According to a Swedish study published in 2001, operating a typical four-stroke, four horsepower lawn mower for an hour produced the equivalent emissions of polycyclic aromatic hydrocarbons (PAHs) as driving a car for 150 kilometers (93 miles). This just goes to show that the environmental impact of lawn mowers can really add up.
Indeed, the Outdoor Power Equipment Institute estimates that, on average, people use their lawn mowers for 25 hours a year; this is the equivalent, then, of driving 3,750 kilometers (R). So, if you’re already concerned about the carbon footprint of your driving, it’s also worth looking at what’s lurking in your shed.
Now, that said, things have improved since the Swedish researchers ran their tests almost two decades ago. For one thing, the U.S. Environmental Protection Agency (EPA) implemented new regulations to curb emissions from smaller ‘non-road’ engines. Their first stab at this was actually back in 1998, with ‘Phase I’ rules mandating a 32% reduction in emission from these engines. This was based on concerns that mowers, leaf blowers, chain saws, and other garden equipment was responsible for almost 9% of some types of air pollution. Phase 3 exhaust emissions standards took effect in 2011 or 2012, depending on the size of the engine. These standards have further helped to reduce emissions, but the fact remains that gas powered lawn mowers produce far more emissions than electric lawn mowers.
Air pollution from gas lawn mowers
Older two-stroke engines are terribly inefficient in terms of fuel burning, with about 30% of engine fuel failing to undergo complete combustion. This means greater levels of air pollutants are produced, including carbon monoxide, nitrous oxide, and hydrocarbons; the latter two leading to increased smog formation. Hydrocarbons also include known carcinogens.
Other emissions from lawn mowers include methane, ethane, ethene, ethanol, and:
- Nitrogen oxides – these contribute to acid rain and result from subjecting nitrogen and oxygen in the air to the high temperature and high-pressure conditions in an internal combustion engine. Nitrogen oxides also react with hydrocarbons in the atmosphere to form ground-level ozone that can damage lungs. Acid rain can damage infrastructure, buildings, wildlife and vegetation (including your lawn!).
- Particulates – microscopic airborne particles emitted in the exhaust from diesel-fueled vehicles. These contribute to smokiness and the smell from gas powered mowers and damage the respiratory system, causing breathing difficulties especially in infants, seniors, and anyone with pre-existing health concerns.
- Carbon monoxide – a colorless, odorless, poisonous gas that results from incomplete fuel combustion.
- Carbon dioxide – the end product when burning gasoline and other carbon-based fuels. While carbon dioxide does not directly damage human health, it is a greenhouse gas and contributes to climate change, which has its own detrimental effects on health.
Cities that have a high number of things such as auto-rickshaws with two-stroke engines have some of the worst air pollution and heavy soot. This was the case in Delhi, in India, but a push to phase out these engines in favor of four-stroke engines has helped lower smog levels.
Unfortunately, as some cities move towards reducing air pollution from small engines, other cities are seeing an uptick in air pollution as more people start using things like gas-powered leaf blowers and snow blowers instead of raking and shoveling by hand. This is a double whammy for respiratory health as people reduce their levels of physical activity while simultaneously making air quality worse.
One 2011 study by car experts at Edmunds found that a consumer-grade leaf blower pumped out more pollutants than a 6,200-pound 2011 Ford F-150 SVT Raptor. In their tests, a two-stroke engine leaf blower emitted nearly 299 times the hydrocarbons of the pickup truck and 93 times the hydrocarbons of a sedan, in addition to many times more carbon monoxide and nitrogen oxides. A four-stroke mower was better than the two-stroke but still emitted more pollutants than either vehicle.
In the US, all gas mowers sold since 2012 should meet the CARB/EPA 50-state emissions standards for emissions. Some mowers exceed these standards, which further lowers emissions and keeps your running costs low.
For example, Honda have consistently exceeded required standards for small engine emissions year after year (R). Beginning with their 2001 model, Honda Class II GX340 and GX390 engines, designated as EPA Class II engines with an engine displacement greater than 225 cc, met the 2005 requirements of CO 12.1 g. As such, I tend to favor Honda for my Leaf Score picks for gas-powered mowers.
One other thing to think about when weighing up the pros and cons of gas mowers versus electric is the need to refill the gas tank. Refilling gas-powered lawn mowers, leaf blowers, and other garden equipment is a messy business and can easily lead to fuel spills. This spilt fuel can get into your lawn, garden beds, groundwater, nearby ponds, and so forth, and some of the oil evaporates into the air as volatile organic compounds. Not exactly great for the environment, or your bank balance.
Are electric lawn mowers really better for the environment?
While it’s clear that electric mowers are quieter and better for the immediate environment (i.e. your back yard) than a gas guzzling mower, they are only really eco-friendly if the electricity that runs them is produced cleanly. If your electric mower is powered by a grid powered by coal plants, this just means the pollution is produced upstream and out of sight.
However, even if your electric mower is powered by electricity produced from burning coal, it will still be somewhat ‘cleaner’ than burning gasoline to mow your lawn. That’s because power plants are more efficient than small engines at using carbon-based fuels and have a range of mechanisms in place to reduce and capture emissions (although these standards are being eroded in the US currently). Using electricity also totally eliminates the risk of fuel spills and gas emissions in your back yard.
Ideally, of course, your electric mower will be both energy efficient and powered by clean energy. In practice, this means having the luxury of choosing a clean energy provider for your whole home, pressuring government to implement more robust clean energy policies, setting up a micro hydro system if you have a stream on your property, operating a wind turbine, or using a solar panel to charge the batteries.
Or, you could use a reel mower like the Earthwise 16-Inch Reel Lawn Mower, which is designed to cut Bermuda and Zoysia grass. This type of mower produces no emissions at all, although a reel mower is clearly impractical for lawns larger than around 10,000 square feet, unless you’re very fit or using lawn mowing as exercise.
Some hybrid solar electric robot mowers are now available, but the solar powered walk-behind mower seems oddly absent from the market. If you’re game, you could rig up a solar panel to charge your mower’s batteries yourself. In most cases, a full charge using a 25-Watt solar panel will take about 2-3 days, assuming good sun exposure, or twice that time if it’s overcast. Before you rig up this system, though, make sure your battery charger is compatible with the solar panel or other clean energy source.
The downside of electric mowers
I should mention one major downside of electric cordless mowers: dead batteries ending up in landfill. Sadly, it is not currently cost effective to strip batteries for parts, which means they are usually frozen, crushed, and sent to landfill when they no longer hold a charge. The chemicals in lithium ion batteries aren’t a big cause for concern when this happens (unlike older lead acid batteries). Nonetheless, dumped rechargeable batteries contribute to waste in landfill.
Depending on where you live, you may be able to take your old batteries to be recycled. It’s best to check with your local government office or do a quick internet search for organizations doing this kind of work in your area. And, of course, treating your batteries well will help delay the need to replace them! This means taking care to avoid storing them anywhere excessively hot or cold, and making sure to charge them up after each use. Storing a lithium ion battery with less than 30% charge can reduce battery life.
Aside from the environmental impact of lawn mowers, there are, of course, plenty of other things to consider when buying a new lawn mower. Cutting deck width and size, noise and vibration output, and even the size of the wheels and the drive function are all key factors that can mean you love or hate your mower. If you’re already familiar with the term ‘measured mean value’, you may want to jump ahead to my recommendations for the best electric mowers, gas powered mowers, and robot mowers.