How trees can help keep us healthy and active in hot weather
By Nadina Galle and Jad Daley
During another summer of record-breaking heat, fueled by climate change, you might have heard that heat kills more people annually than any other extreme weather event. As we take measures to prevent heat-related death we must also address other more subtle public health threats — including the potential for reduced physical activity when our streets and parks are shimmering in the heat. Keeping people healthy and active in the hot months will require a new resolve to create shady and cool spaces for outdoor recreation that are equitably available in every neighborhood.
Heat as a barrier to physical activity is no hypothesis. Heat is already severely affecting our ability to safely engage in outdoor activities during the warmer months. As reported in The Lancet, there has already been a 20% increase this century in the number of days globally when even moderate physical activity poses a health risk due to heat. By 2050, this number is expected to rise by another 40%.
If the idea of runners staying home or empty playgrounds does not grab your attention as a public health threat, it should. Physical inactivity is one of the most widespread sources of illness and mortality, responsible for 1 in 10 deaths worldwide. This is especially alarming among young people, with only 1 in 4 adolescents meeting the recommended levels of physical activity.
Keeping active also benefits our mental health. Research shows that engaging in regular physical activity can reduce the risk of depression by up to 30%. Our bodies are not meant to hibernate through the summer in air-conditioned spaces, especially in regions where it is hot in the summer and cold in the winter — creating two seasons when we might not get the outside time and physical activity we need.
We can’t afford to make exercise any harder for people. Research indicates that nearly 80% of adults in the United States fail to meet the recommended levels of physical activity, largely due to sedentary lifestyles exacerbated by screen time. The last thing we can afford is to allow our recreation spaces — such as city parks, rail trails, and children’s playgrounds — to become dangerously hot.
So, what can we do to keep people moving in the heat? One big step is to more fully utilize nature’s air conditioning provided by trees.
You might not know that trees play a crucial role in combating urban heat. Trees cool the area beneath their canopy by an impressive 20–45 degrees Fahrenheit via shade and a natural form of cooling called “evapotranspiration” that takes place when trees exchange moisture with the air around them. Even better, the cooling from trees can be effective in outdoor spaces where air conditioning cannot.
To fully harness cooling from trees for climate-safe physical activity, we must put the right kinds of trees in the right places. That means mapping out our high-value recreational areas, such as trails, parks, and sidewalks that are heavily used for walking, running, and biking, and use tools such as Tree Equity Score to identify which ones most lack tree cover. Then we must select tree species to plant in those locations that are most ideal to provide long-lasting natural cooling and clean air benefits in that location’s particular geographic context.
One notable example of this approach in action is the “Cool Corridors” initiative in Phoenix, Arizona. The City of Phoenix was the first in the nation to commit to “Tree Equity” so every neighborhood would have the cooling benefits of trees, remedying systemic inequities of tree cover along lines of income and race. But Phoenix went a step further to map out 100 Cool Corridors where trees are most needed in these low tree cover areas to cool people exercising or traveling by foot, bicycle, or other means.
The City of Phoenix and its partners are planting dense stands of trees along these linear corridors and in some cases combining the tree planting with applying high albedo pavements that also contribute to the cooling effect. Cool Corridors provide more than just safe areas to recreate — they offer essential transit networks for shopping, commuting, and maintaining social interactions even during periods of extreme heat.
Recent history has shown that active transit options become particularly crucial when other public transportation systems fail under heat stress, an increasing problem. For example, heat in Portland, Oregon has caused streetcar cables to melt while in Phoenix high temperatures have caused rail lines to warp, leading to delays and service interruptions in both cases. Cool Corridors can help enhance urban heat resilience by keeping residents moving in such instances.
One of us had the opportunity to assist with planting the first Cool Corridor in the Laveen neighborhood of South Phoenix. This mile-long run of dense tree cover connects Cesar Chavez High School to a popular local park and library, creating a shadier route for walking, running, and biking. The planting event turned into a vibrant community celebration, with planting led by dignitaries such as Mayor Kate Gallego, frontline partner organizations, corporate partners such as Salesforce, Aspiration, and iHeartMedia, and enthusiastic students from the Chavez High School. Some community members scuttled their planned trip into the park when they learned they could join in the tree-planting effort.
What about our youngest athletes and the safety of the playgrounds and other spaces they rely on? One of us, a new mother, sees the impact of overheated playgrounds and trails daily in Toronto, Amsterdam, and Willemstad (Curaçao), where she divides her time with her family. For her, a playground or street without shade means she will not take her one-year-old son outside. It is that simple.
Like the elderly, infants and young children are particularly vulnerable to extreme heat. From 2018 to 2023, extreme heat contributed to the deaths of 69 infants in the U.S. — nearly 2 per 100,000 infants — according to NBC News analysis of CDC data. Infants overheat faster due to their higher surface-area-to-volume ratio and less developed sweat glands, making it difficult for them to regulate their body temperature. Additionally, since babies cannot directly communicate discomfort, subtle signs of overheating can be easily missed until it is too late.
Having tree-shaded and cooled public spaces can make a significant difference for vulnerable children, and not only in famously heat-prone areas like Curaçao and Phoenix; heat also increasingly impacts cities not designed for high temperatures, such as Toronto, Amsterdam, and Portland. As global temperatures rise, every city must target shading and cooling measures to the places where infants and other very young people play, such as planting shade trees in playgrounds, to keep our young people active and safe at the same time.
To be clear, efforts to keep people moving in the heat must be complemented by providing natural cooling directly to people’s homes, such as planting trees to benefit lower-income seniors who lack air conditioning or cannot afford to fully use it. According to research from Duke University, if we do not undertake these kinds of actions, we could see tens of thousands more heat-related deaths annually.
Fortunately, a country as prosperous as America does not have to choose between different natural cooling solutions. Our communities need heat-resilient homes as well as cooler parks and trails, and trees can help greatly in both contexts. Already, we are seeing historic new investments in this area, such as the $1.5 billion provided by the Inflation Reduction Act for U.S. Forest Service grants to cities and their partners to increase tree cover in disadvantaged neighborhoods. Now is the time to redouble this kind of investment so we can keep people safe and healthy in a new era of extreme heat.
About the authors
Nadina Galle
Nadina Galle, PhD, an ecological engineer and 2024 National Geographic Explorer, is the author of “The Nature of Our Cities: Harnessing the Power of the Natural World to Survive a Changing Planet,” released in June 2024 by HarperCollins. Galle’s work explores how integrating nature into urban environments can address both environmental and health challenges, providing a rich context for understanding the broader benefits of trees.
Jad Daley
Jad Daley is President and CEO of American Forests, the nation’s old forest conservation non-profit. Under his leadership, American Forests coined the term “Tree Equity”, developed the free online Tree Equity Score tool, and led advocacy efforts to secure $1.5 billion in the Inflation Reduction Act for U.S. Forest Service grants to cities and their partners doing this work. Daley is also an accomplished endurance athlete, having finished in the top 150 overall at the New York City Marathon.
