One hour of grass cutting equals 100 miles worth of auto pollution

May 28, 2001

The air pollution from cutting grass for an hour with a gasoline-powered lawn mower is about the same as that from a 100-mile automobile ride, according to a new study from Sweden, which recommends using catalytic converters on mowers. The report is the first to compare lawn mower pollution with auto mileage, according to the researchers.

The recommendation is reported in the June 1 issue of Environmental Science & Technology, a peer-reviewed journal of the American Chemical Society, the world's largest scientific society.

One significant pollutant from mowers is polycyclic aromatic hydrocarbons, or PAHs, said Roger Westerholm, Ph.D., an assistant professor at Stockholm University in Stockholm, Sweden. He claims such emissions, similar for both riding and push mowers, can be cut more than 80 percent using a catalytic converter like those used in automobiles.

Westerholm found that the worst case of lawn mower PAH emissions totaled more than 4,000 micrograms per hour using unleaded fuel without a catalytic converter. Average emissions dropped to nearly 800 micrograms over the same time period with the addition of a catalytic converter, he said. Some PAHs, including a few in lawn mower emissions, are classified as probable carcinogens by the U.S. Centers for Disease Control and Prevention.

"Obviously, if catalysts will become mandatory on lawn mower engines, and possibly other small engines as well, a significant reduction of exhaust components will be achieved," Westerholm said.

In 1998, the U.S. Environmental Protection Agency issued the so-called "Phase I" rules, which mandated a 32 percent reduction in emissions for small "non-road" engines. This affects all engines less than 25 horsepower produced after 1997, including mowers, leaf blowers and chain saws. According to an EPA study prior to the Clean Air Act of 1990, small engines from lawn and garden equipment make up nearly 9 percent of some types of air pollution. While current mowers meet the reduced emissions standards, catalytic converters would lower emissions levels further, Westerholm said.

In the Swedish testing, the researchers used regular unleaded fuel in a typical four-stroke, four horsepower lawn mower engine and found, after one hour, that the PAH emissions are similar to a modern gasoline-powered car driving approximately 150 kilometers (93 miles). A typical push-type lawn mower is run for an average of 25 hours per year, according to the Outdoor Power Equipment Institute. A higher-octane fuel known as alkylate also was tested and resulted in lower emissions. Alkylate is difficult to find in the United States and significantly more expensive than regular unleaded fuel in Europe.

Catalytic converters are already available on some European mowers, Westerholm reported. The pollution-control devices have been required on U.S. made cars since the late 1970s.

"Using a catalyst would help prevent most emissions from small engines," he said. "Of course, people could also use an electrical-powered lawn mower instead."
The research cited above was funded by the Swedish Environmental Protection Agency.

The online version of the research paper cited above was initially published May 4 on the journal's Web site. Journalists can arrange access to this site by sending an email to or calling the contact person for this release.

Roger Westerholm, Ph.D., is an assistant professor in the department of analytical chemistry at Stockholm University in Stockholm, Sweden.

American Chemical Society

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