To cut to the chase: I use a 21″ push mower with a modern Honda overhead-valve engine. Starting from EPA data on emissions for engines of that type, I calculated two simple rules of thumb, for the pollution generated by my lawn mowing.
If the standard of comparison is the typical car on the road — call it a mid-2010s full-sized sedan — then gas lawn mowers are 100 times dirtier than gas cars, per horsepower. And an hour of mowing generates about as much pollution as an hour of driving.
That’s just the mower. That doesn’t include emissions from your gas can, as outlined in the just-prior post.
Background
As I continue my deep dive into pollution from lawn mowers, I come across ever-more-outrageous comparisons between lawn mowers and cars. Recall that the prior high-water mark was a claim that boiled down to “lawn mowers burn 12 gallons of gasoline per hour”.
But that prior one doesn’t hold a candle to this:
Source: This thing.
This time, it’s the claim — attributed to the EPA without citation as to source — that a single gas lawn mower produces as much pollution as 43 new cars, each driven for 12,000 miles per year. That’s more than a half-million vehicle miles.
That’s a pretty good trick, from burning two gallons of gas per year, in my lawn mower. That doesn’t seem probable, to me. And certainly doesn’t account for the global warming impact.
And at that same time, you can find plenty of seemingly-reputable sources telling you that the EPA says one hour of mowing equals 45 miles in a new car. Like this one, from the State of New Hampshire (reference).
Despite the change of units (one year, one hour), and possibly what is being measured (“pollution”, gasoline vapor and NOX), this is what we in the business term a high variance statistic. Plausibly somewhere between 45 and 500,000 miles.
And neither one was trying to minimize the pollution caused by lawn mowers. So it’s plausible that even the New Hampshire example chose the statistics that made lawn mowers look as bad as possible.
I’m not pro-gas-lawn mower. I’ve already electrified the big-ticket items for my carbon footprint (car transport, home heating). I just want a serious answer to the question, how much pollution does my 21″ gas mower likely emit?
For the environmental impact I truly care about — global warming — I know that the impact of my lawn mowing is rounding error. I use two gallons of gas a year. And I’ve already fixed my mistake of using an ancient pre-CARB vented plastic gas can. See last post. Now I just want a serious, even-handed, non-propaganda answer to the question, how bad is a gas lawn mower, in terms of air pollution?
Recap
In four posts so far, I think I’ve nailed down two things.
First, the often-cited statement that one hour of lawn mowing produces as much pollution as 300 (ish) hours of driving a car appears to derive from a comparison of two EPA standards. That’s the Tier 2 automotive air pollution standards (in place through roughly 2017) and the 2011/2012 EPA standards for non-road spark ignition engines.
These standards determine the maximum amount of certain pollutants that new car or non-road gas engines may emit. They don’t tell you about actual, average emissions by cars and mowers currently in use.
For the two main smog-forming exhaust gasses — unburned hydrocarbons and nitrogen oxides (NOx) — the cleanest “bin” for new passenger cars under the Tier 2 standards is 0.125 grams per mile, while the 2012 EPA standard held a typical gas lawn mower engine to emit no more than 10 grams per kilowatt of peak power, per hour. In the case of my particular lawn mower, which Honda rates for 3.3 KW of output, that works out to 33 grams per hour. Doing the math, an hour of mowing is allowed to emit as much of these smog-forming gasses as (33/.125 = ) 264 hours of driving a car falling into the lowest-polluting “bin” under those Tier 2 standards.
But there were a couple of major loose ends. On the one hand, the California Air Resources Board (CARB) specifies “commercial mower”, suggesting that in practice, that 300-to-1 ratio only holds for very large lawn mowers. But on the other hand, the current Tier 3 standard for cars is so much cleaner than the older Tier 2 standard that one hour of mowing is allowed to emit as much pollution as 1100 miles of driving a car meeting the current Tier 3 pollution standards.
Second, old gas cans can be bad gas cans. In particular, I’ve ditched my old vented plastic can. Per the California Air Resources Board, storing gas and refueling with old gas cans may result in as the emission of as much smog-forming compounds as running the lawn mower. That’s not just due to spilling gas, or having gas vapors go out the vent in the can. The big surprise to me is that the plastic used in those old cans is quite permeable to gasoline. I’m in the process of weighing my old gas can daily. If current trends continue (2 grams per day lost, in a cool garage), the slow, small daily losses through the sides and vent of the can will add up to more annual gasoline vapor release than occurs for the typical U.S. passenger car.
That said, I still haven’t found a good, reliable answer to the core question.
But what about actual emissions?
What I actually want to know is how polluting my own, particular lawn mower actually is. This is a 21-inch push mower with a Honda GCV-160 engine. That’s a mostly-modern engine design with overhead values and 8.5:1 compression, producing somewhat over four horsepower. (This, in contrast to old-school Briggs and Stratton engines, roughly 6:1 compression, no overhead valves, about 3.5 horsepower.)
The best I can do is identify what the EPA used to model a lawn mower like mine. The source document is this:
Exhaust Emission Factors for Nonroad Engine Modeling, — Spark-Ignition, Assessment and Standards Division, Office of Transportation and Air Quality, U.S. Environmental Protection Agency, EPA-420-R-10-019, NR-010f, July 2010.
This 2010 document is based on a survey of what was known about small-engine emissions as of 2008. Per the as of 2008, and per the writeup, that wasn’t much. (“The emission factors used in these prior efforts have been based on a very small number of engine studies, particularly when compared to the large body of data available for highway vehicles.) That said, these factors are what the EPA used (and, I believe, continues to use) in estimating the total fraction of all U.S. air pollution that is attributable to off-road gas engines.
There we find this table:
Source: EPA, full citation above.
In RED, I’m showing you that this is the kind of engine I have on my lawn mower. Not hand-held, four-stroke, overhead valve. I’m using the Phase 3 engine because I think that refers to engines to be sold after the 2012 Phase 3 EPA limits went into place.
In GREEN are three key figures. On the far right, that’s gas consumption (in pounds of gasoline per horsepower per hour). The other two are going to be combined in to grams of unburned hydrocarbons and oxides of nitrogen, so I can compare this to current motor vehicle standards.
The key fact here is that I use about two gallons of gasoline per year. So I’m going to figure out how much total unburned hydrocarbon and NOX an engine of this sort would generate, from two gallons of gas. (The trick there is that “per horsepower” washes out of all the numbers, when I normalize it to gas consumption per hour per horsepower). Then I’ll see how far you’d have to drive a modern car, to create the same amount of air pollution (of these two pollutants). The result will be the number of vehicle-miles that generates the same amount of pollution as one year’s worth of lawn mowing.
Based on the published EPA data (using studies from 2008 and earlier), I’d expect that running two gallons of gas (12 pounds or approximately 5600 grams of gas) would get me about 15 hours of run time. (In reality, I’d guess it should take about 20 to keep the lawn mowed for one year, so that’s ballpark).
Running that gas through my mower would generate a combined 80 grams of unburned hydrocarbons and oxides of nitrogen. That’s based on the rate that the EPA thinks those are created, per pound of gasoline burned.
The cleanest pre-2017 car would have to drive 642 miles to generate that much, and the average modern (Tier 3 standards) car would have to drive 2673 miles to generate that much.
I think pretty much everything about that estimate is plausible. I get roughly the correct run time out of that 12 pounds of gasoline. As importantly, the Tier 2 standard benchmarks well with the New Hampshire government publication above. Based on the cleanliness of Tier 2 cars, each hour of lawn-mower run time equals about 41 miles of driving the cleanest Tier 2 car. Quite close to the 45 miles cited by New Hampshire (among others).
If you think about that Tier 2 result — one hour of mowing is 41 (or maybe 45) miles of driving — that says that a modern overhead-valve small lawn mower is roughly 100 times dirtier than mid-2010s cars, per horsepower. That is, driving a roughly 3 HP lawn mower for an hour generates as much of these two pollutants as driving a roughly 300 HP car for an hour.
Or, again in round numbers, spending an hour mowing your lawn, with a small modern overhead-valve lawn mower, is like spending an hour driving in a typical mid-2010s vehicle.
The typical American has a half-hour one-way commute. And drives solo. So the impact of an hour a week of mowing a half-acre lawn is like adding another day of commuting. Certainly bad for the environment. But hardly the half-million-miles per year that some have blown this problem up to be.
My action items.
The comparison above may modestly overstate current pollution levels from modern overhead-valve lawn mowers, based on the age of the underlying small-engine data. But not enough to make any material difference, I think. Absent the addition of pollution-control equipment, any reductions in pollution would occur at a slow and incremental pace as current engine designs are refined.
What you see above is therefore my final answer to this question. In round numbers, best guess, if you drive a typical mid-2010s vehicle, and run a small overhead-valve lawn mower, your mowing hours are roughly as bad as your driving hours. Which is, in effect, what the government of New Hampshire said.
Your gas can, though, that’s its own source of pollution and a separate issue. Based on a little bit of research and a sniff test, I’ve ditched my old plastic vented pre-CARB gas can.
Beyond that, I’m not feeling any urgency about changing what I’m doing now.
I maintain my gas lawn mower properly. But that’s as much an issue of cheapness (and proper technique) than it is environmentalism. Change the oil, clean the air filter, and sharpen the blade at the start of every season. I could certainly stand to sharpen the blade twice a season. I’m not sure what that would gain me.
But I’ll keep this issue on the radar screen, because it’s clear that electric lawn mowers have a lower carbon footprint and lower overall emissions. That’s after accounting for the energy costs of creating and recycling (?) those big lithium-ion batteries. (I’ve done corded electrics and that’s just a non-starter for this lawn, so if I return to electric mowing, it’ll have to be battery-powered electric).
Really, the only thing left to do is to mow less. We’ve slowly been reducing the amount of lawn area, and this year I’m planting about a quarter of the back yard to vegetables and flowers.
But probably the single biggest improvement I plan to make this year is to propagate Zoysia grass from my front yard to my back yard. Zoysia is a slow-growing warm-season grass. It was originally promoted in the US, by the USDA, as a way to reduce environmental damage associated with fertilizers, weed killers, and mowing. My wife assures me it needs a lot less mowing than whatever-it-is that’s now growing in the back yard. So I’m going to shoot for an all-Zoysia lawn by the end of the summer, using the Pro Plugger below.
So that’s it. New gas can. Keep up the maintenance. Continue to reduce lawn area. Propagate slow-growing grass. And rethink this if and when the current gas lawn mower dies.
