More than 5.5 million people worldwide are dying prematurely every year as a result of air pollution, according to new research, BBC News reports.

Most of these deaths are occurring in the rapidly developing economies of China and India.

The main culprit is the emission of small particles from power plants, factories, vehicle exhausts and from the burning of coal and wood.

The data was compiled as part of the Global Burden of Disease Project.

Scientists involved in the initiative say the statistics illustrate how far, and how fast, some nations must travel to improve the air their citizens breathe.

"In Beijing or Delhi on a bad air pollution day, the number of fine particles (known as PM2.5) can be higher than 300 micrograms per cubic metre," explained Dan Greenbaum from the Health Effects Institute, in Boston, US.

"The number should be about 25 or 35 micrograms."

Breathing in tiny liquid or solid particles can increase the risk of heart disease, stroke, respiratory complaints and even cancer. And while developed nations have made great strides in addressing this problem these past few decades, the number of citizens dying as a result of poor air quality in developing countries is still climbing.

According to the study, air pollution causes more deaths than other risk factors like malnutrition, obesity, alcohol and drug abuse, and unsafe sex. The Global Burden of Disease Project puts it as the fourth greatest risk behind high blood pressure, dietary risks and smoking.