The study was published in the journal Environmental Research Letters.

The pollution is human-caused and it leads to death of people mostly in South Asia and East Asia.

This human-caused pollution increases fine particulate matter. This particles remains in air and enter our lungs causing cancer and other respiratory disease.

Jason West from the University of North Carolina who was the co-author of study says "Our estimates make outdoor air pollution among the most important environmental risk factors for health".

"Many of these deaths are estimated to occur in East Asia and South Asia, where population is high and air pollution is severe," he added.

The pollution has also caused degradation to the Ozone layer. Researchers made 14 models simulating the level of ozone from 1850, when the industrial era began, and in the year 2000. They also prepared six models simulating levels of fine particulate matter.

Human-caused outdoor air pollution may be responsible for over two million deaths worldwide - a large number of them in - each year, US researchers have said.

It also estimated that around 2.1 million deaths are caused each year by increases in, tiny particles suspended in the air that can penetrate deep into the lungs,

Study has found that air pollution is affected by climate change as well. A shift in climate can increase or decrease air pollution. It estimated that a changing climate results in 1,500 deaths due to ozone and 2,200 deaths related to fine particulate matter each year.

For instance, temperature and humidity can change the reaction rates which determine the formation or lifetime of a pollutant, and rainfall can determine the time that pollutants can accumulate.

"Very few studies have attempted to estimate the effects of past climate change on air quality and health. We found that the effects of past climate change are likely to be a very small component of the overall effect of air pollution," West further said.