Two-hundred and three. That’s the number the U.S. Embassy in New Delhi rated the city’s air quality on Friday evening. Translated into English, that means the air was “very unhealthy,” and that even those with robust immune systems may experience “serious health effects” just by breathing.

This number is not an aberration. Earlier in the day, the conditions were even worse, measuring 299, just three points shy of crossing into the most dire air-quality categorization of “hazardous,” which generally means everyone should remain inside.

Despite these harsh readings, which are expected to improve only slightly over the weekend, runners —  including 2017 Boston Marathon winner Geoffrey Kirui and Olympic 10,000-meter gold medalist Almaz Ayana — are still planning to compete in Sunday morning’s Airtel Delhi Half Marathon. This prospect worries some medical professionals in India.

“Those who are practicing for the marathon today need to realize that they are harming themselves,” Dr. K.K. Aggarwal, president of the Indian Medical Association, said in a news statement on Friday, noting that he’d recommend canceling a marathon for any air quality reading above 100.

Even the race’s main sponsor, Bharti Airtel Ltd., India’s largest telecom operator, has expressed concern, noting this week that if the air quality remains poor in the future, it may have to rethink its sponsorship.

“Air pollution poses serious health risks and it is important that these concerns are addressed urgently and appropriately by the authorities for Airtel to continue associating with the event next year and beyond,” Airtel said in a statement (via Bloomberg).

Organizers of the event, a group called Procam International, meanwhile, have no plans to cancel, despite being “conscious that the state of air quality is not conducive.”

“Air conditions are not a constant and the current air quality is slated to improve,” Procam noted on its website this week, adding two of Delhi’s biggest air pollutants, dust and smog produced from traffic congestion, will be controlled on Sunday.

“Our biggest responsibility is to the running community,” the company continued. “These include: the international elite athletes, the Indian elites; thousands of amateur runners, who have spent sweat and tears over months, preparing to run on a particular date. ADHM is not a spectator sport, whose dates can be changed at short notice.”

Poor air quality in and around Delhi is nothing new. A study released in October on world air pollution, concluded an estimated 2.5 million deaths in 2015 could be linked to pollution in India. The study did not parse out how many of those deaths came in Delhi, where the air pollution in some neighborhoods has rated 40 times higher than the World Health Organization’s recommended safe levels, but experts contend the city’s air is acutely dangerous.

“The air pollution in Delhi is a killer, and more needs to be done to help Delhi breathe,” Reecha Upadhyay, who directs the nonprofit Help Delhi Breathe, said in a statement on Friday.

Despite the stats and warnings, however, race participants, including Kirui and Ayana, still plan to run.

“I am ready for a good result,” Ayana, who dodged questions about pollution at a media event on Thursday, told the Hindustan Times. “After the World Championships in August, I have been training hard for the half marathon. The main target will be to give my best.”

Kirui, meanwhile, appeared more concerned about the field of runners than what any air pollution might do to his lungs.

“It will be challenging task as the field is good,” he told the Indian paper. “Since my training has been good, I hope for a podium finish.”

Read more: