Coal is the least efficient of the fossil fuels in terms of the amount of energy gained vs. CO 2 released. Burning it also releases numerous toxic chemicals and particulates, which can exact a cost on a country's population in terms of reduced life expectancy and increased health costs. Figuring out the exact cost of coal use, however, is challenging because of a combination of different pollution controls and the mobility of the population.

Thanks to an unusual combination of policies (some completely unrelated to pollution), China has accidentally provided the opportunity to put an exact number on the human cost of coal use. And that number turns out to be staggering: 5.5 years of reduced life expectancy that, when spread over the half-billion people of northern China, means a loss of 2.5 billion life-years.

The Huai River line

There are two key policies that turned China into a giant natural experiment on the impact of coal. The first is that, until recent years, China has had laws in place that severely limited the mobility of its citizenry. People didn't tend to move around, so they continued to live (and die) near the site of their exposure. That makes lifetime exposures easy to estimate, and it ensures that local health and mortality records could be directly connected to these exposures.

Second, starting in 1950, the Chinese government divided the country along the Huai River, which roughly traces the line where winter temperatures are, on average, freezing—and north of that line, everyone was eligible for free, coal-powered heating. The line cuts across a variety of provinces and political divisions, so there's little else about China that's likely to be divided in the same way.

China uses coal for a variety of purposes beyond heating—with the result that, as the authors of a new study note, current particulate levels in China are five times what they were in the US back in the 1960s, prior to the passage of the Clean Air Act. (These levels are, incidentally, twice the limits allowed under Chinese law.) Despite the existing overall high levels of pollution across the country, the addition of this much heating-related coal burning had a significant impact on the air quality of northern China over the last half century. By 2000, as one crossed the Huai River line, the total particulates in each cubic meter of air jumped by nearly 200 micrograms.

Exposure to particulates has a variety of negative consequences, primarily on the lung and cardiovascular systems, so the authors classified all deaths in 90 Chinese cities between 1990-2000 and collected additional demographic information that covered factors that are associated with life expectancies. Then they ran it all through a statistical model that took into account each city's distance from the Huai River. Their model suggests that life expectancies north of the river line are 5.5 years lower than they are to the south (although their 95 percent confidence interval is rather large). Their estimate suggests that every 100 micrograms of particulates in a cubic meter of air drops life expectancy by about three years.

Based on this drop in life expectancy, the authors calculate a total loss of life for the half-billion citizens that live north of the line as 2.5 billion years of life lost prematurely.

The authors do note that the free coal provided by the government would likely alter behavior in a variety of ways, some of which could influence health. People might spend more of their year indoors, get less exercise, have more disposable income, etc. Still, the magnitude of the effect is rather large and is consistent with what we know about the health impacts of particulates.

China is just beginning to grapple with its heavy use of coal and its lack of pollution controls. But the installed base of coal-burning hardware, from large plants to individual homes, remains enormous, which means that there will be a long legacy of health problems inherited from earlier policy decisions.

PNAS, 2013. DOI: /10.1073/pnas.1300018110 (About DOIs).