In Pictures: What's choking these 10 mega world cities

Riyadh (350-400): Saudi Arabia's capital tops the list with highly prevelant sandstorms, as well as pollutants emerging from heavy traffic and industrial waste.

Delhi (200-250): The Indian capital is rapidly moving up on the list of heavily polluted cities around the world. Smoke from vehicles, industrial activities, burning of waste and household fuel puts the people at risk of several respiratory problems.

Cairo (150-200): The Egyptian capital is covered in a haze of noxious fumes because of a million mostly-old cars, industrial pollutants, construction work dust and the burning of rice straw by farmers.

Dhaka (150-200): Pollutants from brick kilns, burning of tyres, old cars and heavy traffic combine to put the inhabitants of the Bangladesh capital under grave risk of respiratory ailments.

Kolkata (150-200): Coarse dust particles from construction activities has polluted the atmosphere in West Bengal's capital. The Westerlies blowing in from neighbouring Delhi, UP, Bihar and Jharkhand also carry dust particles contributing to the pollution.

Mumbai (100-150): Mumbai recorded an annual average of 117 microgrammes per cubic metre (gm3) which is over five times the permissible limits. Industrial and construction activity, burning of biomass and heavy traffic are also major contributing factors.

Beijing (100-150): China's capital has hazardous levels of air pollution because of emissions from vehicles and heavy industries, construction dust, coal burning in neighbouring regions and dust storms from the north. The government announced in January it would close 2,500 smaller highly polluting firms in 2016 and also develop a network of ventilation ?corridors? to help disperse smog.

Shanghai (50-100): Massive construction and emissions from power, steel and petrochemical plants impact the air quality of one of China's largest cities. The government has set targets for many of these industrial units to cut down on their pollution levels. Also, on the cards is the transformation of almost 10 square miles of the city area to be converted into forest zone.

Istanbul (50-100): Emissions from construction, use of poor quality coal for indoor heating, inadequate use of public transport have led to rising air pollutions. Nor had the rapid urban transformation been matched by environmental planning.

Mexico City (0-50): Mexico's sprawling capital lies in a high altitude valley flanked by smog-trapping volcanic mountains. Incomplete combustion of diesel emissions because of less oxygen contributes to air pollution. Even though the government has relocated big refineries, banned leaded gas, and built more public transportation, these measures have been thwarted by the city's rapidly rising population.