MANILA, Philippines — The World Health Organization on Thursday called on governments to improve air quality in their cities, saying air pollution prematurely kills two million people a year, with more than half the deaths in developing countries.

Reducing the kind of pollution known as PM10 -- or particulate matter with particles of smaller than 10 micrometers -- could save as many as 300,000 lives every year, according to a statement issued by the WHO's regional office in Manila.

PM10 pollution is caused primarily by the burning of fossil and other types of fuel. Its particles are too small to be filtered in the nose and throat, and they settle in the lungs where they cause health problems.

The WHO said that in many cities, the average annual levels of PM10 exceed 70 micrograms per cubic meter (yard) while recent new WHO guidelines called for lowering that level to less than 20 micrograms to prevent ill health.

That could cut the deaths from air pollution by about 15 percent, said Maria Neira, the WHO director for public health and the environment.

It could also cut the global burden of disease from respiratory infections, heart disease and lung cancer, she said.

Particulate matter pollution is considered the biggest health risk. But the WHO Air Quality Guidelines also recommended lowering the daily allowed limits for ozone from 120 down -- under previous WHO guidelines -- to 100 micrograms per cubic meter (yard).

That represents a challenge especially for developing countries with numerous sunny days when ozone concentrations reach the highest levels, causing respiratory problems and asthma attacks, WHO said.

The guidelines also cut the sulfur dioxide limits from 125 to 20 micrograms per cubic meter (yard).

Many countries around the world do not have regulations for air pollution, which makes control virtually impossible.

WHO said the new guidelines, which were established after worldwide consultation with more than 80 leading scientists, provide the basis for all countries to build their own air quality standards and policies supporting health.