But figures on dangerous smog particles rated as PM2.5 conflict with higher measurements by others, observers say

Environmental authorities in Beijing, China's notoriously smoggy capital, have started releasing more detailed pollution data in response to public pressure.

But one expert said measurements from the first day were low compared with data US officials have been collecting for years.

The initial measurements were low on a day where blue sky was visible. After a week of smog the skies over the city were being cleared by a north wind.

The readings are of particles rated as PM2.5 (PM stands for particulate matter), less than 2.5 micrometers in size or about 1/30th the average width of a human hair. The figures were being posted on Beijing's environmental monitoring centre website. This size of particle can penetrate deep into the lungs, so measuring them is considered a more accurate reflection of air quality than other methods.

It is the first time Beijing has publicly revealed PM2.5 data and follows a social networking clamour by citizens.

The US embassy measures PM2.5 from a device on its rooftop and releases the results. Some residents have tested the air around their neighbourhoods and posted the results online.

The Beijing monitoring centre's website said it was releasing hourly readings of PM2.5 that were taken from one monitoring site about four miles (7km) west of Tiananmen Square. It said the data was for research purposes and the public should only use it as a reference.

The reading at noon Saturday was 0.015 milligrams per cubic metre, which would be classed as "good" for a 24-hour exposure at that level, according to US Environmental Protection Agency standards.

The US embassy reading taken from its site on the eastern edge of downtown Beijing said its noon reading was "moderate." Its readings are posted on Twitter.

Beijing interprets air quality using less stringent standards than the US Embassy, so often when the government says pollution is "light" the embassy terms it "hazardous."

Steven Andrews, an environmental consultant who has studied Beijing's pollution data since 2006, said he was "already a bit suspicious" of Beijing's PM2.5 data. In the 24 hours to noon Saturday seven of the Beijing monitoring centre's hourly figures were "at the very low level" of 0.003 milligrams per cubic metre.

"In all of 2010 and 2011 the US embassy reported values at or below that level only 18 times out of over 15,000 hourly values or about 0.1% of the time," Andrews said. "PM2.5 concentrations vary by area so a direct comparison between sites isn't possible, but the numbers being reported during some hours seem surpisingly low."

The Beijing centre says it has six sites that can test for PM2.5 and 27 that can test for the larger, coarser PM10 particles that are considered less hazardous. The center is expected to buy equipment and build more monitoring sites to test for PM2.5.

Beijing is not expected to include PM2.5 in its daily roundups of the air quality any time soon. Those disclosures, for example "light" or "serious", are based on the amount of PM10, sulfur dioxide and nitrogen dioxide in the air.

"There has been tremendous amounts of attention in the Chinese media whichever newspaper you pick up, whichever radio station you listen to, channel you watch they are all talking about PM2.5 and how levels are so high," Andrews said.

"What has been so powerful is that people are sceptical, and I think rightly sceptical [about the government's descriptions of data]," he said.