It rolled into town earlier this week and has hung around like a bad smell ever since. The worst smog to engulf the Indian capital this season has stung the eyes of rickshaw drivers, clouded a champagne-laden celebration on the lawn of the Australian High Commission and, according to local media, caused a multi-car pile up on a Delhi highway.

Doctors have likened the impact of being under Delhi’s smog to inhaling 50 cigarettes a day, and the Indian Medical Association declared a “state of medical emergency,” according to the Guardian. They warned of “disastrous health consequences” should a planned half-marathon go ahead in the city. Delhi’s chief minister Arvind Kejriwal described the city as a “gas chamber,” the Hindustan Times reports.

Here’s what you need to know about the Delhi smog.

How bad is it?

Nearly off-the-charts-bad. The Air Quality Index (AQI), a gauge for measuring the level of pollutants air contains, reached an alarming level of 833 by Wednesday afternoon, according to the U.S. Embassy in Delhi. The mission warned of “hazardous” levels of PM2.5 — ultra fine particles that can carry carcinogens such as arsenic and mercury and are small enough to permeate most of the body’s defensive filters.

The recorded levels are more than 90 times the average of New York and absurdly far beyond the safe limit set by the World Health Organization — ranging from 51-100. Dr. Arvind Kumar, a physician cited by the Guardian, said such pollution has “terrible effects” on every part of a person’s body. “It can precipitate an acute asthma attack and in the long run it will increase their risk of lung cancer,” Kumar said.

Pollution contributes to 2.5 million deaths in India each year, according to a report published by peer-reviewed journal the Lancet in October.

Why is it happening?

Smog is a perennial problem in India, particularly in the capital region. In fact, this time last year, New Delhi was engulfed in pollution that closed schools and made it difficult to tell the color of some traffic lights. Though the air quality has been deteriorating in the capital for years, the 2016 haze was the most severe in decades. This year’s could rival it.

Read more: India’s Capital Chokes Under the Worst Smog in Decades

There are a variety of reasons for that. Longterm causes such as heavy industry and car exhaust fumes, road dust, and agricultural field torching in neighboring states — which sends thick plumes of smoke over Delhi — have been exacerbated by slow winds and cold temperatures this year, according to the BBC.

But air pollution is not isolated to Delhi. Half of the world’s worst 20 cities for air quality are in India, and the entire north Indian plane, including parts of Pakistan, are afflicted by smog.

What are authorities doing about it?

In October, India’s government enacted a plan to try to limit air pollution. It included restricting traffic and shutting down a major power station, the BBC reports. The government also imposed a ban on the sale and distribution of fireworks in Delhi, which are often used to celebrate the Hindu festival of Diwali.

However, these actions appear to have been largely ineffective. “Every year this happens” Arvind Kejrival wrote on Twitter. “We have to find a [solution] to crop burning in adjoining states,” he wrote.

On Wednesday, authorities warned residents to stay inside, and announced that primary schools in the capital would remain closed until Sunday.

Delhi’s Central Industry Security Force (CISF) ordered the issuance of more than 9,000 face masks to workers dispatched to the Indira Gandhi International Airport, the Delhi Metro, and other government installations, the Times of India reports. But for others in Delhi — particularly workers in the informal sector — there was no option but to suck it up.

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Write to Joseph Hincks at joseph.hincks@time.com.