Regular cycling keeps a person fit.  Those of us who regularly commute or do errands zip along city streets feeling strong and healthy – even a bit smarter than the stuck-in-traffic drivers we pass – and safe so long as we pay careful attention to what’s going on around us.

But even though we may have the skills to avoid accidents in heavy traffic, there are circumstances where cycling may be dangerous to our health.  We may not break our arms or necks, but we may damage our lungs, hearts, and even our brains.

The culprit is the car and truck exhaust spewing into our faces.  Cyclists, because of their more intense level of exertion-influenced breathing, absorb up to six times as much air and its load of pollution as the car driver creating it. The heavier the traffic and the closer we are to the exhaust, the worse its impact – long term exposure has been shown to double a person’s chances of dying from lung cancer and heart disease, perhaps even contributing to earlier-than-otherwise dementia.

Despite these concerns, there are two pieces of good news.  First, researchers estimate that the benefits of shifting from car to bike, for both individuals and society as a whole, substantially outweigh the statistical risks.  Second, recent studies show that there are ways to diminish our exposure – although the “vehicular cyclists” who think bikes are just vehicles and should travel on the open road like everyone else won’t like the suggestions.

How can we reduce our exposure?  First, individually, we need to consciously avoid heavy-traffic roads as much as possible.  If you can find a route with fewer cars, take it.

Second, from an infrastructure perspective, we need to create separate bike-path “greenways” that provide a fully-connected off-road network of routes around the metropolitan area and within urban areas.  (These can be “multi-use paths” for both cyclists and pedestrians, but if so it’s best to have separate areas for slow and fast movers – as is already done in Boston’s Southwest Corridor Linear Park.)

Third, on heavily trafficked roads, we need to give bikes the ability to be as far away from car traffic as possible.  The best way is to create a physical barrier between the bikes and the traffic that is high enough to deflect the exhaust flow.  Moving parked cars out into the street and the bike lane next to the sidewalk – as NYC is doing to create “cycle tracks” that feel safe enough for beginners to use – has already been found to reduce exposure by up to 50%.  But fences, Jersey barriers, and other obstacles might work as well.  On streets lacking space for a physical barrier but that don’t have parking on one side, the bike lane should be located on the no-parking side of the street – as has been done along the Back Bay sections of Boston’s Commonwealth Ave.

Fourth, city or state agencies need to create “pollution level” maps that show us where and at what times we would have the highest exposure.  This would be useful not only for cyclists but also for runners, walkers, and even residents whose close-to-the-road homes are also at risk.

Finally, we need to push for “cleaner” vehicles, electric or otherwise, as well as for making it easier and more advantageous for people to leave their cars at home in favor of transit, walking, or cycling.


According to Wig Zamore, from the Somerville Transportation Equity Partnership (STEP), now that lead has been eliminated from the gasoline supply, a major threat comes from extremely small, combustion-caused particles which cause up to 15% of all US death each year and take about 2 years off the average life span.  Based on studies examining people living near highways, families with the heaviest exposure have up to double the rate of childhood asthma, up to quadruple the risk of impaired lung function, up to double the adult rate of hear attacks and the same for lung cancer.  The latest research shows that air pollution, much of it traffic related, is also associated with the appearance of brain lessons denoting reduced blood flow and developing dementias, including Alzheimer’s.

This is serious stuff.  It has been estimated that almost 85% of the total health benefit of all US environmental regulation comes from its reduction of particulate pollution.

Pollution particulates are commonly divided into three categories based on size.  “Coarse” includes everything up to 10 microns in diameter – there are about 25,000 microns in an inch – and has been regulated since 1987.  “Fine” particles cluster around 2.5 microns wide (and are therefore called PM2.5), and have been regulated since 1997.  And “UltraFine” are smaller than that, and are not yet regulated.  PM2.5 and larger particles comprise about 95% of the mass of particulate emissions.  Ultra Fines comprise about 95% of the total number of particulates.

Ultra Fines are created by the condensation of emission gasses.  They have about a two minute life-span, depending on temperature (they’re produced more readily in cooler air), and spread along with the wind with the biggest danger within about 100 meters (300 feet) of creation.  Housing, for example, should preferably be located at least 400 meters away from heavily trafficked roads, or should have HEPA filters in all air-intakes.  (So far, only California has passed regulations dealing with this issue, which has a huge Environmental Justice impact because it is low-income people who are most typically forced to live within the exhaust plume of major roads.)

The smaller the particle the deeper into the lungs it penetrates and the more likely it is to remain – we retain about 80% of the particles we inhale, which given the preponderance of Ultra Fines means that they are the majority of what stays inside us.  While the PM2.5 particulates tend to cause respiratory problems, the Ultra Fine pass into the blood stream and cause cardiovascular and, perhaps, neurological damage.  (There is some evidence that particulates also pass from the nose directly through the smell neurons into the brain.)

However, a recent study in Portland, Oregon, showed that switching the location of the bike lane and the row of parked cars – creating a type of “cycle track” or “protected bike lane” as NYC is now doing on many major streets – can cut the exposure of cyclists by nearly 50%.

Finally, in Vancouver and some other cities it is possible to consult a “pollution density map” that shows the streets and time of day with the worst air quality.  This allows individual cyclists to avoid those locations as much as possible.  In fact, even without a formal map, most of us know what streets have the heaviest traffic and would probably be smart to avoid them.

US environmental regulations have slowly reduced the number and size of allowed particulate emissions.  Because better technology existed to measure the total weight of emissions rather than the size of the particulates, the original emphasis on reducing the total amount led to a focus on getting rid of the larger particles which contributed most of the mass.  Over the years, as research showed that smaller particles were the most dangerous, regulation expanded its reach.  This year, EPA is now gearing up for another look at emission rules.  This is an important opportunity for the cycling community to raise its voice – as well as runners, walkers, those concerned about public health and environmental justice.

In the meantime, we shouldn’t park our bikes.  A recent paper in an international research journal used statistical projections to quantify the impact of air pollution and accidents on mortality rates.  Their conclusion:  “For individuals who shift from car to bicycle, we estimated that beneficial effects of increased physical activity are substantially larger (3 – 14 months gained) than the potential mortality effect of increased inhaled air pollution doses (0.8 – 40 days lost) and the increase in traffic accidents (5 – 9 days lost).  Societal benefits are even larger because of a modest reduction in air pollution and greenhouse gas emissions and traffic accidents.”

So….ride on; but breath carefully!


“Do the Health Benefits of Cycling Outweigh the Risks?” by H.deHartog, H.Boogard, H.Nijland, G.Hoek, Environmental Health Perspectives, August 2010

“Destination Brain: Inhaled pollutants may inflame more than the lungs,” by Janet Raloff, Science News, 5/22/10

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