Air pollution levels are unsafe across the country, all-year round. While pollution levels spike to dangerously high levels during the winter in north India, those in several parts of the country are poor or worse for large parts of the year. High pollution levels are not restricted to cities; several industrial areas along with rural areas across the Indo-Gangetic plain are also polluted. There are several kinds of pollutants in the air: particulate matter, carbon monoxide, ozone, oxides of nitrogen and sulphur. Fine particulates (PM2.5) form a useful proxy indicator for air pollution. The population-weighted annual average concentration of PM2.5 across the country, estimated using satellite data, was 91 microgram/m3 in 2017 – more than twice the national standards.