Most recently reports of smog across Asia have been making the headlines. In fact, in the Chinese capital Beijing scientists have deemed the air quality so bad that they consider the city "barely suitable" for living.
The word "smog" dates from the early 20th century and is a construction of the English words, smoke and fog. Smog arises most commonly when a cold front hits a heavily populated city. The cold air moves in below the warmer air that has previously been in the area. The cold front is heavier and sits on the city like a blanket.
Before too long, the warm air on top starts to work like a lid on a jar and movement of air between the two levels almost stops. This so-called "inversion weather" is crucial for smog to form. It is also the reason why smog forms more readily in winter than in summer.
The other 'ingredient' in smog is pollution, something which normally exists in big cities in large amounts. Pollutants are emitted into the air from the burning of fossil fuels in power plants and vehicle engines as well from industrial and household fumes and exhaust.
The importance of limits
For years, the World Health Organization has warned that fine particulate matter, ozone, nitrogen monoxide and sulfur dioxide are particularly dangerous, repeatedly urging that specific limits for each of these be introduced in order to protect the residents of big cities.
During winter smog periods, fine particulate matter, which comes from vehicle emissions, is a major issue. The particles are 2.5 micrometers in diameter and can enter the lungs and attack the heart.
In summer smog, ozone becomes more of a problem. Nitrogen Oxide and hydrocarbons, which are part of car emissions, undergo a chemical reaction when subjected to sunlight and form ozone. The gas is colorless and – although it protects us against the sun's rays when in the upper atmosphere – it is highly poisonous down at ground level.
Risk of air pollution
Most people don't know how dangerous smog actually is," says Benedikt Steil from the Max Planck Institute for Chemistry in Mainz. Every year, in Germany, more than 40,000 people die of diseases related to air pollution. That figure is considerably higher than the death toll from traffic accidents.
Long term exposure to fine particulate matter, nitrogen dioxide and sulfur dioxide can lead to chronic respiratory problems, according to the International Agency for Research on Cancer. Lung cancer and bladder cancer, for example, can increase with exposure to smog.
The health risks associated with smog are also worse in developing countries than in industrial nations. Many of the pollutants are generated during energy production processes.
"New types of energy production are needed, or at least cleaner methods of burning fuels using filter systems and catalytic converters," Steil told DW.
In order to keep the air clean in cities like Berlin and New York, authorities have set up low emission zones and built more public transport over the last decade. Steil says that, in developing nations, more people should use bicycles for short trips. It wouldn't solve the problem, he says, but it would be a start.