions into ozone.

According to a study by the Max-Planck Institute in Mainz, some 15,000 people die every year in Dhaka due to air pollution. Researchers found the world's highest concentration of sulfur dioxide there.

Even if it looks the same the world over, smog is different, depending on the city. Smog in Moscow, for instance, is characterized by high amounts of hydrocarbons. The westerly winds which regularly blow across Moscow mean that the western part of the city generally has better air quality.

The smog in Mexico City is made worse by the geographical location. The city is surrounded on three sides by mountains. Due to the high levels of sulfur dioxide and hydrocarbons in the air, Mexico City was long considered one of the most polluted cities in the world. The situation is now improving due to new transport policies and certain factories being shutdown.