The cities of China have wowed the world with their astonishingly rapid industrialization, but they're hardly famous for clean air. The hundreds of millions that have undertaken one of the great historical migrations came seeking jobs, not a pristine environment. Yet new research reveals that for millions of Chinese rural residents, a shift to the city is a breath of fresh air, actually lowering their pollutant exposure.
The air in China's cities is hardly clean. The Chinese government has turned to solar and wind more out of concern for the astonishing rates of urban pollution than fears of global warming. It has sometimes resorted to more drastic measures, but pollutants are still estimated to kill a million people a year. Nevertheless, exposure to small particulates is so much lower in some cities than in the countryside that a paper in Science Advances estimates 450,000 premature deaths were avoided between 1980 and 2010 as a result of the great urban migration.
The explanation for this apparently nonsensical claim lies in the fuel used to heat most rural Chinese houses. The particulates released in burning wood, crop residue, or low-quality coal are so thick indoors that they outweigh the benefits of fresh air outside.
Pollution comes in many forms, but Professor Shu Tao and colleagues at Peking University used particulates smaller than 2.5 micrometers in diameter (PM 2.5) as a proxy measure. Since these fine particles do much of the damage from air pollution themselves and are a good indication of other problems, this is a widely adopted measure.
Using questionnaires of the energy used to heat houses, and already available measures of the PM 2.5 concentrations of indoor and outdoor environments, Tao and co-authors mapped the particulate exposure across China. They concluded that the average national exposure of 58.6 micrograms per cubic meter was 3.9 μg/m3 lower than it would have been had people stayed in the rural areas where they were born. This gap should more than double by 2030.
Nevertheless, it matters which cities people migrate to. “Megacities such as Beijing and Shanghai experienced increases in PM 2.5 exposure associated with migration because these cities received massive immigration, which has driven a large increase in local emissions,” the paper notes. Consequently, the new arrivals have worsened existing residents' exposure. The authors factor this in, but find it is insufficient to offset the benefits of getting away from rural houses' smoky rooms.
The net effect was a saving of 36,000 lives in 2010, and a cumulative total of almost half a million over the whole period. Along with vindicating urbanization, the paper provides an indication of just how staggering the benefits will be if China can eliminate coal and biomass from its fuel mix entirely.