Does using a gasoline-powered lawn mower produce as much pollution as driving an SUV 300 miles?

Dear Cecil: Are gasoline-powered lawn mowers really all that bad? I keep reading that cutting my grass with one is roughly comparable to burning a tire in the backyard or driving an SUV for 300 miles. Is there any truth to this? My mower holds about a quart of gasoline, and it uses less than half of that to cut my yard in 20 minutes. And like most mowers, it has a 3.5-horsepower four-stroke engine, so it doesn’t burn oil like a two-stroke would. An SUV would burn about 30 gallons in 300 miles. Is there really that much available pollution in a pint of gasoline? Steve

Cecil replies:

You’d be surprised, my friend. However, I agree this is a perplexing subject. Browsing around online, I find the following alarming statements:

Claim 1. Operating a gasoline-powered mower for an hour produces as much pollution as driving a car 200 miles. OK, 200 is less than 300, and the vehicle is an unspecified but presumably average auto rather than a gas-guzzling SUV (although even a Hummer H3 these days gets 13-14 miles per gallon in the city, not ten as you suggest). Still, that’s a lot of pollution.

Claim 2. One mower-hour = “40 automobiles driving.” That sounds even worse than one car traveling a few hundred miles.

Claim 3. Per hour, a gasoline push mower = 11 cars, a riding mower = 34 cars. Let’s strike an average and say one mower-hour is equivalent to 20 car-hours. Still not great, but we’re talking 100 percent improvement for a 30-second investment in Google. Not bad.

Claim 4. One mower-hour = 350 miles in a car. Now mowers are back in the toilet. This page even cites a source, a book called Redesigning the American Lawn (Bormann et al, 1993). However, a parenthetical note says, “This information, though valid at the time of publication, is no longer accurate,” and that “based on current calculations,” one mower-hour produces “the amount of pollution emitted by a car driven for approximately 20 miles.”

Wait a minute. We’ve gone from 350 to 20. That means either lawn mowers have gotten 17½ times cleaner or cars have gotten 17½ times worse. Neither seems likely. We’d better consult with the experts, you may think. How about the Environmental Protection Agency?

Oh, wait. The EPA website is where all these contradictory numbers appear.

I called anyway. “We now prefer not to compare one [pollution] source to another,” an EPA spokesperson told me. Translation: Having thoroughly confused things, we’re walking. So once more it was up to my assistant Una to run the numbers.

To simplify matters, we compared the maximum pollution allowed by federal law for mowers versus cars, and assumed our benchmark grass cutter was a six-horsepower push mower operated at half throttle. We were interested in two types of pollutants: carbon monoxide, or CO, and hydrocarbons plus nitrogen oxides, which we’ll call HC+NOx.

Under current standards, in an hour a push mower will produce the same HC+NOx as a car driven 257 miles, and the same CO as one driven 401 miles. To put it another way, assuming a car averages 40 miles per hour, a push mower produces more HC+NOx than six cars and the same CO as ten.

Things will improve when federal emissions standards for lawn mowers are tightened in 2012. Under the new standards, a push mower may produce as much HC+NOx as a car driven 160 miles — in other words, one lawn mower would equal four cars.

Big deal, you say. I run my lawn mower 20 minutes a week. How much damage could I be doing?

This is narrow thinking. Looking at the big picture, we realize mower emissions are only the beginning of what’s wrong with American lawn care. Consider:

Estimates vary wildly, but it’s likely Americans burn more than 600 million gallons of gasoline a year cutting the grass. Hell, the EPA estimates at least 17 million gallons of gasoline are spilled annually just filling lawn mowers.
In 2009, according to the Consumer Product Safety Commission, 86,000 injuries involving lawn mowers required a trip to the emergency room; in 6,400 of these cases the victim was dead on arrival or wound up hospitalized. The CPSC estimated in 2003 that lawn mower accidents cost us $5.4 billion per year.

But perhaps you remain blasé. Who needs all those toes? OK, one last point:

In a time of dwindling water supplies, somewhere between a third to half of residential water use is for lawn and garden irrigation, and about half of that water is wasted by poor watering practices.

Fact is, unless you’re a croquet fanatic, you don’t need all that grass. The green parts of the planet generally manage to stay green on their own. My natural plantings look like weeds to you? Fine, be a Neanderthal. I’m just saying there’s another way.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.