In an unsettling testament to the severity of Beijing’s pollution problem, thousands of runners hit the streets for the city’s annual marathon on Sunday decked out in gas masks.

Organizers of the 34th Beijing International Marathon warned runners in advance to expect a slight to moderate smog risk, but upgraded the warning to ‘hazardous’ early Sunday, according to the BBC. Channel News Asia reports that air pollution on race day was 16 times the recommended level.

(MORE: China's Smog Worsens)

As the marathon began at Tiananmen Square, thick smog hung over the racecourse, creating an odd backdrop for more than 25,000 athletes. Participants told the BBC that the air smelled like burnt coal.

Organizers rejected calls to postpone the race and instead equipped the 26-mile course with more medical staff and stocked supply stations with thousands of sponges for runners to wipe pollution off their skin, the Associated Press Reports.

"On a normal day, nobody would run in such conditions," said participant Liu Zhenyu, a computer engineer. "But the event is happening today, so what can we do?"

The kind of pollution in smoke is made up of tiny particles known as PM2.5, which can become embedded deep within the lungs and damage the body, according to CNN. The WHO says the daily level of exposure to PM2.5 shouldn’t exceed 25 micrograms per cubic meter, but levels had reached more than 400 micrograms per cubic meter in Beijing by the start of the race on Sunday.

Several runners were forced to pull out because they feared for their health, and many top long-distance runners avoided participating in the race all together, Channel News Asia reports.

"When I looked at the state of the mask after 10 kilometres (6 miles), I decided enough was enough," British runner Chas Pope, 39, who dropped out of the race, told the BBC. "It felt pretty ridiculous given we're meant to be running for health and fitness."

A combination of heavy use of coal, a booming population and little environmental regulations has made China the poster child for air pollution in recent years. The government has declared a ‘war on pollution,’ but poor air quality persists, according to Channel News Asia.