As Nepal is celebrating Everest Day today as a big day for the world’s mountaineering community, there are growing concerns over deaths rise in Mount Everest.

May 29 is marked as Mt Everest Day every year in memory of the first summit of the world’s highest peak on this day in 1953. New Zealand mountaineer Sir Edmund Hillary and Tenzing Norgay Sherpa made successful ascend of the Everest on May 29, 1953.

According to a news report by Associated Press reports quoting seasoned mountaineers write Nepal’s reluctance to limit the number of permits it issues to scale Mount Everest has contributed to dangerous overcrowding, with inexperienced climbers impeding others and causing deadly delays.

During the short period this season when the weather was clear enough to attempt the summit, climbers were crammed crampon-to-crampon above South Col’s sharp-edged ridge, all clipped onto a single line of rope, trudging toward the top of the world and risking death as each minute ticked by.

“There were more people on Everest than there should be,” said Kul Bahadur Gurung, general secretary of the Nepal Mountaineering Association, an umbrella group of all expedition operators in Nepal.

Eleven people have died this season, the highest number since 2015. Most are believed to have suffered from altitude sickness, which is caused by low amounts of oxygen at high elevation and can cause headaches, vomiting, shortness of breath and mental confusion.

Once only accessible to well-heeled elite mountaineers, Nepal’s booming climbing market has driven down the cost of an expedition, opening Everest up to hobbyists and adventure-seekers. They are required to have a doctors’ note deeming them physically fit, but not to prove their stamina at such extreme heights.

“Because of the altitude, climbers have just hours to reach the top before they are at risk of a pulmonary edema, when the lungs fill with liquid. From Camp Four at 8,000 meters (26,240 feet) to the 8,850-meter (29,035-foot) peak, the final push on Everest is known as the “death zone,” reports Associated Press.

The deaths this year on Nepal’s side of the mountain included Don Cash, a sales executive from Utah, and Christopher Kulish, an attorney from Colorado, who both died on their way down from the peak writes AP.

According to AP, this year, permits were issued to 381 people, the highest number ever, according to the government. They were accompanied by an equal number of guides from Nepal’s ethnic Sherpa community.