Why Do Trees Help Us Breathe? From Oxygen to Clean Air

Trees help you breathe in two fundamentally different ways: they generate the oxygen your lungs need, and they scrub harmful pollutants out of the air before you inhale them. The oxygen story traces back hundreds of millions of years to the evolution of photosynthesis, but the air-cleaning story is more nuanced and, for city-dwellers, arguably more relevant to daily health. Trees intercept fine particles, absorb toxic gases, and even release compounds that appear to boost immune function. They also, under certain conditions, make air quality worse. Understanding both sides helps explain why the species, placement, and management of trees matter as much as their sheer number.

How Photosynthesis Built the Atmosphere You Breathe

Every green leaf on every tree runs a solar-powered chemical reaction that splits water molecules apart. The hydrogen atoms get funneled into building sugars from carbon dioxide, while the leftover oxygen escapes into the air. The molecular machinery responsible is a protein complex called Photosystem II, which uses a small cluster of manganese and calcium atoms to crack water apart using sunlight. This single enzyme, embedded in the membranes of plant cells, algae, and cyanobacteria, is responsible for virtually all the oxygen in Earth’s atmosphere.¹

The earliest version of this process started in ancient cyanobacteria in the ocean billions of years ago, but it was the colonization of land by the first plants, roughly 470 million years ago, that drove oxygen toward present-day levels. Modeling work suggests these early land plants, which resembled mosses more than oaks, could have reached about 30 percent of today’s global land-plant productivity by around 445 million years ago. Because their tissue had a far higher carbon-to-phosphorus ratio than marine organisms, they dramatically increased the amount of organic carbon buried in sediments, which is the net long-term source of atmospheric oxygen. The result was a sustained rise in oxygen to roughly modern concentrations by 420 to 400 million years ago.² That link between land plants and atmospheric oxygenation has been reinforced by interdisciplinary reviews connecting the Paleozoic spread of vegetation to a stepwise rise in oxygen levels and a drawdown of carbon dioxide.³

Today, the oxygen contribution of any individual tree or city park is negligible compared to the vast global pool already in the atmosphere. You will not suffocate if someone cuts down the trees on your block. But the air-cleaning functions of trees have a much more immediate, local effect on what enters your lungs.

Catching Particles on Leaves

Fine particulate matter, especially particles smaller than 2.5 micrometers (PM2.5), is one of the most dangerous components of urban air pollution. These particles are small enough to slip past your nose and throat and lodge deep in lung tissue. Trees act as living air filters: their leaves intercept and trap particles that would otherwise stay suspended in the air you breathe.

Not all leaves are equally good at this job. Research comparing different tree species found that needle-shaped leaves captured the most PM2.5, followed by narrow lance-shaped leaves. At the microscopic level, the leaves that performed best had dense grooves and tiny hair-like structures called trichomes on their surfaces. The amount of PM2.5 trapped on a leaf was positively correlated with trichome density and groove density, meaning trees with rougher, more textured foliage function as better filters.⁴ This is why conifers like pines and spruces are often discussed as strong candidates for pollution barriers along roads.

Trees along busy roads also accumulate heavy metals from vehicle exhaust and tire wear on their leaf surfaces. Analysis of particles deposited on roadside leaves has found high concentrations of iron, silicon, and individual particles rich in metals like lead, zinc, chromium, and manganese.⁵ In one study of Mediterranean cypress trees planted along a road in Istanbul, a dense row of trees reduced pedestrian exposure to cadmium and lead by more than half compared to the same road with no trees.⁶ Wild plant species growing along roadsides absorb significant amounts of heavy metals into their own tissues, effectively pulling contaminants out of the air and soil.⁷

Absorbing Gases Through Leaf Pores

Beyond trapping particles, trees absorb gaseous pollutants directly through tiny pores on their leaves called stomata, the same openings they use to take in carbon dioxide for photosynthesis. Ground-level ozone is one of the most important gases trees remove this way. Ozone irritates airways and worsens conditions like asthma, and its uptake through tree stomata is considered an important process for improving urban air quality.⁸ Trees also absorb sulfur dioxide, nitrogen dioxide, and various volatile organic compounds through the same mechanism.

There is a catch, though. Trees emit their own volatile organic compounds, known as biogenic VOCs or BVOCs. Some of these react with nitrogen oxides in sunlight to produce ozone, the same pollutant the trees are removing. So trees can simultaneously scrub ozone from the air through their leaf surfaces and contribute to ozone formation through their chemical emissions.⁹ In practice, though, the BVOC contribution from additional urban trees is often swamped by the much larger volume of human-generated VOC emissions from vehicles and industry. A modeling study found that VOC emissions from added urban trees had no detectable effect on ozone concentrations, less than one part per billion, because the area was already saturated with anthropogenic and background biogenic emissions.¹⁰ The concern becomes more real as cities reduce tailpipe pollution from vehicles. In cleaner-air scenarios where nitrogen oxide levels keep dropping, BVOC emissions from trees could play a larger role in ozone chemistry.

The Street Canyon Problem

If trees are air filters, you might assume planting as many as possible along city streets is always better. But in narrow urban “street canyons,” those corridors flanked by tall buildings, dense tree canopies can actually trap pollution at pedestrian level by blocking the wind that would otherwise carry exhaust up and away.

Wind-tunnel experiments have shown that adding trees to a street canyon shifts the pollution pattern from a simple two-dimensional distribution to a more complex three-dimensional one. However, the overall average pollution level in the street did not consistently rise or fall with tree density; instead, the concentration field redistributed in ways that depend heavily on geometry and airflow.¹¹ The direction of wind matters enormously. A study modeling conditions on a London street found that when wind blew parallel to the road, trees had a beneficial effect, reducing pedestrian-level concentrations by up to 18 percent by channeling flow and distributing pollutants vertically. But when wind blew perpendicular to the road, the same trees could increase local pollutant concentrations by more than 100 percent near the canopy by creating stagnant recirculation zones that trapped exhaust.¹²

The implication is not that street trees are bad, but that their placement needs to account for building heights, street widths, and prevailing wind patterns. On wide, open boulevards, tree canopies clean the air unambiguously. In tight canyons, urban foresters need to consider whether dense canopy cover might block ventilation more than it filters pollution.

Trees, Asthma, and Respiratory Health

Multiple epidemiological studies have linked neighborhoods with more trees to lower rates of respiratory illness, particularly in children. One study found that an increase of about 343 trees per square kilometer was associated with roughly a 29 percent lower prevalence of childhood asthma after adjusting for income, housing, and other confounders.¹³ A separate ecological study in Connecticut found that every 10-percentage-point increase in tree canopy cover corresponded to about a 4 percent decrease in pediatric asthma emergency visits and hospitalizations, though the relationship was U-shaped, suggesting that extremely high canopy cover may reduce the benefit or introduce other factors like pollen.¹⁴

These are observational studies, so they cannot prove that trees directly caused the improved health outcomes. Wealthier neighborhoods tend to have both more trees and better healthcare access, and teasing apart those factors is difficult. Still, the consistency of the association across different cities and study designs makes it unlikely that trees are doing nothing. The particle-trapping and gas-absorbing mechanisms described earlier provide a plausible route through which more trees could mean fewer respiratory flare-ups.

Phytoncides and Your Immune System

Trees release airborne chemicals called phytoncides, essentially essential oils that help protect them from insects and decay. Research on “forest bathing,” the Japanese practice of spending extended time in forested areas, has found that exposure to these compounds appears to boost a specific part of the human immune system. In controlled studies, people who took multi-day trips to forests showed significantly higher natural killer cell activity compared to before the trip, and this boost lasted for more than 30 days. In contrast, equivalent sightseeing trips in urban areas without forest exposure produced no such change.¹⁵

Laboratory experiments have clarified part of the mechanism. When human NK cells were directly exposed to phytoncides, they showed increased killing activity in a dose-dependent manner, along with higher levels of the proteins they use to destroy target cells.¹⁶ In a separate experiment simulating forest air exposure with volatile compounds from hinoki cypress, participants showed increased NK cell activity alongside decreased stress hormones in their urine, suggesting a dual pathway of direct immune stimulation and stress reduction.¹⁷

This line of research is still fairly young, and most studies have been small. But the consistency of results across lab and field settings, and the plausible biological mechanism involving stress hormone reduction, make it one of the more intriguing ways trees may support respiratory and overall health beyond simple air filtration.

Carbon Storage and Old-Growth Forests

Trees do not just cycle oxygen and filter pollutants in real time. They also lock away carbon for decades or centuries. This matters for breathing in the long run because the carbon dioxide trees absorb would otherwise contribute to climate change, which in turn worsens air quality through heat-driven ozone formation, longer pollen seasons, and more intense wildfires.

Old-growth forests store substantially more carbon per hectare than younger stands. An analysis of U.S. forests found that old-growth forests averaged about 224 metric tons of carbon per hectare, compared to 201 for mature forests and 178 for young ones.¹⁸ Research in the Pacific Northwest found that old-growth watersheds were more drought-resistant and sustained water availability better than young forest plantations, while also accumulating more carbon over time.¹⁹

Young forests do grow faster and can pull carbon out of the air at higher rates per year than old forests, especially in the tropics, where regrowing forests can absorb CO2 up to 20 times faster than old ones. But this rapid uptake does not make up for the massive carbon stocks released when old forests are cut. Between 2010 and 2020, forests undergoing stand replacement lost an estimated 0.38 billion tons of above-ground carbon per year. The young forests that replaced them will not recover the carbon stocks of old forests within any timeframe relevant to current climate policy.²⁰

When Trees Hurt Air Quality

Trees are not an unqualified good for the air you breathe. In certain scenarios, they actively make things worse.

The most dramatic example is wildfire. When forests burn, they release enormous quantities of fine particulate matter, and this smoke turns out to be far more dangerous per microgram than typical urban pollution. A study in Southern California estimated that PM2.5 from wildfire smoke was up to 10 times more harmful to respiratory health than PM2.5 from other sources, driving respiratory hospital admissions up by roughly 10 percent per 10-microgram increase in wildfire-specific particles.²¹ A California-wide analysis found that smoke event days were associated with a 10.3 percent increase in emergency visits for asthma, with the largest effects in children under five.²² Among older adults nationally, asthma-related hospitalizations on wildfire smoke days were about five times higher than on non-smoke days.²³

Pollen is a quieter, more chronic problem. In recent decades, urban forestry has trended toward planting male trees because they do not drop seeds or fruit, making streets cleaner to maintain. But male trees produce pollen, and the widespread use of male clones has dramatically increased pollen loads in many cities. After Dutch elm disease killed millions of elms across North America, the U.S. Department of Agriculture recommended replacing them with male ash and maple trees, further skewing the urban forest toward pollen production.²⁴ For allergy and asthma sufferers, this preference for “litter-free” male trees has made city air harder to breathe, not easier.

Microbes, Green Spaces, and the Skin You Breathe Through

An emerging area of research looks at how time spent near trees and in green spaces changes the community of microbes living on your skin and in your nasal passages. After exposure to urban green spaces, study participants showed increased microbial diversity on their skin and in their noses. Skin microbes shifted to become more similar to soil microbiota, while nasal microbes became more similar to the air sampled in those green areas.²⁵ The idea, rooted in what researchers sometimes call the “biodiversity hypothesis,” is that exposure to a rich environmental microbiome may help train the immune system and reduce the kind of overreactive immune responses behind allergies and asthma. This is speculative territory compared to the well-documented particle-trapping work, but it adds another dimension to why proximity to trees and green spaces tracks with better respiratory outcomes in population studies.

Species Selection in Urban Forestry

Because different tree species vary so widely in their ability to filter particles, absorb gases, emit BVOCs, and produce allergenic pollen, the choice of which trees to plant in a city is a genuinely complex optimization problem. Urban forestry researchers have begun developing composite scoring methods that weigh a species’ carbon storage, pollution removal, and BVOC emissions together to identify the best candidates for specific locations.²⁶ A tree that excels at trapping particles but emits high levels of isoprene, a potent ozone precursor, might not be the right choice in a city already struggling with summer smog. Similarly, a species that stores carbon well but produces enormous pollen loads may do more harm than good near schools or hospitals.

Indoor and semi-indoor applications are being explored as well. Researchers at Harvard tested a plant-based living wall for its ability to absorb volatile organic compounds indoors, evaluating 12 plant species for their VOC absorption performance to guide selection for indoor air-cleaning installations.²⁷ The scale of removal from a few potted plants or a living wall is modest compared to a mechanical ventilation system, but as part of a broader design strategy these approaches are gaining traction in building design, especially in spaces where opening a window means letting polluted city air in.