India, Nov. 7 -- The colours of firecrackers in Delhi on Diwali depend on metal salts that burn into fine particles, whether these are the 'green ones' or the conventional ones sold illegally.

One of those salts, barium, has been banned in fireworks since 2018. But, on Diwali night in 2019, a year after the ban, a team from IIT Delhi and other institutes measured barium in Delhi's air at up to 190 times its level of the week before. Strontium, which burns red, rose by up to 145 times.

The Supreme Court has allowed certified green firecrackers in Delhi-NCR on November 7 and 8, between 6am and 7am and between 8pm and 10pm. A day before the first window opened, HT found nearly 30 street vendors selling fireworks without licences in Old Del...