Fall will soon be here and with it, clusters of trees with tri-color leaves. As leaves fall, some wonder whether electric leaf blowers, lawn mowers or rakes are best for the environment. (Tracy A. Woodward/The Washington Post)

To most people, falling leaves are a beautiful sign of the changing seasons. But for many Washingtonians, the rite of fall is just another reason to have an argument. Residents of some localities have even pushed for a ban on gasoline-powered leaf blowers, which they find noisy and say are bad for the environment. Their opponents argue that banning the blowers would make leaf management arduous and prohibitively expensive. Failure to manage leaves means unattractive lawns, falling home prices — and perhaps the end of Western civilization.

I’ll leave the question of leaf blower noise to neighborhood discussion groups, but I can help resolve the environmental question: Exactly how bad for the Earth are gas-powered leaf blowers?

Much of the argument has to do with the two-stroke engine found in many of them. The two-stroke engine — so named because it completes one cycle of internal combustion in two movements of the piston — is lightweight, cheap, compact and simple, which makes it a handy motor not just for leaf blowers but also for chain saws, lawn mowers and jet skis. (There are also four-stroke leaf blowers, which use the same type of engine that powers your car and offer more complete combustion and less air pollution, but they are typically larger and more expensive.)

The two-stroke engine has developed a reputation as an environmental hazard. Because the engine lacks an independent lubrication system, fuel has to be mixed with oil. More important, about 30 percent of the fuel the engine uses fails to undergo complete combustion; as a result, the engine emits a number of air pollutants. Carbon monoxide, nitrous oxides and hydrocarbons escape from the engine in large quantities. Everyone knows the acute effects of carbon monoxide, but the other gases are equally worrisome. Both nitrous oxides and hydrocarbons contribute to smog formation. Hydrocarbons can be carcinogenic, and nitrous oxides can cause acid rain.

Cities where two-stroke engines are in particularly wide use suffer terribly from air pollution. Some of India’s urban centers, for example, are draped in heavy soot, a problem due in large part to auto-rickshaws powered by two-stroke engines. More than a decade ago, Delhi phased out tens of thousands of auto-rickshaws with two-stroke engines in favor of those with four-stroke engines that run on natural gas. This alleviated the pollution somewhat, but few cities have followed Delhi’s lead.

How a two-stroke engine works

In leaf blowers, two-stroke engines have been shown to emit contaminants comparable to large automobiles. A 2011 test by the car experts at Edmunds showed that “a consumer-grade leaf blower emits more pollutants than a 6,200-pound 2011 Ford F-150 SVT Raptor.” The company subjected a truck, a sedan, a four-stroke and a two-stroke leaf blower to automotive emissions tests and found that under normal usage conditions — alternating the blower between high power and idle, for example — the two-stroke engine emitted nearly 299 times the hydrocarbons of the pickup truck and 93 times the hydrocarbons of the sedan. The blower emitted many times as much carbon monoxide and nitrogen oxides as well. The four-stroke engine performed significantly better than the two-stroke in most of the categories, but still far worse than the car engines.

The takeaway is that if you fret about the air pollution coming out of your car’s tailpipe, you should avoid gas-powered leaf blowers. While it’s true that their contribution to overall air pollution is modest, that’s largely because so few people rely on them. As their usage grows, so will the environmental effects.

Leaf blowers are less of a concern with regard to carbon dioxide emissions. The numbers vary widely, but a leaf blower uses significantly less gasoline per hour than a car, and they spend far less time in use.

All of this goes to say that, if you use a leaf blower, you should opt for an electric model if possible. As with cars, an electric model will not eliminate your contribution to the production of greenhouse gases. Electric motors shift fuel combustion from the device to a power plant, but the power still has to come from somewhere, which means carbon dioxide will be released. (More than 80 percent of U.S. electricity comes from fossil fuels.)

But switching to electric would sharply reduce air pollution. Power plants are equipped with scrubbers to filter out pollutants, the kind of technology that could never be attached to a hand-held leaf blower. Consumer Reports says that, for ordinary yards, electric blowers perform comparably to gas-powered models.

Before anti-leaf-blower activists get too high and mighty, though, a brief word to keep this all in perspective. As mentioned above, leaf blowers aren’t the only household tool that uses a two-stroke engine. Many lawn mowers also rely on the technology. A Swedish study released in 2001 showed that one hour of using a gas lawn mower releases approximately the same amount of air pollutants as 100 miles of driving.

For reasons I’ve never fully understood, the campaign to ban gas-powered lawn mowers hasn’t been nearly as intense as the one over leaf blowers. It can’t be the lack of alternatives: The hand-push lawn mower has been around since 1830 and is still widely available, even if it doesn’t produce a lawn with the putting-green feel that a gas-powered mower does. I suspect it’s merely a matter of inertia. Leaf blowers aren’t as entrenched as lawn mowers. But if we’re going to get serious about yard work and pollution, let’s not let a thin layer of leaves obscure an equally significant problem.