Fourteen Jordanian pilgrims died as they performed rituals related to Hajj, a holy trip to Mecca in Saudi Arabia that Muslims are encouraged to take once in their lives, Jordan’s official news agency said on Sunday.

The pilgrims died as a result of exposure to extreme sun and heat, the agency said, based on a report from Jordan’s Foreign Ministry. In Mecca, temperatures reached nearly 110 degrees Fahrenheit on Sunday, and recent studies have indicated that climate change will increase health risks there.

Another 17 pilgrims were also missing, the agency said.

The Hajj is one of the largest mass gatherings in the world, with Muslim pilgrims traveling from near and far for the spiritual experience, which is also a physical and mental challenge.

This year, Hajj started on Friday and will end on Wednesday; 1.8 million pilgrims were expected to take part, according to the General Authority for Statistics, a Saudi government agency.

The Saudi Press Agency reported on Saturday that the country’s medical center for heat exhaustion had treated 225 pilgrims for heat stress and fatigue.

Deaths have also occurred during previous pilgrimages, including from stampedes. In 2015, more than 700 people died from a stampede. Many pilgrims, who are often older, have also experienced heat stress in recent years, with scores dying from the heat.

The pilgrimage is one of the five pillars of the Muslim faith, and many of the rituals take place outdoors, in Mecca and the surrounding desert. These rituals include praying outside the Great Mosque of Mecca and spending the day in prayer at Mount Arafat, often under the blazing sun.

Scientists have warned that weather conditions will be severe when Hajj, which follows the lunar calendar, falls during the summer, as it did this year.

Relief measures have helped reduce cases of heat stress, scientists say. Saudi authorities have used water mist sprays to cool the air, they said, and they have provided water, umbrellas and air-conditioned transportation for the pilgrims.

Vivian Nereim contributed reporting from Riyadh.