We spend 2 percent of our total commuting time stopped at intersections, but the concentration of tailpipes while we wait is so bad that we get 25 percent of our total exposure to air pollutants there, says a new study.

The big problem is particulate matter, a key emission from diesel engines. The Occupational Safety and Health Administration (OSHA) warns that short-term exposure to high concentrations “can cause headache, dizziness, and irritation of the eye, nose and throat severe enough to distract or disable” workers. Prolonged exposure “can increase the risk of cardiovascular, cardiopulmonary and respiratory disease and lung cancer.” The World Health Organization links air pollution to seven million premature deaths annually (one-eighth of total global deaths).

Waiting for the light to change in Bangkok. Inhaling particulate matter is a big problem. (Photo: Joan Campderrós-i-Canas/Flickr)

Waiting for the light to change in Bangkok. Inhaling particulate matter is a big problem. (Photo: Joan Campderrós-i-Canas/flickr)

The revelations are contained in a new study by Anju Goel and Prashant Kumar, both scientists the University of Surrey in England. They monitored exposure to air pollution at various points of a typical commuter’s journey, and found that intersections with traffic lights were the biggest “hot spots,” thanks to drivers accelerating and de-accelerating to meet the demands of the signal. “Peak particle concentrations were found to be 29 times higher than during free-flowing traffic conditions,” they found.

Another problem with traffic lights is that cars are bunched up at them, so your exposure is worse than it otherwise would be. Dr. Kumar tells me, "At traffic lights, we found that when we closed the windows and switched off the fan, this gave us the lowest exposure. When the windows were closed but the fan was on, the exposure was at its highest. This is due to the fact that the air outside the vehicle at red lights is generally much more polluted than the air inside the car. Switching on the fan sucks the dirty outside air to the inside the vehicle, and the air inside takes some time to dilute or escape out of the vehicle, resulting in an accumulation of pollutants inside."

The smog in New Delhi is sometimes called

The smog in New Delhi is sometimes called "fog." (Photo: Stephen Rebernik/flickr)

I asked Dr. Kumar about New Delhi, believed to be the dirtiest city in the world in terms of air pollution. Driving there, would traffic lights actually make a really bad situation even worse? He gave an interesting answer:

One of the interesting points in cities like Delhi is that commuters normally switch off their engines due to long queues on the roads. They're trying to save fuel, but it indirectly helps cut emissions. In case of congestion, most of the road becomes hotspots rather than just the red lights. However there are a number of flyovers been built in recent years in Delhi, and that helps reduce traffic congestion and hence emissions.
At last! The light turns green!

At last! The light turns green! (Photo: Tilemahos Efthimiadis/flickr)

Some ways to reduce your exposure:

  • Keep your windows shut at traffic lights.
  • Turn fans off and make sure the circulation system is set on a closed loop, rather than taking in outside air.
  • Keep your distance from other cars at intersections.
Traffic agencies can do their part by synchronizing lights around speed limits, which creates more flowing traffic and reduces drivers getting caught at intersections. Driving an electric or zero emission fuel-cell car would help, too, as would taking public transportation, biking and walking!

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Jim Motavalli ( @jmotavalli ) writes about cars, technology and the environmental world to anyone curious enough to ask.