Beijing has recorded an API of 500 at least once before, on November 1, 2003. The "inhalable particles" are more commonly known as RSPs, or respirable suspended particulates.

Interestingly, the API in Beijing is only given for the worst pollutant on a given day. Shanghai's Environmental Bureau (http://www.semc.com.cn/home/index.asp) gives numbers for all three tracked pollutants. RSPs, NO2, and SO2. I'm not sure why this is, but it may be that Shanghai has a more complex pollutant mix than Beijing, where RSPs are far and away the most common pollutant.

November 2003 Data from the Chaoyang District Environmental Protection Department:

http://hb.bjchy.gov.cn/hjzl/hjzl_kqzlgg.jsp?offset=180&doSearch=true&reportDateFrom=2003-11-01&reportDateEnd=2004-05-01

A correction: The Chinese API does not cover RSPs, but TSPs (total suspended particulates).

The yesterday's air pollution level 5 or 500 microgram of particulate matter (PM10) per cubic meter was not an all time high. Level 5 was reached for the 7th time this year. Thereof 3 times at least 500 mgr/m3. They stop counting above 500. You can check the history of Beijing's air by entering the Beijing and 2006-01-01 to 2006-12-13 at this link: http://www.zhb.gov.cn/english/air-list.php3

The Insider’s guide to Beijing once published a statement of a Doctor from the Beijing United Family Hospital saying that you should avoid going outside when 150 mgr/m3 are reached.
Others see it even more critical. E.g., the Australian Department of Environment and the German Ministry of Environment aim for a daily maximum concentration of 50 micrograms of PM10 per cubic metre (level 1). In Germany every city is allowed to exceed this goal only 35 days per year while in Australia apparently only five days. So in Beijing do not look only at the terrible ‘500’-days but count all the days in 2006 when the ‘50’-goal was exceeded: Year to date, there were 347 polluted days in Beijing. Only 25 days where ‘healthy’ days.

Sorry, I have to correct my previous comment: There were 322 polluted day vs. only 25 good days. Together 347 days ytd.

Does anybody know what an individual can do to attempt to protect their health if they live in Beijing? I can't tell everybody to stop driving their cars!

You should try smoking Zhongnanhai 0.8 cigarettes. The herbal blend protects your lungs against pollution, SARS and other dangers.

@Chip. Cyclists’ fume masks are designed to filter out stuff like sulphur and lead oxide. Never seen them sold in China though, you’d probably have to get one online. Other than that, I’d get friendly with a non-smoker on death row, if you know what I mean...

The only place I've seen cyclist masks in the region are in Hong Kong (the Flying Ball bike shop in Cheung Sha Wan).

An API of 500 does not indicate a particulate concentration of 500 micrograms per cubic meter. In Mainland China, An API of 300 indicates a concentration of 625 ug/m^3. I don't have the specific numbers for an API of 500, but it would be fair to assume it would be well over 700 ug/m^3.

For comparison, the Canadian government sets the 24-hour Maximum Tolerable Concentration of TSPs at 400ug/m^3 (http://www.environnement-canada.ca/soer-ree/English/Indicators/Issues/Urb_Air/Tech_Sup/uasup5_e.cfm)