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Ask the Ecologist-May 2008

Q: I heard that walking behind a lawn mower for an hour is like hanging your head behind your car tailpipe for 100 miles. Could this be true? – Nick A: According to the California Air Resources Board (CARB), 2006 lawn mower engines contribute 93 times more smog-forming emissions than 2006 cars. Why are lawn mowers so much worse than cars? Mowers use a single-stroke spark engine with no catalytic converter. That means these engines burn a lot more oil and gas and there is virtually no filter for their emissions.  Assuming the 93 multiplier rate from CARB is correct, then running your lawn mower for an hour is more like having your head behind a car tailpipe for a few days – much worse than what you heard. While the mower’s particular blend of emissions is different from cars, the major disparity between the two motor types’ emissions is the real take-home message: lawn mowers are really bad polluters. But if the US Environmental Protection Agency (EPA) finalizes its proposed regulations, lawn mowers will get a lot better after this summer. In 2007, EPA proposed to regulate small land and marine engines by requiring catalytic converters and other improvements. EPA estimates that by 2030, these emission reductions would prevent 450 particulate matter-related premature deaths, approximately 500 hospitalizations, 52,000 work days lost, and other quantifiable benefits every year. The total estimated annual benefits of this rule in 2030 are approximately $3.4 billion. The engines and vehicles covered by EPA’s proposal are significant sources of smogforming pollution and other health impacts. They account for about 25 percent of mobile source hydrocarbon emissions and 30 percent of mobile source carbon monoxide emissions. This is a disproportionate amount when you consider that these engines make up only 1 to 2% of the total single engines in use presently – including cars and trucks. The answer to your question also somewhat depends on what type of car you are driving, how well tuned it is, and how you drive it. Manual transmission, well-tuned engines, and more recently produced cars have fewer emissions. But the gist of the answer to your question is that lawnmowers currently produce a heck of a lot of bad stuff. Hopefully they’ll get better soon after EPA’s regulation is put into law. Try a push mower this year – help make our air cleaner, save some dough, and get some exercise while you mow. Send your ecological inquiries to our resident ecologist at