From the NYT:

The municipal government reported levels as high as 500 on Saturday evening from some monitoring stations. The Chinese system does not report numbers beyond 500. Nevertheless, readings in central Beijing throughout the day were at the extreme end of what is considered hazardous according to the United States Environmental Protection Agency standards. (By comparison, the air quality index in New York City, using the same standard, was 19 at 6 a.m. on Saturday.)

Pollution levels in Beijing had been creeping up for days, and readings were regularly surging above 300 by midweek. The interior of the gleaming Terminal 3 of the Beijing Capital International Airport was filled with a thick haze on Thursday. The next day, people working in office towers in downtown Beijing found it impossible to make out skyscrapers just a few blocks away. Some city residents scoured stores in search of masks and air filters.

The magnitude of the pollution is somewhat hard to comprehend. We often see levels as high as these peaks in unventilated, indoor spaces where people cook using solid fuels (like wood, grass, or dung). To imagine concentrations like that at ambient levels is terrifying from a public health perspective. Health data from Beijing in the coming weeks should back this up — if it is made accessible. Indicates a clear need for some sort of action to preserve population level health, especially amongst the most vulnerable.