Residential and transportation energy consumption in urban areas contributes significantly to ambient fine particulate matter with a diameter below 2.5 μm (PM_2.5). In a context of massive rural-urban migration, these emissions have evolved both in terms of magnitude and geographic distribution. In the absence of measurements, fine PM_2.5 concentrations were expected to rise—it turns out that they haven't.

An international collaboration involving LSCE has reconstructed the history of the map of migrations to cities, also representing the emissions of associated pollutants. The scientists have studied the impact of these emissions on the health of urban populations, extrapolating it from 1980 to 2030. They show that, despite the increase in urban emissions, the effect of migration in China has been a paradoxical reduction of PM_2.5 exposure, primarily due to unequal energy use between urban and rural areas. Once settled in the city, the populations significantly reduce the impact of their consumption, though increasing, as they generally switch to cleaner fuel types compared to rural areas.

The national average PM_2.5 exposure in 2010 was reduced by 3.9 μg/m³ (between 3.0 and 5.4 ± µg/m³), i.e. 6.5%, corresponding to an annual reduction of 36,000 premature deaths (between 19,000 and 47,000).

This favorable evolution of mortality is the result of two factors: the migrants swarming into cities live in precarious conditions with high population density, and then have access to better housing conditions. The arrival of new migrants in cities induces an increase in deaths by 142,000 (± 17,000). Thankfully, this is largely outweighed by the beneficial effects of the use of cleaner energy, with decreases in deaths by 148,000 (between 76,000 and 194,000) and improved living conditions for migrants after a settling-in period, so about 29,000 more lives saved (between 15,000 and 39,000).