Does China, in short, truly deserve its reputation as an environmental pariah?

Ramon Guardans, a biologist who studies air pollution and co-chairs a global monitoring plan on toxic chemicals for the United Nations Environment Program, is skeptical.

"The air concentrations of several pollutants in sites in China are certainly comparable to the levels observed in heavily industrialized areas in Europe and North America before the 1970's," he wrote by email. But pound for pound, person for person these pollutant emissions were, and generally still are, much greater than China's.

"So in relation to the population density we see in China," Guardans continues, "the U.S. and Europe did a much dirtier job industrializing."

Archival records indicate that throughout the 1960's daily average PM 10 levels in Los Angeles routinely exceeded 600 micrograms per cubic meter of air (µg/m3). For a city a fraction of the size of Beijing, these levels are very high; far higher, in fact, than Beijing's usual daily averages (which are below100 µg/m3) and well above currently acceptable levels. No PM exposure is safe, but the World Health Organization describes exposure to daily average PM 10 levels below 50 µg/m3 , as well as annual average levels below 20 µg/m3, as "acceptable."

The average annual PM 10 levels in even China's most polluted cities is generally much lower than the peak levels that have grabbed headlines. For example, the average annual PM 10 level in Lanzhou (China's worst offender) was 150 µg/m3 in 2010, while Xining ranked second at 141 µg/m3 . These levels, while dangerous to human health, are pretty comparable to those once witnessed in Eastern Europe's manufacturing centers. The Czech cities of Prague and Mostecko, for example, each achieved average annual PM 10 levels greater than 150 µg/m3 and 130 µg/m3, respectively, in the 1980's.

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Data for historical outdoor air pollution comparisons is hard to come by because many pollutants, including ultra-fine particulate matter (PM 2.5), were not widely monitored until recently. Indeed, large gaps in the monitoring of important pollutants still exist around the world for a variety of reasons, most notably the tremendous cost of monitor installation and maintenance; a single mercury monitor, for example, can cost more than $100,000. Nevertheless, comparisons of historical emissions data can tell us a little bit about how polluted a country's air is or used to be.

China's current emissions of a variety of air pollutants is huge: recent estimates hold that nearly 34 million tons of sulfur dioxide and 11 million tons of nitrogen dioxide were released into China's airways in 2010, mostly through automobile exhaust, power plant emissions, and biomass burning. In addition, China emits these pollutants at some of the highest levels in the world - the highest for sulfur dioxide. But, for a country with four times the population of the United States, these are not much higher than our own emissions used to be. In 1980, near peak U.S. output, the country released 26 million tons of sulfur dioxide into the air (about 30 percent less than China currently does) and 27 million tons of nitrogen dioxide (about 68 percent more than China in 2010).