Dangerous levels of air pollutants, linked to increased risk of heart attack, are being released in Mecca during Haj, the world's largest annual gathering, according to a new study.
During the annual holy pilgrimage, millions of Muslims on foot and in vehicles converge on the Saudi Arabian city of Mecca.
"Haj is like nothing else on the planet. You have 3 to 4 million people - a whole good-sized city - coming into an already existing city," said Isobel Simpson, a University of California Irvine research chemist in the Nobel Prize-winning Rowland-Blake atmospheric chemistry laboratory.
"The problem is that this intensifies the pollution that already exists. We measured among the highest concentrations our group has ever measured in urban areas - and we've studied 75 cities around the world in the past two decades," said Simpson.
Scientists from UCI, King Abdulaziz University in Saudi Arabia, the University of Karachi in Pakistan, the New York State Department of Health's Wadsworth Centre, and the University at Albany in New York captured and analysed air samples during the 2012 and 2013 Haj 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 Mecca.
"The worst spot was inside the Al-Masjid Al-Haram tunnel, where pilgrims on foot, hotel workers and security personnel are exposed to fumes from idling vehicles, often for hours," researchers said.
The highest carbon monoxide level - 57,000 parts per billion - was recorded in this tunnel during October 2012. That's more than 300 times regional background levels.
Heart attacks are a major concern linked to such exposure: The risk of heart failure hospitalisation or death rises sharply as the amount of carbon monoxide in the air escalates, researchers said.
"There's carbon monoxide that increases the risk of heart failure. There's benzene that causes narcosis and leukaemia," Simpson said.
"But the other way to look at it is that people are not just breathing in benzene or CO, they're breathing in hundreds of components of smog and soot," said Simpson.
The scientists detected a stew of unhealthy chemicals, many connected to serious illnesses by the World Health Organisation and others.
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.
Once the Haj was over, concentrations of all contaminants fell but were still comparable to those in other large cities with poor air quality, researchers said.
The research was published in the journal Environmental Science and Technology.
