A recent air quality monitoring survey - released on Monday by Greenpeace - have found that the deadly PM2.5 levels in the capital are 10 times higher than the safety limit prescribed by the World Health Organisation (WHO).

Washington: A recent air quality monitoring survey — released on Monday by Greenpeace — has found that the deadly PM2.5 levels in the capital are 10 times higher than the safety limit prescribed by the World Health Organisation (WHO), and four times higher than even the Indian safety limit.

Delhi’s air is the most toxic in the world due to high concentrations of PM2.5 — particulate matter less than 2.5 micrometres in diameter — that is believed to pose the greatest health risk because it penetrates deeply into lungs.

The PM2.5 limit prescribed by WHO is 10 microgrammes per cubic metre, and the Indian limit is 40 microgrammes per cubic metre. PM2.5 are miniscule particles in the air that reduce visibility, cause the air to appear hazy, and affect respiratory tracts, reports the Daily Mail.

Air pollution is killing 3.3 million people a year worldwide, according to a new study that includes this surprise: Farming plays a large role in smog and soot deaths in industrial nations.

Scientists in Germany, Cyprus, and Saudi Arabia and at Harvard University calculated the most detailed estimates yet of the toll of air pollution, looking at what caused it. The study also projects that if trends don't change, the yearly death toll will double to about 6.6 million a year by 2050.

The study, published on Wednesday in the journal Nature, used health statistics and computer models. About three-quarters of the deaths are from strokes and heart attacks, said lead author Jos Lelieveld, at the Max Planck Institute for Chemistry in Germany.

The findings are similar to other, less-detailed pollution death estimates, outside experts said.

"About six percent of all global deaths each occur prematurely due to exposure to ambient air pollution. This number is higher than most experts would have expected, say, 10 years ago," said Jason West, a University of North Carolina environmental sciences professor who wasn't part of the study but praised it.

Air pollution kills more than HIV and malaria combined, Lelieveld said.

With nearly 1.4 million deaths a year, China has the most air pollution fatalities, followed by India with 645,000 and Pakistan with 110,000.

The United States, with 54,905 deaths in 2010 from soot and smog, ranks seventh highest for air pollution deaths. What's unusual is that the study says that agriculture caused 16,221 of those deaths, second only to 16,929 deaths blamed on power plants.

In the northeastern United States, all of Europe, Russia, Japan, and South Korea, agriculture is the No. 1 cause of the soot and smog deaths, according to the study. Worldwide, agriculture is the No 2 cause with 664,100 deaths, behind the more than one million deaths from in-home heating and cooking done with wood and other biofuels in the developing world.

The problem with farms is ammonia from fertilizer and animal waste, Lelieveld said. That ammonia then combines with sulfates from coal-fired power plants and nitrates from car exhaust to form the soot particles that are the big air-pollution killers, he said.

In London, for example, the pollution from traffic takes time to be converted into soot, and then it is mixed with ammonia and transported downwind to the next city, he said.

"We were very surprised, but in the end it makes sense," Lelieveld said. He said the scientists had assumed that traffic and power plants would be the biggest cause of deadly soot and smog.

Agricultural emissions are becoming increasingly important but are not regulated, said Allen Robinson, an engineering professor at Carnegie Mellon University who wasn't part of the study but praised it.

Ammonia air pollution from farms can be reduced "at relatively low costs," Robinson said.

In the central United States, the main cause of deaths from soot and smog is power plants; in much of the West, it's traffic emissions.

Jason West and other outside scientists did dispute the study's projections that deaths would double by 2050. West and others said it's likely that some places, such as China, will dramatically cut their air pollution by 2050.

With inputs from AP