During Hajj in the Saudi Arabian city of Makkah, the pilgrims get exposed to dangerously high levels of air pollutants which can increase their heart attack risk, shows a study.
The Hajj is annual holy pilgrimage in which millions of Muslims converge in Makkah.
"Hajj is like nothing else on the planet. You have three to four million people - a whole good-sized city - coming into an already existing city," said Isobel Simpson University of California, Irvine (UCI).
"The problem is that this intensifies the pollution that already exists," Simpson added.
"There is carbon monoxide that increases the risk of heart failure. There is benzene that causes narcosis and leukaemia," Simpson said.
The researchers analysed air samples during the 2012 and 2013 Hajj pilgrimages on roadsides - near massive, air-conditioned tents and in narrow tunnels that funnel people to the Grand Mosque, the world's largest, in the heart of Makkah.
The worst spot was inside Al-Masjid Al-Haram tunnel, where pilgrims on foot, hotel workers and security personnel are exposed to fumes from idling vehicles, often for hours.
The highest carbon monoxide level - 57,000 parts per billion - was recorded in this tunnel during October 2012.
In addition to the high smog-forming measurements, the team in follow-up work found alarming levels of black carbon and fine particulates that sink deep into lungs.
Besides UCI, scientists from King Abdulaziz University in Saudi Arabia, the University of Karachi in Pakistan, the New York State Department of Health and the University at Albany in New York were also involved in the research.
The findings were presented at the American Geophysical Union meeting in San Francisco, US.
