Despite the smog, Ethiopia's Girmay Birhanu Gebru won the marathon in a time of two hours, 10 minutes and 42 seconds. Many elite runners chose not to participate in the race. AFP / Getty Images

Thousands of runners participated in the marathon, and some chose to risk running in the polluted air — deemed "hazardous" by the U.S. Embassy — without a mask. AFP / Getty Images

A man gives two-thumbs up, despite wearing a mask to protect from pollution. One runner told Bloomberg that he "was basically a vacuum cleaner." AFP / Getty Images

Runners chat as they jog through the thick smog of Beijing. Andy Wong / AP

A runner in a fancy costume takes part in the 34th Beijing International Marathon, which began at Tiananmen Square and ended more than 26 miles later at the Olympic Park. AFP / Getty Images

Many participants wore masks during a polluted day at the Beijing International Marathon. Despite the air, one man gives a thumbs-up in front of Tiananmen Square on Oct. 19, 2014. Reuters

Thousands of runners battled thick smog at the 34th Beijing International Marathon on Sunday, with some athletes donning masks as air pollution soared to 16 times the maximum recommended level.

The level of small pollutant particles known as PM2.5, which can embed themselves deep in the lungs, reached more than 400 micrograms per cubic meter in parts of Beijing as the racers lined up, according to the U.S. Embassy, which categorized the air as "hazardous." The World Health Organization's recommended daily maximum average exposure is 25.

Many top long-distance athletes stayed away from the race, which was won by Ethiopia's Girmay Birhanu Gebru in two hours, 10 minutes and 42 seconds.

“I was basically a vacuum cleaner,” William Liu, a 30-year-old banker, told Bloomberg after completing the marathon.

Organizers made 140,000 water-soaked sponges to available athletes, advising runners to scrub down their skin after it was “exposed to the air,” the Beijing News reported.