Dangerously high levels of air pollutants are being released in Mecca during the hajj, the annual holy pilgrimage in which millions of Muslims on foot and in vehicles converge on the Saudi Arabian city, according to findings reported today at the American Geophysical Union meeting in San Francisco. "Hajj 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 UC 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."

Scientists from UCI, King Abdulaziz University in Saudi Arabia, the University of Karachi in Pakistan, the New York State Department of Health's Wadsworth Center, and the University at Albany in New York captured and analyzed air samples during the 2012 and 2013 hajjes 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. 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 hospitalization or death rises sharply as the amount of carbon monoxide in the air escalates, the researchers note in a paper published in the journal Environmental Science & Technology. Headaches, dizziness and nausea have also been associated with inhaling carbon monoxide.

"There's carbon monoxide that increases the risk of heart failure. There's benzene that causes narcosis and leukemia," 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."

The scientists detected a stew of unhealthy chemicals, many connected to serious illnesses by the World Health Organization and others.

"Air pollution is the cause of one in eight deaths and has now become the single biggest environmental health risk globally," said Haider Khwaja of the University at Albany. "There were 4.3 million deaths in 2012 due to indoor air pollution and 3.7 million deaths because of outdoor air pollution, according to WHO. And more than 90 percent of those deaths and lost life years occur in developing countries."

Khwaja experienced sooty air pollution firsthand as a child in Karachi, Pakistan, and saw his elderly father return from the hajj with a wracking cough that took weeks to clear. He and fellow researchers braved the tunnels and roads to take air samples and install continuous monitors in Mecca.

"Suffocating," he said of the air quality.