Environmental Exposure and Human Health: How Every Breath Shapes the Body

My early career was spent examining hearts and lungs directly through an open chest in the operating room. As a heart and lung transplant surgeon, I witnessed these organs at their most vulnerable—failing hearts, scarred lungs, impaired circulation, and inadequate oxygen. In that setting, disease is never abstract; it is an intensely physical reality.

Over time, and later through twelve years in the U.S. Senate, I began to think more broadly about what makes people sick. We rightly focus on blood pressure, cholesterol, smoking, genetics, infection, diet, and exercise. Yet another set of equally powerful forces acts on us daily: the air we breathe, the heat we endure, the smoke we inhale, the landscapes around us, and the chronic stress created by environmental disruption.

Modern science now allows us to see these forces at work inside the body. Climate change and environmental degradation increasingly manifest in the heart, lungs, kidneys, nervous system, hormonal stress response, and immune system. This is medical science, and it affects every one of us. Environmental quality is a personal health issue that should shape how physicians, communities, and policymakers respond.

The heart, where I focused most of my medical career at Stanford and Vanderbilt, offers one of the clearest examples. Fine particulate pollution, or PM2.5, is small enough to penetrate deep into the lungs and trigger inflammation, vascular dysfunction, and autonomic stress. The American Heart Association has long recognized particulate pollution as a major cardiovascular risk factor. More recent research links extreme heat, worsening air quality, and severe weather with higher rates of heart attack, stroke, heart failure, and cardiovascular death.

Heat creates its own cardiovascular burden. As the body attempts to cool itself, blood vessels dilate, the heart works harder, and fluid losses increase. For older adults, outdoor workers, or those living with heart disease, a stretch of extreme heat can quickly become an urgent medical event.

The author, Sen. Bill Frist, MD, performing a heart surgery at Vanderbilt University Medical Center.

Vanderbilt University Medical Center

After the heart, the lungs were the organ I most frequently operated on. Lungs are our most intimate interface with the environment. Every breath—we take over 20,000 each day—brings the outside world directly into the body. Wildfire smoke, ozone, particulate pollution, and longer pollen seasons inflame airways, worsen asthma and COPD, increase respiratory infections, and drive hospitalizations. Children deserve special attention here because their lungs are still developing.

One of the most encouraging studies in this field followed children in Southern California as air quality improved over nearly two decades. Those living in communities with the largest reductions in pollution experienced the greatest gains in lung function. Consider what this means: cleaner air changed the physical development of children’s lungs, leading to healthier lives. This is preventive medicine practiced at a community scale.

And the evidence extends well beyond the chest.

The kidneys depend on stable blood flow, hydration, and internal chemistry. Repeated heat exposure and dehydration can produce acute kidney injury and, over time, may contribute to chronic kidney disease. This risk grows for outdoor workers and agricultural laborers as temperatures rise. Air pollution also takes its own toll. A major study of roughly 2.5 million U.S. veterans found that long-term PM2.5 exposure was associated with greater risk of chronic kidney disease, faster loss of kidney function, and progression to end-stage renal disease.

Beyond individual organs, environmental factors also affect the systems that coordinate how those organs work together.

The autonomic nervous system constantly balances sympathetic activity—our “fight or flight” response—with parasympathetic activity, the “rest and recover” system that slows the heart, regulates blood pressure, and helps restore equilibrium. Pollution pushes that balance toward physiological stress. Studies have linked particulate exposure with reduced heart rate variability, a marker of impaired autonomic regulation.

Nature, however, can also move us in a more positive direction. A large Japanese field study found that people walking in forest environments showed increased parasympathetic activity and reduced sympathetic activity compared to those walking in urban settings. Other research suggests that even viewing green environments can improve physiological recovery after stress. I find this especially compelling. Anyone who has walked through a forest, sat beside a river, or spent time in nature understands the feeling of the body settling into a calmer, more relaxed state. Science is now measuring what most of us have experienced intuitively.

The same pattern appears in the hormonal stress system. The hypothalamic-pituitary-adrenal axis releases cortisol and other hormones when we perceive a threat. That response can save a life, but repeated or sustained activation wears on the body and can measurably contribute to heart and blood vessel disease, metabolic dysfunction, anxiety, and depression. One elegant randomized crossover experiment compared periods of purified indoor air with typical unfiltered exposure and found that higher particulate pollution was associated with increases in cortisol, cortisone, epinephrine, and norepinephrine. Dirty air, in other words, can register as biological stress and disease.

Temperature matters here too. A 2023 systematic review and meta-analysis of 114 studies found that higher temperatures were associated with worse mental health outcomes, more psychiatric hospital visits, and higher suicide rates. A warming environment reaches far beyond heat exhaustion and physical health; it can affect the systems that govern mood, behavior, and resilience.

The immune system adds another layer. Air pollution, extreme heat, wildfire smoke, and changing allergen patterns can promote chronic inflammation and disrupt immune regulation. One population study of more than 80,000 people linked long-term particulate exposure with higher rates of autoimmune disease. Other recent research describes effects on barrier defenses in the lungs, skin, and gastrointestinal tract.

Here the story begins to turn from damage toward restoration. My perspective is colored by my direct experiences as board chair of The Nature Conservancy globally.

Healthy, natural, biodiverse environments help support more balanced immune function through exposure to a richer microbial world. Green space has also been associated with lower mortality, even after researchers account for factors such as air pollution and noise. The pathways are likely several: more physical activity, lower stress, stronger social connection, and physiological recovery.

This is why we should move beyond thinking of nature simply as something beautiful to protect. Our natural systems should be viewed as part of the infrastructure of human health. It is not just conservation for conservation’s sake; it is conservation for humanity’s sake.

For physicians, that means widening the environmental history when we first see a patient. Where does this person work? How much heat are they exposed to? Are they breathing wildfire smoke, or living near heavy traffic? Do they have shade, trees, parks, and safe places to exercise?

For health systems, resilience increasingly belongs in preparedness. Heat waves, smoke events, storms, and prolonged power outages create predictable patterns of illness. Hospitals and community health organizations can actively plan around them.

For policymakers, the opportunity is larger still. Clean air standards are preventive medicine, and efforts in Washington to undo them are misguided. Urban trees and shade reduce heat exposure, and parks and greenways support exercise, stress reduction, and social connection. Investments in healthier environments can lower disease burden long before a patient reaches an emergency room or winds up on the operating table.

Cleaner air, safer temperatures, healthier landscapes, and greater access to nature improve the conditions in which human biology operates. They complement medical care while reducing the heavy burden that eventually arrives at the clinic, the hospital, and the operating room.

I began my career looking inside the chest, caring for hearts and lungs at their most vulnerable. Decades later, I see health through a wider lens, and I know the real objective is to intervene long before a person ends up on an operating table. The environment shapes our health, literally with every single breath we take.

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