Some Motor Vehicles Pose Hidden Dangers to Cyclists

Health Effects Of Pollution

Most road cyclists worry about getting hit by motor vehicles. They imagine themselves sustaining any of the various types of injuries known to be produced by the trauma of colliding with a vehicle weighing several tons. To that end, cyclists learn how to ride defensively and how to position themselves to minimize their risk of getting hit by the cars around them.

This type of danger is foremost in cyclists’ minds because it is visible. And it represents immediate pain and, occasionally, imminent death.

Certain vehicles tower over cyclists and inspire fear by their sheer magnitude. Included in this class are large trucks and buses. Few cyclists survive crashes with these vehicles. For that reason, experienced cyclists do their best to steer clear of them.

Cyclists rarely talk about the hidden dangers posed by vehicles. For instance, one of the most harmful substances humans can encounter on the roads are the toxins produced by diesel fuel — spewed into the air by buses, trucks and heavy machinery used in farming and construction.

Even though diesel fuel has been “cleaned up” somewhat in recent years, by reducing sulfur content,” it still produces microscopic particles which are known to be toxic and carcinogenic. When horsepower is equal, diesel exhaust is 100 times more toxic than gasoline exhaust, even when carbon monoxide is considered.[1] According to the Environmental Protection Agency, truck exhaust accounts for 20% of all vehicle-produced microscopic soot and 30% of all smog-causing chemicals in the United States.[2] Less than 1% of new American cars have diesel engines. However, diesel engines power 37% of all new cars sold in Europe.[3]

The components of diesel exhaust include: carbon monoxide, carbon dioxide, nitrogen oxides, sulfur oxides, hydrocarbons, and unburned carbon particles.[1] Diesel engines emit other toxins in greater quantities than gasoline engines such as organic molecules, which are highly toxic.

A number of health problems have been associated with the inhalation of these toxins. People who exercise or otherwise exert themselves in the vicinity of diesel exhaust are at higher risk of ill effects due to their higher intake of oxygen.

Some studies have demonstrated the development of cardiac (heart) problems, which appeared after acute exposure to diesel exhaust. Pulmonary (lung) problems have also been observed. Ozone and other substances in diesel exhaust can contribute to the destruction of lung tissue. And people with diseases such as asthma may have their illnesses aggravated by contact with diesel exhaust.

What’s most disconcerting is the animal lung studies which have shown that diesel exhaust particles directly damage DNA and result in carcinogenesis.[4] Studies in humans have shown that those whose occupations expose them to higher than average rates of diesel exhaust exposure have a significantly higher-than-normal incidence of death from lung cancer.[5]

On the bright side, the World Health Organization’s (WHO) cancer agency has concluded that diesel fumes cause cancer. This ruling could make exhaust as important a public health threat as secondhand smoke. Although the risk of getting lung cancer from diesel fuel is small, the WHO raised the status of diesel exhaust to carcinogen from “probable carcinogen.”

The U.S. government still classifies diesel exhaust as a likely carcinogen because new diesel engines emit fewer fumes and further studies are needed to assess potential dangers. Experts say that lung cancer is caused by multiple factors and point out that other things, such as smoking, are more dangerous.

On a cautionary note, other experts warn of the extreme dangers posed by the microscopic particulates emitted by diesel exhaust. Such particles are so small, they can only be seen by an electron microscope.

These tiny particles can bypass nasal hairs, the body’s first line of defense, and settle deep into a person’s lungs, where they can remain and cause irritation and inflammation. Their small size also allows them to get past various bodily defenses and migrate into the bloodstream. In addition, they affect blood vessels’ ability to contract, which can lead to a heart attack.

What does all of this mean for cyclists, particularly those who use their bikes for transportation?

It’s impossible to avoid diesel exhaust exposure entirely. Any road that permits trucks or buses will contain diesel fumes. Fast riding cyclists who breathe more heavily will increase their exposure to the fume’s ill effects.

To reduce risk, cyclists should avoid traffic congestion whenever possible. Idling vehicles spew more emissions than moving vehicles.

Positioning oneself away from trucks and buses, even if it means slowing down and hanging back, is another good defense against inhaling diesel exhaust. Whenever possible, cyclists should take side streets with less traffic, just as they might do to avoid getting hit by a car.

All of these things are temporary measures. What really needs to be done is to reduce our dependence on diesel fuel and to decrease the number of motor vehicles on the roads.

This can be accomplished through advocacy and personal practices. Advocates can lobby for more infrastructure for cyclists and pedestrians to provide alternatives to motor vehicle travel. They should also promote the development and use of cleaner burning fuels.

Individuals can make a habit of walking, cycling, or using public transportation whenever possible. Even making 25% of daily trips by alternative methods of transportation would help to reduce air pollution in and around cities.

In the daily struggle between cars and bicycles over who has a right to use the roads, cyclists should assert their importance by pointing out how their use of the roads improves public health —  not only by increasing their rate of exercise —  but by lowering the public’s risk of developing air pollution related diseases. While some see cars as more modern, it’s really bicycles which are leading the way to a future with a healthier populace and less air pollution. Fuel independent transportation is the next step in creating a world better designed to promote good human health and a higher quality of life.





1. Zielinska B, Sagebiel J, McDonald JD, Whitney K, Lawson DR. Emission rates and comparative chemical composition from selected in-use diesel and gasoline-fueled vehicles. J Air Waste Manag Assoc 2004; 54: 1138-50.

2. Gilman P. Health assessment document for diesel engine exhaust, EPA/600/8-90/057F. Washington, DC: US Environmental Protection Agency; 2002.

3. Heinrich J, Wichmann HE. Traffic related pollutants in Europe and their effect on allergic disease. Curr Opin Allergy Clin Immunol 2004; 4: 341-8.

4. Dybdahl M, Risom L, Bornholdt J, Autrup H, Loft S, Wallin H. Inflammatory and genotoxic effects of diesel particles in vitro and in vivo. Mutat Res 2004; 562: 119-31.

5. Jarvholm B, Silverman D. Lung cancer in heavy equipment operators and truck drivers with diesel exhaust exposure in the construction industry. Occup Environ Med 2003; 60: 516-20.

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