New Delhi air pollution hits 30 times WHO limit

Toxic air pollution in New Delhi rose on Wednesday to more than 30 times the World Health Organization's recommended daily limit, air monitor data showed.

Monitoring firm IQAir recorded PM2.5 levels above 461 micrograms per cubic metre in parts of the Indian capital. PM2.5 particles are small enough to enter the bloodstream.

The deterioration came three days after authorities tightened measures, including restrictions on traffic and construction, to curb smog in the wider metropolis, which is home to more than 30 million people.

The pollution spike also came ahead of Sunday's Diwali festival. India's Supreme Court this month allowed limited use of so-called green firecrackers, which are designed to reduce emissions, during the festival.

India's environment ministry says vehicles account for 23 percent of Delhi's winter pollution, biomass burning including farm stubble fires for 20 percent, industry including coal-fired power plants for about nine percent, and secondary particles for 27 percent.

Jacob Koshy, science editor of The Hindu newspaper and author of a new book on pollution, said ammonia released by fertilisers used around the capital helps form secondary particulate matter when it reacts with other pollutants.