A new study links heavy air pollution from coal burning to shorter lives in northern China. Researchers estimate that the half-billion people alive there in the 1990s will live an average of 5 years less than their southern counterparts because they breathed dirtier air.

China itself made the comparison possible: for decades, a now-discontinued government policy provided free coal for heating, but only in the colder north. Researchers found significant differences in both particle pollution of the air and life expectancy in the two regions, and said the results could be used to extrapolate the effects of such pollution on lifespans elsewhere in the world.

The study by researchers from China, Israel and the United States was published Tuesday in the Proceedings of the National Academy of Sciences.

While previous studies have found that pollution affects human health, “the deeper and ultimately more important question is the impact on life expectancy,” said one of the authors, Michael Greenstone, a professor of environmental economics at Massachusetts Institute of Technology.

“This study provides a unique setting to answer the life expectancy question because the (heating) policy dramatically alters pollution concentrations for people who appear to be of otherwise identical health,” Greenstone said in an email. “Further, due to the low rates of migration in China in this period, we can know people’s exposure over long time periods,” he said.

The policy gave free coal for fuel boilers to heat homes and offices to cities north of the Huai River, which divides China into north and south. It was in effect for much of the 1950-1980 period of central planning, and, though discontinued after 1980, it has left a legacy in the north of heavy coal burning, which releases particulate pollutants into the air that can harm human health. Researchers found no other government policies that treated China’s north differently from the south.

The researchers collected data for 90 cities, from 1981 to 2000, on the annual daily average concentration of total suspended particulates. In China, those are considered to be particles that are 100 micrometers or less in diameter, emitted from sources including power stations, construction sites and vehicles.

The researchers estimated the impact on life expectancies using mortality data from 1991-2000. They found that in the north, the concentration of particulates was 184 micrograms per cubic meter ― or 55 percent ― higher than in the south, and life expectancies were 5.5 years lower on average across all age ranges.

The researchers said the difference in life expectancies was almost entirely due to an increased incidence of deaths classified as cardiorespiratory ― those from causes that have previously been linked to air quality, including heart disease, stroke, lung cancer and respiratory illnesses.

Total suspended particulates include fine particulate matter called PM2.5 ― particles with diameters of no more than 2.5 micrometers. PM2.5 is of especially great health concern because it can penetrate deep into the lungs, but the researchers lacked the data to analyze those tiny particles separately.

The authors said their research can be used to estimate the effect of total suspended particulates on other countries and time periods. Their analysis suggests that every additional 100 micrograms of particulate matter per cubic meter in the atmosphere lowers life expectancy at birth by about three years.

The study also noted that there was a large difference in particulate matter between the north and south, but not in other forms of air pollution such as sulfur dioxide and nitrous oxide.

Francesca Dominici, a professor of biostatistics at Harvard School of Public Health who has researched the health effects of fine particulate matter in the U.S., said the study was “fascinating.”

China’s different treatment of north and south allowed researchers to get pollution data that would be impossible in a scientific setting.

Dominici said the quasi-experimental approach was a good approximation of a randomized experiment, “especially in this situation where a randomized experiment is not possible.”

She said she wasn’t surprised by the findings, given China’s high levels of pollution.

“In the U.S. I think it’s pretty much been accepted that even small changes in PM2.5, much, much, much smaller than what they are observing in China, are affecting life expectancy,” said Dominici, who was not involved in the study.

一项新的研究表明由于烧煤引发的严重空气污染将缩短中国北方居民的寿命。研究人员估计，因为呼吸更污浊的空气，上世纪90年代居住于北方的5亿居民要比南部的居民平均少活5年。

中国可以用自身来做一个比较：几十年来，国家政策只允许给寒冷的北方地区提供免费的煤燃料供暖，现在这项政策已经被废止。研究人员发现，在南北两个地区，空气中颗粒物污染和人们的预期寿命都存在显著差异，而且研究结果可以用来推断在世界其他地方这种污染对寿命的影响。

这项研究由来自中国，以色列和美国的研究人员共同完成，并于周二发表在《美国国家科学院院刊》（Proceedings of the National Academy of Sciences）上。

虽然之前的研究发现，污染会影响人体健康，但是“更深层和根本上更重要的问题是对预期寿命的影响。”该项研究的作者之一，麻省理工学院环境经济学教授迈克尔·格林斯通（Michael Greenstone）说。

“这项研究为回答有关寿命的问题提供了一个独特的情境，因为（供暖）政策极大地改变了看似健康状况相同的人生存环境的污染浓度，”格林斯通在一封电子邮件中说道，“此外，由于中国在此期间的低迁移率，我们就可以知道，人们长期以来都是暴露在一定污染浓度的环境中。”

因淮河为中国区分南北的界限，故之前的政策为中国淮河以北城市的家庭和办公室提供免费的煤燃料锅炉供暖。该政策在中央计划的1950年-1980年期间开始实行，虽然在1980年后被废止，但大量的煤燃烧却给北方的空气留下了危害人体健康的颗粒污染物。研究人员并没有发现其他不同对待中国南北部的政策。

研究人员收集了90个城市多年总悬浮微粒日平均浓度的数据，时间跨度从1981年到2000年。在中国，悬浮微粒被认为是指由包括发电厂、建筑工地和车辆发出的，直径小于或等于100微米的粒子。

研究人员通过1991年-2000年的死亡率数据来估计对预期寿命的影响。他们发现，在北方，微粒的浓度为每立方米184微克——比南部数据高55%，而且在所有年龄段的平均预期寿命比南方短5.5年。

研究人员说，预期寿命的差异几乎完全是由于心肺死亡类疾病的发生率增加——这些通常被人们认为是空气质量导致的疾病包括心脏病、中风、肺癌和呼吸系统疾病。

总悬浮粒子包括被称为PM2.5的细颗粒物，即直径不超过2.5微米的颗粒。因为它可以渗透到肺部深处，PM2.5引起人们对健康的极大担忧，但研究人员缺乏数据来单独分析这些微小颗粒的实际影响。

该项研究的作者还说，他们的研究可以用来估计总悬浮微粒对其他国家和时段的影响。他们的分析表明，大气中每立方米每增加100微克的颗粒物，则降低出生时3年左右的预期寿命。

该研究还指出，南北之间在颗粒物上有很大的差异，但在其他形式的空气污染，如二氧化硫和氮氧化物上并没有这种差异。

弗朗西斯卡·多米尼西（Francesca　Dominici）认为这项研究“有着巨大吸引力”。身为哈佛大学公共卫生学院的生物统计学教授，多米尼西在美国研究细颗粒物对健康的影响。

中国南北方受到的不同对待使得研究人员能够获取到不可能在任何科学场景中得到的污染数据。

多米尼西说准实验法是近似一个随机实验的很好方法，“尤其是在这种不可能做随机实验的情况下。”

她说，考虑到中国的高污染水平，她对调查结果并不感到惊讶。

“在美国，我想，哪怕PM2.5的数值发生很小的变化，远远小于他们在中国观测到的数据，大部分人也会认为这影响到人们的预期寿命。”并没有参与这项研究的多米尼西说道。