Reporting this week at the annual American Geophysical Union, scientists from UC Irvine discussed air quality results from samples taken during the 2012 and 2013 Hajj.
The annual pilgrimage brings between 3-4 million people to the holy city of Mecca. Isobel Simpson, the lead researcher on this project, stated that, "The problem is that this intensifies the pollution that already exists. We measured among the highest concentrations [of smog-forming pollutants] our group has ever measured in urban areas – and we've studied 75 cities around the world in the past two decades."
The worst locations were tunnels leading into the Grande Mosque where carbon monoxide levels were up to 300 times higher than baseline measurements, and pedestrians were often walking in large numbers alongside idling motor vehicles. Increases in carbon monoxide are linked to increased numbers of hospitalizations and deaths from heart failure. In addition to carbon monoxide, the team found elevated levels of benzene, black carbon, and other fine particulates that can affect lung function.
The main culprit here that can be addressed by the Saudi government is a lack of regulation over automobiles, gasolines, and exhausts. Currently, there is a significant lack of public transportation in the area, and nearly everyone owns a car. Those cars don't have the devices that are currently required and built into vehicles in the Unites States to limit pollution.
The easiest thing to fix would be separating pedestrians from cars in the tunnels, or at least spreading vehicles out more evenly among the tunnels leading to the Mosque, so that pollutants don't build up as much and negatively affect those walking in. Alternatively, improving ventilation in the tunnels would reduce deadly carbon monoxide build-up.
In the meantime, if you are planning to join the Hajj in the near-future, consider bringing an air-filtering face mask.