Outdoor pollution caused by humans kills over two million people worldwide every year, a new study has found.

US scientists used climate models to simulate the concentrations of ozone and fine particulate matter in the environment.

Fine particulate matter is a general term used to describe any tiny solid or liquid particles that are suspended in the air. They can penetrate deep into the lungs causing respiratory illnesses and cancer.

Particulate air pollution includes the pollution emitted by motor vehicles (in exhaust fumes) and some power plants.

Ozone is a gas which occurs naturally in the upper atmosphere. It protects the earth from the sun's dangerous ultraviolet (UV) rays. However, in the lower atmosphere, near the earth's surface, ozone is created by chemical reactions between air pollutants from vehicle exhaust, gasoline vapours and other emissions. At ground level, high concentrations of ozone are toxic to people and plants.

The study found that an estimated 2.1 million people die every year as a result of fine particulate matter caused by humans. A further 470,000 people die from increases in ozone caused by humans.

"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," explained the study's co-author, Jason West, of the University of North Carolina.

The study also found that a changing climate, which had been thought to worsen the effects of air pollution, actually has only a small effect. In fact, the number of air pollution deaths that can be attributed to climate change is relatively small - around 1,500 ozone-related deaths and 2,200 particulate matter-related deaths per year.

"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," Mr West explained.

Details of these findings are published in the journal, Environmental Research Letters.