July 31 (Reuters) - Beijing has promised clean air for the Olympic Games it hosts in August, but experts say the way the Chinese capital measures its pollution means that even "blue sky days" may not be entirely safe for athletes.

Below find details of major problems with its air quality standards, and how its safety levels match up to international guidelines and those of other major polluters.

THE AIR POLLUTION INDEX

This is Beijing's general guideline to whether the city's air is safe to breathe, and is used to measure if the capital has met its "blue skies" target for the number of days with safe air. But scientists say it has several key flaws.

-- It omits at least two dangerous pollutants, ozone and fine particulate matter.

-- The overall value is calculated by taking an average of readings across the city, meaning some areas could have dangerous levels even on "blue sky" days.

-- The index does not take into account the cumulative effect of different pollutants, measuring only whether levels of each is within its own guidelines.

AIR STANDARDS

China's air quality standards are not as tight as most U.N. guidelines, but are stricter than many of the U.S. rules.

They do not however cover particulate matter 2.5, fine particles that scientists say damage the lungs and may also be able to seep into the bloodstream.

The country has separate limits for different types of area. Class I is protected areas, and Class II is for residential areas and Class III is industrial areas, where the government allows dirtier air because people spend less time there.

For a full comparison of standards see the table below. Data from the report Urban Air Pollution in Asian Cities, published by Earthscan. All quantities in micrograms per cubic metre.

Chinese US EPA EU WHO

Class II Sulphur Dioxide one year 60 78 20 ~ 24 hours 150 365 125 20 one hour 500 ~ 350 ~ Nitrogen Dioxide one year 80 100 40 40 24 hours 120 ~ ~ ~ one hour 240 ~ 200 200 Ozone eight hours ~ ~ 120 100 one hour 160 235 ~ ~ Particulate matter 10 one year 100 50 40 20 24 hours 150 150 50 50 Carbon monoxide 24 hours 4,000 ~ ~ ~ eight hours ~ 10,305 10,000 10,000 one hour 10,000 40,000 ~ 30,000

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