More than 40,000 marathoners are preparing to compete on Sunday in the Indian capital New Delhi, where air pollution has reached dangerous levels.Images of the city this week show it blanketed in a thick yellow smog, some buildings barely visible through the haze. The air quality index, which ranks pollution on a scale of 0 to 500, reached 227 on Friday — classed as “very unhealthy.” This score reflects the concentration of fine particle matter, known as PM2.5, per cubic meter. According to the World Health Organization, an acceptable air quality index score for humans to breathe regularly is 25. Unsurprisingly, running in this environment can be dangerous. Microscopic pollutants like PM2.5, which are smaller than 2.5 micrometers in diameter, can enter deep into a person’s lungs and bloodstream, increasing risk of respiratory problems, cancer, stroke and heart attack. Physical exercise can increase the amount of pollutants breathed in by up to five times.But still, a record high of 40,633 people are planning to run the Airtel Delhi Half Marathon, which includes other categories like a 10-kilometer race (about 6.2 miles) and a senior citizens’ race. All the races will start and end at the Jawaharlal Nehru Stadium in the south of the city, following different looped routes.

Organizers are taking a number of measures to try and combat the health risks. Ahead of the race on Sunday, they will spray the entire course with water mixed with “ecologically safe” chemical reactants, which is meant to reduce and remove dust and pollutants like particulate matter, according to the marathon website. They will also attempt to use a more high-tech, largely unproven method — transmitting Wi-Fi pulses through the air to push pollution particles in different directions, similar to the push of a magnet, the site said.Organizers told CNN there would also be medical facilities and medical staff throughout the route.Some runners are still worried. Jonathan Gil Harris, who has been running marathons in India since 2012, says the air has worsened noticeably in the past four years.”It hasn’t stopped people from running but it is a very real challenge and concern with the levels of pollution reaching what they have,” he said. “You see more runners running with the masks, looking to get out of Delhi for their running.”He added that although he was glad organizers were trying to go green, the Wi-Fi technology felt like “a band aid on a great gaping wound.””It would be nice to see actual progress,” he said. “(The government) keep telling us they’re doing so but the empirical evidence for a runner suggests otherwise.”CNN has reached out to the Department of Environment in the Delhi government for comment.

The Yamuna River on a polluted day in New Delhi on October 18, 2019.There are some ways runners can better prepare themselves for the pollution — face or gas masks with air filters are often used during marathons in other polluted cities. During the 2014 Beijing Marathon, many runners were geared up with in masks and protective eyewear like swimming goggles.Other leading runners stayed away entirely or dropped out halfway. British runner Chas Pope, 39, pulled out after 10 kilometers (6.2 miles) after seeing the blackened air filters on his face mask.According to Airtel representatives, no runners have dropped out as of Friday.

The world’s most polluted city

New Delhi has been the world’s most polluted cityfor years, according to Greenpeace and AirVisual, which found it had an average air quality index of 135.8 in 2018 — almost three times the level which the US Environmental Protection Agency regards as healthy.Chief Minister of Delhi Arvind Kejriwal has long campaigned against air pollution, and called the city a “gas chamber” in 2017.The pollution has intensified in recent weeks — which officials say is because of fires caused by crop burning in neighboring states like Haryana and Punjab. Every year, farmers across these fertile regions set fire to their fields to clear them for the next season.Similar tactics have also caused devastating wildfires in Indonesia and the Brazilian Amazon this summer.

Kejriwal has called for an end to the crop-clearing practice, as well as action in other sectors such as transport — last month, he and other officials announced a new road rationing measure that if implemented, would require odd- and even-numbered vehicles to only operate on odd or even dates, in an attempt to cut back on emissions, CNN-News18 reported.He also said at a press conference Thursday that the Delhi government was working to distribute masks and implement pollution mitigation measures like the sprinkling of water on roads. Earlier in September, he had said the government was trying to procure up to 6 million masks.Air quality typically plunges in the winter, exacerbated by the combination of lower temperatures, crop-clearing, and Diwali, the Hindu Festival of Lights. Diwali is celebrated with firecrackers and has posed a persistent pollution problem — last year, the Indian Supreme Court banned the sale of most firecrackers, but to no avail. People continued to buy and use them, and parts of the capital after Diwali reported levels of pollution almost 40 times those considered “safe” by the World Health Organization.

People burst firecrackers for Diwali on November 7, 2018 in New Delhi, India.In efforts to combat the Diwali pollution, New Delhi authorities this year are also granting fewer licenses to pyrotechnic vendors than usual, and the Health Ministry urged the public to use more environmentally friendly firecrackers, according to CNN affiliate CNN-News18.Usually, the Airtel races takes place in November — but this year, organizers moved it to October, taking place before Diwali. This has reassured some runners — Harsh Vardhan Sahni, 33, called Diwali “the big culprit” of pollution.”What’s made a difference to me is that they’ve scheduled it before Diwali,” said Sahni, who is competing in the Airtel races for the first time. He added that he wasn’t too concerned about pollution, and that “the air quality is getting worse but it doesn’t alarm me.””This is the air we live and breathe in anyway so it’s not like a few hours on one particular morning will make a difference,” he said.