Soaring air pollution in India’s capital, New Delhi, has reached “hazardous” levels this week, causing state officials to limit movement and revive plans to stimulate artificial rain to clear toxic smog.

Last week, authorities in the city ordered schools to shut down. Businesses and offices were also ordered to function at half-capacity from Monday.

Monitors earlier in the week recorded a “severe plus” reading (493) of the 24-hour air quality index (AQI), which is more than 30 times the World Health Organization’s (WHO’s) limit. This indicated extremely high levels of unsafe particulate matter called PM2.5 in the air, the highest reading this year.

The WHO guidelines state 24-hour average concentrations of PM2.5 should not exceed 15 microgrammes per cubic metre more than three or four days per year.

Delhi’s toxic smog season usually emerges every winter and contributes to the capital region of 33 million people being consistently ranked among the most polluted areas in the world.

Speaking to the news media on Wednesday, Delhi Environment Minister Gopal Rai said the city’s government was ready to go ahead with an emergency plan to stimulate artificial rain to clear the smog but blamed the federal government for delaying approvals.

Here’s what you need to know about Delhi’s smog challenge and what artificial rainfall might do:

What’s the air pollution level, and what’s AQI?

Air pollution levels have risen rapidly since the end of October and have remained hazardous every day since November 12, according to monitoring start-up IQAir.

Officials on Tuesday said the AQI had slightly dipped to 460. The AQI measurement is a tool that governments around the world use to communicate their air quality status.

Experts said the scores vary because of a difference in the scale countries adopt to convert pollutant concentrations into AQI, so the same quantity of a specific pollutant may be translated as different AQI scores in different countries. On the AQI scale in the United States, for example, Delhi’s pollution reached 1,100, which is in the “hazardous” category.

In India, AQI is rated poor at more than 200, very poor at more than 300, and severe or hazardous at more than 400. The recommended limit is 0 to 100.

What have authorities done?

Officials shut schools and asked students to go online. Offices are also meant to function at half-capacity.

In addition, authorities banned nonessential construction work and restricted the movement of nonessential trucks. The stay-at-home orders are expected to minimise vehicle traffic and construction emissions, which contribute significantly to pollution.

Water tankers have been patrolling parts of the city to sprinkle water in a bid to reduce the smog. Officials have also deployed mechanised sweeping to help settle dust.

Several other Indian cities on Tuesday recorded AQI levels from the poor to hazardous range, according to IQAir. They included Gurugram, Patna, Jaipur, Lucknow, Hyderabad, Ghaziabad, Kolkata, Mumbai and Bengaluru.

Why does smog engulf the Delhi region every winter?

General air pollution is linked to India’s burning of coal to generate more than half of its electricity. In Delhi, that combines with emissions from millions of cars and fumes from the construction industry, which has no pollution controls.

The crisis deepens from October to January when colder temperatures coincide with widespread crop stubble burning. Smoke travels from the neighbouring states of Uttar Pradesh, Punjab and Haryana as thousands of farmers burn off agricultural waste after the harvest season. The heavier winter air, however, traps pollutants close to the ground, worsening the smog.

Some also blame the widespread use of firecrackers during Diwali, the Hindu festival of lights, at the end of October. Authorities banned the use of traditional firecrackers in 2017 and only allowed people to use environmentally friendly lights, but the rule is not properly enforced.

PM2.5, the tiny particulate matter present in polluted air, lodges deep in the lungs and causes health problems, especially for children, the elderly, and people with respiratory or heart conditions.

Every year, hundreds of thousands of people die from respiratory conditions in India. An IQAir world air quality report in 2021 found that no city in India met the updated WHO safety standards of 5 microgrammes of PM2.5 per cubic metre of air. Nearly half of India’s states surpassed this limit by more than 10 times.

What’s stopping authorities from addressing the crisis? Will artificial rain work?

In Delhi, a political blame game between the city’s government and central authorities has often emerged.

The city government is currently controlled by the Aam Aadmi Party (AAP), a largely Delhi-based group. It’s in opposition to the ruling Bharatiya Janata Party (BJP), which dominates the federal government.

Delhi’s environmental minister in several statements this week blamed Prime Minister Narendra Modi and Indian Environment Minister Bhupender Yadav for being silent on the crisis, saying the smog was their “moral responsibility”.

Rai said he wrote to Yadav four times in the past month to seek clearance for a cloud-seeding experiment that he said would help combat the crisis by generating artificial rain to clear the smog. Since last year, Delhi’s government has worked with scientists at the Indian Institute of Technology Kanpur, an elite technical school in Uttar Pradesh, seeking to carry out a trial.

“The central government cannot remain silent like this. Artificial rain has now become an emergency need for Delhi,” Rai told journalists.

Scientists, however, have expressed scepticism about the plan and cloud seeding, a complex weather modification process that has been used in some cases to increase rainfall and disperse smog. Some said it’s too experimental and fails to address the root causes of constant pollution.

Will cloud seeding work?

Typically, cloud seeding entails spreading chemicals, commonly silver iodide or common salt, into clouds with the aid of ground generators or aircraft. The agents allow snowflakes to form and fall back to the Earth’s surface as artificial rain.

There isn’t a consensus yet among scientists on whether cloud seeding is effective. One of the sceptics, Sachin Ghude, an air quality forecaster at the Indian Institute of Tropical Management in Pune, told The Indian Express, a New Delhi-based newspaper, that Delhi’s plan “fails to address the root causes of pollution and practical challenges of cloud seeding, particularly in the absence of suitable weather conditions”.

Many scientists also worry about the effects of silver iodide on the environment. However, countries like China regularly seed clouds for rain or to avoid precipitation. During the 2008 Olympics, organisers fired more than 1,000 rockets into the sky to ensure there would be no clouds or rain.

In India, cloud-seeding experiments have been ongoing since the 1950s. From 2003 to 2009, the state of Andhra Pradesh seeded clouds to battle prolonged dry spells. Authorities at the time said the project had boosted rainfall by 15 to 45 percent.

What other policies have Indian authorities rolled out?

Nationwide, authorities have also embarked on other initiatives to solve the crisis.

In 2000, the Indian government launched the Bharat Stage Emission Standards (BSES) which regulates the output of air pollutants from ignition engines, including motor vehicles.

BSES is modelled after European standards. Progressively stringent phases of the regulations have been rolled out over time, restricting the sale and registration of noncompliant engines or cars.

The National Clean Air Programme (NCAP) was also launched in 2019, targeting 100 cities. The aim was to significantly enhance the air quality in chosen locations by 2026 by improving monitoring. The NCAP promised to reduce particulate matter concentrations by 20 to 30 percent and overall pollution by 40 percent compared with 2017 levels.

Analysts, however, said the programme was badly designed because it focuses too much on individual cities and does not jointly integrate state-level activities.

In Delhi, for example, where only one-third of its pollution comes from within the state itself, some officials said they have no faith in the NCAP.