In a village in Uttar Pradesh's Reusa block this January, I met Pushpa Devi, a young mother. As we spoke, she fed her tiny daughter milk from a spoon—the baby was too weak to breastfeed. Even at one month old, she looked to be under 2.5 kilograms, the limit below which newborns are considered to be low birthweight. Several factors contributed to the fragility of Pushpa Devi's daughter, and the air she was breathing was certainly one of them.

Every winter morning and evening, in thousands of villages across north India, people without adequate housing or warm clothing huddle around small fires to try and keep the chill at bay. I saw countless such fires in the week I spent in Reusa, as well as farmers burning sugarcane thrash to clear their fields after the harvest, and small factories burning sugarcane pulp to make gur. Closer to Lucknow, 90 kilometres to the south, coal-fired power plants and factories such as brick kilns emitted constant streams of smoke. These and other sources of air pollution leave rural north India under a thick layer of smog throughout winter.

According to India's National Ambient Air Quality Standards, the permissible outdoor concentration of particles smaller than 2.5 micrometres in diameter, averaged over 24 hours, is 60 micrograms per cubic metre. These are particles so small they can enter deep into the lungs, and even into the bloodstream. Levels of outdoor air pollution in Reusa regularly hovered at two to four times this limit during my visit, and rose to around ten times the limit one night. Air this polluted can kill babies and old people, impede healthy development in children, and give everyone else respiratory infections.