Toxic compounds, hazardous chemicals and aggressive gases are not only produced in laboratories with restrict protection laws and well planned ventilation systems. They are, unfortunately, allowed to be produced and emitted as well as to keep circulating in our living environments and in the atmosphere around us. Though most of our concern is focused on “green-house” gases and the “green-house” effect, air pollution and air quality are among important environmental issues because of their severe impacts on health.

Many capital cities around the world suffer from “smog”. “Smog” has different origin and composition. Heavy industries using oil, metals and natural gas in their production can be potential sources for smog formation. Domestic fireplaces with coal and wood can contribute to major parts of the smog formation in some cities; this can be also the case with high volume of road traffic, rubbish incineration and dust from the surrounding deserts. In some mega cities the number of cars has increased to tens of millions in the last 30 years. However, fossil fuels, in particular coal, powered plants can still be major sources causing the biggest problem. Apart from man-made pollution, natural processes, e.g. sandstorms in hot arid and semi-arid regions/deserts can contribute to smog forming. In combination with intense ultra-violet rays industrial and automobile emissions can be transformed into, as transported as, ozone.

But, according to the most recent figures from the World Health Organization (WHO), the megacity doesn’t even rank among the top 10 cities for smog. Most of the worst afflicted are smaller cities across the developing world.

Poor air quality causes a number of illnesses for city residents, like chronic respiratory problems and lung cancer. 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. Smog can contain high levels of toxic chemicals, e.g. hydrocarbons, heavy metals, fine particulates, S-/N-/C- compounds, ozone, …. and other hazardous compounds.

The Chinese capital, Beijing, isn’t the only big city suffering from smog. From Asia to the Middle East to the Americas, here’s a look at the 10 worst cities for bad air. Beijing, China; Ahwaz, Iran; Ulan Bator, Mongolia, Lahore, Pakistan, New Delhi, India; Riyadh, Saudi Arabia; Cairo, Egypt; Dhaka, Bangladesh; Moscow, Russia; Mexico City, Mexico

http://www.dw.de/top-10-worst-cities-for-smog/g-17469135