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 Beijing and 2006-01-01 to 2006-12-13 at this link: Air Quality Daily Report for 84 Major Cities In China
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. 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 only look at the terrible ‘500’-days but count all the days in 2006 when the ‘50’-goal was exceeded: Year to date, there were 322 polluted days in Beijing vs. only 25 ‘healthy’ days. :-(
See also Every breath you take in Beijing by DANWEI.