Our Better Nature: Do Trees Cause Air Pollution?

To ask if trees are bad for the environment seems absurd. One may as well ask if water is unhealthy for us. And yet, drinking too much water in a short period can cause “water intoxication,” leading to brain swelling, coma, and on occasion, death. Although water can be harmful in exceptional cases, we should keep drinking it, clearly.

On the other hand, the question of whether trees hurt the planet is a knotty one. When President Ronald Reagan said in 1981 that “Trees cause more air pollution than automobiles do,” he was widely mocked. However, he had a point. On hot sunny days, trees give off volatile chemicals that indirectly cause serious air quality issues.

In the presence of sunlight, the compounds trees emit can react with nitrogen oxides from auto and truck exhaust to form ground-level ozone. It’s a major lung irritant and contributes to smog as well. In the stratosphere, ozone protects us from getting fried by ultraviolet radiation; down low, it can fry our lungs.

Trees worsen ground-level ozone only if car exhaust and sunshine are plentiful. Though it’s not good news, it doesn’t inspire me to run out and paint all trees with the same critical brush. For one thing, I’ve got other stuff to do, plus I’m sure the bristles would wear out pretty quick.

Trees cause environmental damage in other ways, as well. It turns out that many trees liberate methane from the soil, where it normally remains locked up. The mechanism by which trees do this isn’t clear, but it’s a measurable effect. Methane is a greenhouse gas at least 28 times more potent than carbon dioxide in its ability to warm the Earth.

And the news gets worse for trees: Some species, notably cottonwoods, actually make methane, thanks to microbes that colonize their live tissues. We’ve long known that dead trees — and vegetation of all sorts — create this planet-warming gas. Scientists must now consider living forests as methane sources in their climate-change models.

Uh-oh. Maybe we’ve been thinking too highly of trees!

This is where it feels like Mother Nature is pulling our leg. In 2024, scientists found that the corky outer layers of tree bark absorb around 50 million tons of methane per year. No other process on Earth removes more of this gas from the atmosphere. Although trees can release soil-based methane, and sometimes create a bit of their own, they are still net methane sinks.

So, score a point for the trees. But they’re not out of the woods yet. In terms of proving that trees aren’t shady characters, we’ll have to beat the bushes for more evidence in their favor.

Fortunately, we don’t have to look far. According to the USDA Forest Service, trees reduce overall sulfur dioxide pollution by 14 percent. They also take a lot of nitrogen oxides out of the air. To give these things real-life context, breathing sulfur dioxide and nitrogen oxides can aggravate asthma, and even cause irreversible lung damage.

According to one study, trees vacuum out more than 50 percent of airborne particulate matter. Particulates matter because every year, about 6.4 million deaths worldwide are caused by this kind of pollution. Breathing airborne particles raises our chances of asthma attacks. The smaller-size ones are the worst, because they lodge deep in our lungs, often entering the bloodstream. This is especially bad for developing fetuses, and puts adults at a greater risk of heart disease and stroke.

Trees clean the air by drawing in airborne particles through a weak electromagnetic charge on leaf surfaces and holding them until rain washes them off. This is much like how a special air filter called an electrostatic precipitator cleans pollution in heavy industry, except those systems cost a lot of money.

In addition, a group of sweet-smelling compounds, called terpenes, given off by conifer trees are more than a piney air freshener. They drift up to about 3,000 feet, where they make clouds over forested areas twice as dense as clouds above other terrain. It’s like corn starch for clouds, I guess. Denser clouds reflect around 5 percent more sunlight, which doesn’t sound like much, but apparently it makes a real difference in helping to moderate the climate. I’d say that’s a rather cool trick.

Perhaps the best-known “ecosystem service” trees provide is that they take carbon dioxide from the air and store it as wood, which is roughly 50 percent carbon. Worldwide, more than 17 million tons of carbon dioxide are taken out of circulation annually and sequestered by trees. In the U.S., forests store about 14 percent of our yearly carbon dioxide emissions. That’s important, as we burn more fossil fuel per capita than any other nation. Healthier, more diverse forests stash a lot more carbon than plantation forests do; another reason to safeguard species diversity. We’ve long known that trees do wonders for our mental and physical well-being, and it looks like we can put to rest any claim that they sully our planet. I encourage everyone to drink water daily, and to get out and enjoy the shade of a tree whenever possible.