In July 2009, China's Foreign Ministry made a demand of the American embassy: Stop taking measurements of air pollution in Beijing available to ordinary Chinese since they conflicted with official data and could lead to "confusion" among the public and undesirable "social consequences."

The Americans refused to comply. For the past 2½ years, they have continued to announce hourly their findings of the air quality over the embassy, which is in a not-that-polluted area of the capital, and to put them on an embassy-managed Twitter site.

The trouble is that while the Americans were measuring — and disclosing — the concentration of "fine" particulate of less than 2.5 micrometers in diameter (PM 2.5), which pose the greatest risk to human health, the Beijing Environmental Protection Bureau's air pollution index reflected only the concentration of the much larger PM 10 particulates.