Delhi's air quality remained in the emergency category despite efforts by the state government and the Centre. Air pollution has emerged as a major challenge and the problem becomes more complex due to multiplicity and complexity of the mix of emission sources, such as industries, automobiles, generator sets, domestic fuel burning, roadside dusts and construction activities. In 2017, the city's PM 2.5 annual average was 143 micrograms per cubic metre, more than three times the national safe standard, while the PM 10 average was 292 micrograms per cubic metre, more than 4.5 times the national standard.