Are you an ultra-marathoner?

These are people who run races longer than the standard marathon distance of 26.2 miles. It is a growing sport; more than 600,000 people participated in an ultrarace in 2018, an increase of nearly 350 percent during the past decade, and nearly 1,700 percent from the 1990s. But it can be quite dangerous.

Last week 21 runners died as a storm descended on a high mountain pass during a 62-mile race in northwestern China. Last year, Kateryna Katiuscheva of Ukraine, known as the “Iron Lady” of ultrarunning, collapsed six miles from the finish of a 42-mile trail race. Organizers found her after an eight-hour search but she died in the hospital, at age 33, the following day. And last summer, the veteran ultrarunner Kim McCoy lost her leg when she was struck by a car while crossing a highway 70 miles from the finish line of a 340-mile race across the American South.

Most runners are on their own. Few organizers provide nourishment. Some organizers require runners to have their own crew supporting them along the way. It is not clear how strict organizers are about making runners carry an emergency pack with warmer clothing, something that has become increasingly common at races that pass through high-altitude regions in the United States and Europe.