A thick layer of smog shrouded New Delhi on Friday, after revelers celebrating Diwali, the Hindu festival of lights, flouted a ban on firecrackers.

The air quality index stood at over 345 shortly after dawn, according to Swiss firm IQ Air, driving pollution in the sprawling Indian capital to "hazardous." It also put New Delhi at the top of a real-time list as the world's most-polluted city, above Lahore in neighboring Pakistan.

In recent years, authorities in New Delhi have outlawed the use and sale of traditional firecrackers during Diwali and have urged people to instead celebrate the light festival with more environmentally friendly options.

Why isn't the firecracker ban working?

The ban has proven difficult to enforce and is often flouted.

Some Hindu groups have criticized the measure, saying it interferes with their ability to observe the light festival which celebrates the Hindu goddess Lakshmi.

Authorities, meanwhile, say the ban aims to save lives.

In many areas on Friday, levels of deadly particulate matter — that can lodge deep in the lungs and cause a number of health problems  — surged to well over the daily maximum considered safe by the World Health Organization.

According to the AFP news agency, police in New Delhi seized nearly two tons of fireworks in the leadup to this year's Diwali, but revelers could still travel to neighboring states to buy them.

Other sources of pollution

New Delhi, home to 33 million people, is regularly ranked as having one of the most polluted urban areas in the world.

Fumes from its traffic-clogged streets and factories only add to the problem. 

Diwali also often coincides with the burning of crop residues on farms in neighboring states. This can aggravate air quality at the beginning of winter, with cooler temperatures trapping smoke.

nm/kb (AFP, AP, Reuters)