Engineers: in big picture, gas, cordless mowers equal polluters

March 21, 2005

GAINESVILLE, Fla. — Gasoline mowers pollute more than their electric cousins – or do they?

A new study by University of Florida engineers confirms that traditional gas-powered mowers belch far more smog-forming pollution than battery-powered or corded mowers. But the study, described in an article in the current issue of Environmental Engineering Science, also finds that lead pollution stemming from production of the battery in cordless mowers may contribute significantly more carcinogens and other toxins to people and the environment.

“I think what surprised me the most is the fact that the battery-powered mowers have the potential to cause as much impact as the gas-powered variety,” said Angela Lindner, a UF assistant professor of environmental engineering and author of the study.

Nongasoline-powered mowers, which are becoming more available and popular, are often touted as environmentally benign, “zero emission” machines.

That’s true if the only emissions considered are those generated while the mower is cutting grass. But the UF study is much broader. Known to engineers as a “life cycle analysis,” the study is the first to compare pollution from cordless, corded and gasoline-powered mowers in the context of their entire lifespan — from when miners retrieve the raw materials for their construction through the mowers’ use on lawns and on to when owners haul them to the dump.

“What this kind of study does is give you an honest assessment of the differences, and sometimes the results may surprise people,” Lindner said.

The study took into account, for example, that corded and cordless mowers are not entirely emissions free during their use, because coal-fired electricity plants are significant polluters. For gas mowers, it considered pollution not only from the mowers themselves but also from gasoline refineries.

The researchers focused on 10 commercially available gasoline mowers, one battery-powered mower and one corded mower. To ensure a representative sample, they chose some gasoline-powered mowers built to the Environmental Protection Agency’s latest emissions-limiting standards, which became effective in 2001, and others built under less stringent standards enacted in 1997.

To gauge the mowers’ emissions during use, Lindner and co-author Deepak Sivaraman drew on testing data from the EPA’s National Vehicle and Fuel Emissions Laboratory. The Ann-Arbor, Mich.-based laboratory tests mowers’ emissions as part of the EPA’s regulatory efforts.

The researchers also made some assumptions. For example, they estimated, based on past studies, that the average American mows a half-acre lawn 26 times each year while walking about 2 mph. Assumptions that the average working lifespan of a gasoline mower is eight years and of a lead-acid battery about 4 1/2 years helped the engineers calculate how much fuel or electricity each model of mower would use, and thus how much pollution it would cause.

With all that taken into account, the study confirmed the conventional wisdom in some ways. Most prominent, it found that gasoline-powered mowers result in as much as 1,500 times more carbon monoxide, 31 times more hydrocarbons and nitrogen oxides and 18 times more carbon dioxide than the electric varieties. All the compounds are ingredients of smog, which has been tied to health problems including asthma in schoolchildren in heavily polluted cities.

Nationwide, gas-powered mowers are estimated to produce as much as one-tenth of the smog-forming pollutants from all mobile sources, said Phil Carlson, an engineer at the National Vehicle and Fuel Emissions Laboratory. Although the EPA’s regulations have reduced mowers’ pollution by 70 percent since 1997, the agency is working on a third set of regulations expected to make mowers still cleaner after 2010, he said.

“A lot of people say, ‘They’re such small engines,’ which is true, but there are a lot of them out there” so they are significant contributors to air pollution, Carlson said. The UF study also found that the mining process used to produce the material for the cordless mowers’ batteries makes these mowers big contributors of lead emissions.

As a result, “the larger airborne lead emissions from the battery-powered lawn mower’s life cycle compared to the gasoline-powered mower’s implies a far greater risk of both noncarcinogenic and carcinogenic health effects from the battery-powered lawn mowers,” the study says.

Lindner said the problem is compounded when consumers don’t dispose of the battery properly, with the result of a potential leak from a landfill. “One conclusion of this story is we need a better policy for recycling these batteries,” she said.

By far the least polluting mowers were the corded variety, where pollution was limited to that caused by the power plant. But these mowers seem to suffer from an intractable image problem, Lindner noted. “Talk to some guys and gals about corded mowers and they’ll just curl their lips in protest,” she said.

Sivaraman, who worked on the study while a master’s student at UF and is now a doctoral student at the University of Michigan, said one consideration for consumers trying to make an intelligent choice may be that gas-powered mowers cause more pollution locally.

“I would still suggest the battery mowers because the lead emissions are not released during the usage stage but during the battery manufacturing stage, implying that it is not directly near people,” he said. “The emissions from the gas mower are near people and most importantly, children.”