Those who regularly walk among the trees know what the forest does—how it slows the mind, deepens the breath, and leaves a quiet imprint on the body. They know why it is needed. But beyond this personal experience, scientific inquiry is essential to understanding the broader impact of forests on human health. A growing body of research, including Urban Trees, Air Quality, and Human Health by David J. Nowak, reveals that trees do more than provide a moment of peace; they actively shape the air we breathe, protect us from disease, and help regulate the very climate that sustains us.

The Unseen Threats: Air Pollution and Urban Heat

The modern world is filled with invisible dangers—air pollution being one of the most pervasive. According to the World Health Organization (WHO), air pollution contributes to one in nine deaths globally, making it the largest environmental risk factor for human health. Cities, with their dense traffic and industrial activity, act as amplifiers of pollution and heat, often at the expense of human well-being.

Heatwaves, increasingly frequent due to climate change, are another silent threat. In 2003, more than 70,000 people perished in Europe due to extreme heat. Urban areas exacerbate this issue, trapping heat in concrete and asphalt, creating what is known as the “urban heat island” effect. But in the midst of this, forests and urban trees stand as quiet sentinels, offering solutions woven into their very nature.

How Trees Become the Breath of the City

The role of trees in mitigating pollution and heat is profound, yet often overlooked. Through a symphony of natural processes, they:

1) Cool the environment – Trees reduce temperatures through shade and transpiration, with cooling effects reaching up to 1.3°C in shaded areas.

2) Filter the air – Trees remove pollutants such as nitrogen dioxide (NO2), sulfur dioxide (SO2), and fine particulate matter (PM2.5 and PM10), improving air quality. A single hectare of urban tree cover can remove up to 200 kg of pollutants per year in heavily polluted areas.

3) Reduce energy consumption – By shading buildings and reducing heat, trees lower air conditioning use, decreasing fossil fuel emissions from power plants. In the U.S. alone, urban forests save $5.4 billion annually in energy costs.

4) Support mental well-being – Beyond physical health, forests act as restorative spaces, reducing stress, improving cognitive function, and enhancing emotional resilience.

The Public Health Connection: A Forest’s Gift to the Body

Forests have long been called the “lungs of the Earth,” but their relationship with human health extends far beyond oxygen production. Recent studies highlight how urban greenery reduces asthma rates, lowers mortality, and prevents thousands of respiratory-related hospital visits each year.

For example, research shows that trees in the United States alone prevent more than 850 deaths annually by improving air quality. They also help avoid over 670,000 cases of acute respiratory symptoms and 430,000 asthma-related incidents. Moreover, increased tree cover has been associated with lower levels of mental distress, proving that their influence is not just physiological but deeply psychological. And you wonder or are even sceptical why the practice of Forest Therapy, Forest Bathing or Shinrin Yoku is needed.

When Tress Can Harm?

Yet, not all interactions with trees are inherently beneficial. Certain species release volatile organic compounds (VOCs), which, under specific conditions, contribute to ozone formation and secondary organic aerosols (SOAs). Oaks, poplars, and eucalyptus, for instance, are known for their high VOC emissions. Additionally, tree pollen can exacerbate allergies and respiratory conditions, posing challenges for sensitive individuals.

As a Forest Therapy, Shinrin Yoku, or Forest Bathing Guide, you might find this perspective surprising—a small twist in the narrative of nature’s healing power. After all, if you’ve studied the science behind Shinrin Yoku, you’re likely well-versed in the benefits of phytoncides (among them terpenes)—the forest’s natural antimicrobial compounds known to reduce stress, enhance immune function, and promote overall well-being. But as guides, we must remain open, critical, and adaptable to evolving scientific research. Nature, after all,  is a dynamic system, complex and sometimes contradictory, yet always deeply interwoven with human health.

A stark example of the complexity of air quality and forest interactions can be found in northern Thailand, where seasonal air pollution reaches extreme levels, particularly between February and April. In 2019, Chiang Mai recorded the worst air pollution in the world. By 2021, it was ranked the third most polluted city globally. In February 2023, air quality indices (AQI) ranged from moderate (96) to hazardous (294), with PM2.5 concentrations far exceeding Thailand’s national air quality standard of 50 μg·m−3. The consequences are alarming: WHO estimates that air pollution contributes to approximately seven million premature deaths annually, with Southeast Asia and the Western Pacific region accounting for 45% of the global mortality rates associated with poor air quality.

VOCs are an essential part of the atmospheric equation, acting as precursors for ozone (O3) and secondary organic aerosols (SOAs). While anthropogenic VOCs (AVOCs) stem from human-made sources such as vehicle emissions and industrial activities, biogenic volatile organic compounds (BVOCs) originate from forests and vegetation. BVOCs contribute up to 90% of global VOC emissions, with isoprene, monoterpenes, and sesquiterpenes playing significant roles in atmospheric chemistry. Their presence enhances SOA formation, which constitutes a large fraction of fine particulate matter (PM2.5).

Research suggests that BVOCs are not passive contributors—they actively shape air quality, sometimes in unexpected ways. For instance:

– 20% of global ozone formation and 76% of SOA production are attributed to BVOCs.

– Under high-NOx conditions, glyoxal (CHOCHO), a byproduct of isoprene oxidation, increases SOA formation, exacerbating haze and particulate pollution.

– In VOC-limited environments, a reduction in NOx (such as from vehicle restrictions) can paradoxically increase sulfate aerosols, altering air quality in unpredictable ways.