FE2006—The Ecology of Healthy Air
1.0 Introduction—What Makes Air Feel Healthy?
Most approaches to indoor air quality begin with a straightforward objective: identify what may harm us and remove it. Smoke, particulate matter, allergens, molds, pathogens, and toxic chemicals are filtered, ventilated, disinfected, or otherwise excluded. This approach has produced enormous benefits. Yet it also defines healthy air largely by what is absent.
Anyone who has walked through a forest, stood beside a body of water, entered a lush garden, or stepped outdoors after rain may recognize another quality of air that is harder to describe. The air can smell and even feel different—fresh, earthy, pleasant, sometimes invigorating. Humidity and temperature undoubtedly contribute to that sensation, but so do compounds produced by plants, soil, water, fungi, and microorganisms. The atmosphere surrounding living ecosystems is itself biologically active.
Growing appreciation for the importance of natural light provides an instructive example. People who deliberately spend more time outdoors often report feeling calmer, sleeping better, and experiencing other improvements in well-being, and there are sound biological reasons for expecting daytime light exposure to influence circadian rhythms and sleep.[1] Yet going outdoors changes more than light exposure. At the same moment, a person begins breathing different air—air with different humidity, temperature, pollutants, plant compounds, microorganisms, and other biological constituents. Physical activity, visual surroundings, and sound may change as well.
This creates a basic problem of causation. If someone spends more time outdoors and subsequently feels better, it is tempting to attribute the improvement to the variable being deliberately changed: natural light. But correlation does not establish which component of the outdoor environment produced the observed effect. Was it the light? Was it the air? Was it another environmental change—or an interaction among several of them? The established importance of natural light should not cause us to overlook another exposure occurring simultaneously with every outdoor breath.
This raises a complementary question: Is healthy air simply air from which harmful things have been removed, or might it also contain characteristics that contribute positively to human health?
FE2006 explores air as an ecological environment rather than merely a medium through which oxygen reaches our lungs. Humans evolved surrounded by biological diversity. If some characteristics of that environment contributed to the development and maintenance of healthy biological systems, understanding them may offer new approaches to indoor environmental health.
2.0 Air Is an Ecosystem
Air appears empty, but biologically it is anything but empty. Bacteria, fungi, spores, pollen, fragments of organisms, and other biological material continually move through the atmosphere. Collectively, the biological component of this environment is increasingly discussed as the aerobiome—the airborne microbiome.[2]
The composition of the aerobiome varies with geography, vegetation, weather, season, soil, bodies of water, animals, human activity, and the built environment. A forest therefore does not merely contain different trees from a city street or office building. It exists within a different biological atmosphere.
Buildings create microbial ecosystems of their own. Ventilation, humidity, temperature, occupants, surfaces, filtration, and connection to outdoor air all influence which microorganisms are present. Research comparing mechanically ventilated rooms with rooms receiving outdoor air through open windows has demonstrated differences in the diversity and composition of airborne bacterial communities.[3] The built environment is therefore not biologically neutral. How we construct and operate buildings helps determine the microbial environment in which their occupants live.
This suggests that breathing has an ecological dimension as well as a respiratory one. We do not merely exchange oxygen and carbon dioxide with an inert atmosphere. We continually interact with a complex biological environment.
3.0 The Chemistry of a Living Environment
The biological character of natural air extends beyond the organisms themselves. Plants, bacteria, fungi, soil, and other components of living ecosystems produce enormous numbers of volatile compounds that can enter the surrounding atmosphere.
Trees and other plants release compounds including terpenes such as α-pinene and β-pinene. Collectively, some biologically active volatile substances released by plants have been described as phytoncides. Microorganisms also produce microbial volatile organic compounds, or mVOCs.[4]
One familiar example is geosmin, an intensely detectable compound associated with the characteristic earthy smell of soil. Geosmin can be produced by several groups of microorganisms and is strongly associated with Streptomyces and other Actinobacteria. Humans can detect extraordinarily small concentrations of it.[5] Geosmin contributes to a sensory experience many people associate with soil, forests, gardens, and rain, although its presence should not itself be interpreted as evidence of healthfulness.
Nor should geosmin be confused with the entire experience. What people perceive as fresh natural air is probably not the product of one chemical. It may instead represent an environmental signature created by combinations of plant volatiles, microbial metabolites, moisture, temperature, airborne biological material, and countless other variables.
The aroma may tell us that the air is biologically and chemically different, and generate pleasing sensations, without telling us which of those differences cause that feeling or whether they are actually beneficial to health.
4.0 Forest Bathing and the Health Hypothesis
An intriguing body of research has examined what the Japanese call shinrin-yoku, commonly translated as forest bathing. The practice involves spending relaxed time immersed in a forest environment rather than literally bathing in water.
Research published in 2006 provides a particularly appropriate historical connection to FE2006. Qing Li and colleagues investigated phytoncides derived from trees and reported effects on human natural-killer-cell activity in laboratory experiments.[7] Subsequent small human studies compared physiological measurements before and after forest exposure and reported changes including increased natural-killer-cell activity and reductions in some stress-related measures.[8]
Researchers also detected phytoncides such as α-pinene and β-pinene in forest air. Particularly interesting experiments subsequently attempted to separate the forest from its chemistry. In one study, volatile compounds derived from hinoki cypress oil were introduced into urban hotel rooms occupied by participants. The researchers again reported changes in natural-killer-cell activity and stress-related measures.[9]
These experiments do not establish that forest air prevents disease, extends life, or that particular forest compounds should simply be added to indoor air. Many variables change when someone enters a forest: exercise, noise, visual surroundings, temperature, humidity, pollution exposure, psychological state, microbial exposure, and chemical composition all change simultaneously.
That is precisely the methodological problem raised earlier. Someone who spends two hours walking through a forest has simultaneously received forest light, forest air, forest sounds, forest temperatures, forest microorganisms, forest chemistry, exercise, and psychological exposure to nature. Assigning the resulting physiological response to any one component requires experiments specifically designed to separate them.
Nevertheless, this research establishes an important question: Could some components of natural air exert measurable effects on human physiology?
If so, the pleasant sensation associated with certain natural environments may deserve investigation rather than dismissal as merely aesthetic.
5.0 From the Aerobiome to the Human Microbiome
The ecological question becomes still more interesting when the aerobiome is considered alongside the human microbiome.
Humans are themselves ecosystems. Our skin, mouth, respiratory tract, gastrointestinal tract, and other body surfaces support complex microbial communities. These communities are not completely isolated from the environments around us. Throughout life we encounter microorganisms and their products through food, water, soil, animals, surfaces, other humans—and air.
The biodiversity hypothesis proposes that reduced contact with diverse natural environments may alter interactions between environmental microbiota, the human microbiome, and immune regulation. A notable 2012 study by Ilkka Hanski and colleagues found associations among surrounding environmental biodiversity, skin microbiota, and allergic sensitization.[10] Such findings do not prove that greater microbial exposure is universally beneficial, but they raise the possibility that separation from environmental biodiversity can have biological consequences.
This does not mean that every environmental microorganism is beneficial, nor that microorganisms encountered in the air necessarily establish permanent residence within the human microbiome. Some are harmful; many are transient; others may have little effect at all.
The more important principle is that the human microbiome exists within a larger ecology. It continually receives environmental exposures and signals.
That creates an important connection with FE2013—Our Adaptive Microbiome. If our microbiome adapts to its environment, understanding the human microbiome without understanding the biological environment surrounding it may reveal only part of the story.
6.0 Have We Made Indoor Air Too Biologically Simple?
Modern buildings protect us from many environmental hazards. They provide temperature control, shelter from weather, filtration of particulates, protection from some infectious exposures, and separation from smoke and outdoor pollution. None of those achievements should be discarded.
But another possibility deserves investigation.
In attempting to eliminate undesirable biological exposures, have we sometimes assumed that the ideal indoor environment is one that approaches biological sterility?
Other ecosystems suggest caution with that assumption. Healthy soil is not sterile. Healthy natural water is not sterile. The healthy human intestine is emphatically not sterile. In each case, health depends partly upon relationships among organisms, competition, diversity, environmental conditions, and biological balance.
Air may prove different. But that should be discovered rather than assumed.
Research on buildings already demonstrates that indoor microbial communities differ according to ventilation source and other environmental conditions.[3] Buildings therefore do not simply remove microorganisms; they create ecological conditions that favor some organisms over others.
The relevant objective may eventually prove to be neither maximum microbial exposure nor maximum microbial elimination, but an understanding of what constitutes a healthy indoor ecology.
7.0 Healthy Air and Healthspan
Air differs from many environmental exposures because we encounter it continuously. We can choose particular foods and limit contact with particular substances, but breathing accompanies virtually every moment of life.
Even modest physiological effects associated with environmental quality could therefore become important when exposures accumulate over decades. The established harms of chronic exposure to air pollution already demonstrate that the composition of inhaled air can influence long-term health.[11] The complementary question is less well understood: can particular characteristics of healthy environments contribute positively to maintaining health?
This creates a connection between FE2006 and FE2011—Reframing Life/Health Spans: Maturation vs. Aging. If healthspan depends partly upon maintaining physiological resilience rather than merely treating diseases after they appear, then lifelong environmental quality deserves attention.
That changes the research question from merely:
How do we prevent unhealthy air from shortening life?
to:
Could healthier air help support health throughout life?
The distinction matters. Longevity research frequently concentrates on interventions introduced after deterioration has begun. Environmental health offers another possibility: improving the conditions within which the organism must maintain itself throughout its lifetime.
8.0 From Removal to Restoration
The conventional approach to indoor-air quality can be summarized as removal. Identify particulate matter, smoke, allergens, toxic compounds, pathogens, or excessive moisture and remove or control them.
That work should continue.
But the ecology of healthy air suggests a second strategy: restoration.
Before anything can be restored, however, we must determine what is actually beneficial. Researchers could compare air from forests, gardens, meadows, shorelines, post-rain environments, ordinary outdoor locations, and conventional indoor spaces. Modern analytical methods could characterize microbial populations, plant and microbial volatile compounds, particulate composition, humidity, temperature, allergens, pollutants, and other environmental variables.
Human responses could then be measured independently. Which environments do people describe as fresh, pleasant, relaxing, or invigorating? Do those perceptions correspond with measurable physiological differences? Which environmental variables correlate with those differences?
Research should also deliberately separate air from the other benefits of being outdoors. Natural-light exposure provides an especially useful example. An experiment could compare people exposed to conventional indoor light and air, natural-spectrum or appropriately timed light with conventional indoor air, conventional light with experimentally reproduced characteristics of natural air, and the combination of both interventions. Such designs could begin separating effects attributable to light, effects attributable to air, and interactions between the two.
The same principle applies to exercise, visual exposure to nature, sound, temperature, humidity, and psychological expectation. Controlled experiments should progressively separate these variables rather than assuming that whichever variable motivated someone to go outdoors caused everything subsequently observed.
Such research must also distinguish pleasantness from healthfulness. Evolution may have given humans preferences for environmental signals that historically correlated with favorable conditions, but a pleasant smell is not proof that something is healthy. Conversely, an environmental factor need not have a noticeable aroma to influence physiology.
The objective should therefore be discovery rather than confirmation:
Identify what distinguishes environments associated with favorable human responses, and then determine experimentally which characteristics matter.
9.0 Engineering the Ecology of Healthy Air
Once those characteristics are understood, engineering can follow.
One possibility is a Microbial Air Filter or broader Biological Air System. Microorganisms are already employed in biological water treatment, and biofiltration can also use bacteria and fungi to metabolize certain contaminants in polluted air. A future indoor system might similarly maintain selected microorganisms within a contained biological medium through which air circulates.
Those organisms would not necessarily need to be released into the occupied environment. They could remain inside a replaceable biological cartridge, where they might metabolize selected contaminants or generate carefully characterized biological compounds. If particular plant-derived or microbial compounds prove beneficial, the system could reproduce them at controlled concentrations. Humidity, airflow, conventional particulate filtration, and other physical variables might be managed simultaneously.
The eventual technology might not use living microorganisms at all. Research could reveal that particular combinations of plant compounds, humidity, filtration, and other characteristics account for most of the desired effect.
That uncertainty is a virtue rather than a weakness.
The sequence should be:
First discover what healthy air is. Then determine how best to reproduce it.
The product should follow the evidence rather than the evidence being selected to justify a predetermined product.
10.0 Conclusion—Bringing Healthy Environments Indoors
Humans have spent most of their evolutionary history surrounded by living environments. Only comparatively recently have large portions of human life moved into sealed, mechanically regulated spaces in which temperature, humidity, ventilation, filtration, illumination, and biological exposure are deliberately controlled.
Indoor environmental science has appropriately concentrated on protecting people from what can harm them. FE2006 proposes adding a complementary question: What if healthy natural environments contain characteristics that actively contribute to human health?
The aerobiome, plant volatiles, microbial metabolites, environmental biodiversity, humidity, natural light, and other characteristics of outdoor environments provide starting points for investigation. Their importance should neither be assumed nor dismissed.
The experience of going outdoors illustrates the challenge. When someone steps into morning sunlight and subsequently sleeps better, light is an obvious explanation—and an important one. But that person has simultaneously entered a different atmospheric, microbial, chemical, thermal, visual, and acoustic environment. The outdoors is an integrated biological environment, not a collection of isolated variables.
Healthy air may therefore prove to be more than air containing adequate oxygen and fewer contaminants. It may be an ecological environment with which human biology continually interacts.
If so, the next generation of indoor-air technology may move beyond purification alone. Rather than asking only how to remove what is harmful, we can also ask what may be beneficial, determine why it is beneficial, and investigate whether those characteristics can safely be restored.
The goal would not simply be to make indoor air smell like the outdoors.
It would be to understand why healthy outdoor environments sometimes make us feel different—and how much of that difference may be carried in the air we breathe.
Notes:
1. Natural Light and Circadian Biology. Light is the principal environmental signal entraining the human circadian system, and the timing, intensity, duration, and spectral characteristics of light exposure can influence circadian timing and sleep. This provides an established biological basis for expecting outdoor daytime light to affect sleep while leaving open the question of what additional outdoor exposures may contribute. See Blume, C., Garbazza, C., & Spitschan, M. (2019), “Effects of light on human circadian rhythms, sleep and mood,” Somnologie, 23, 147–156.[Return]
2. The Aerobiome. The term aerobiome is increasingly used to describe airborne biological communities and their interactions with environments and health. Air contains bacteria, fungi, viruses, pollen, spores, biological fragments, and other bioaerosols whose abundance and composition vary considerably across environments.[Return]
3. Built-Environment Microbiomes. Kembel, S. W., et al. (2012), “Architectural design influences the diversity and structure of the built environment microbiome,” The ISME Journal, 6, 1469–1479. The investigators found that ventilation strategy and architectural characteristics were associated with differences in airborne bacterial communities, with naturally ventilated rooms showing microbial communities more closely related to outdoor air.[Return]
4. Biological Volatile Compounds. Plants release large quantities of biogenic volatile organic compounds, including terpenes such as α-pinene and β-pinene. Bacteria and fungi likewise produce diverse microbial volatile organic compounds (mVOCs). These compounds can participate in ecological signaling and interactions and contribute to characteristic environmental odors.[Return]
5. Geosmin and Streptomyces. Geosmin is a volatile compound produced by various microorganisms, notably including species of Streptomyces. Humans possess an exceptionally low odor-detection threshold for geosmin, which contributes strongly to the characteristic earthy aroma associated with moist soil. Geosmin should not itself be interpreted as evidence that an environment is healthy; in FE2006 it is important primarily as an example of how microbial metabolism can produce compounds that humans readily perceive in natural environments. See Jiang, J., He, X., & Cane, D. E. (2007), “Biosynthesis of the earthy odorant geosmin by a bifunctional Streptomyces coelicolor enzyme,” Nature Chemical Biology, 3, 711–715.[Return]
6. Pine-Sol and Pine Oil. Pine-Sol traces its origin to chemist Harry A. Cole, who developed the cleaner in 1929 while living near pine forests outside Jackson, Mississippi. According to the manufacturer’s historical account, Cole recognized pine oil’s usefulness as a disinfectant and deodorizer and incorporated it into the product. See Pine-Sol, “Our History,” The Clorox Company.[Return]
7. The 2006 Phytoncide Study. Li, Q., et al. (2006), “Phytoncides (wood essential oils) induce human natural killer cell activity,” Immunopharmacology and Immunotoxicology, 28(2), 319–333. This laboratory study exposed human NK-92MI natural-killer cells to phytoncides, including α-pinene and related compounds, and reported increased NK-cell activity and expression of several cytolytic proteins. Because this was primarily an in-vitro experiment, it should not be described as demonstrating that breathing forest air improves human immune function. Its importance to FE2006 is that it provides an early experimental investigation of biological activity associated with compounds characteristic of forest environments—and provides the historical rationale for the 2006 essay number.[Return]
8. Human Forest-Bathing Studies. Li and colleagues subsequently conducted small human studies in which participants spent time in forest environments. A 2007 study involving 12 healthy men reported increased natural-killer-cell activity following a three-day/two-night forest trip. Subsequent work in men and women reported similar immune changes along with reductions in some stress-related measures. These studies are suggestive but small and cannot independently determine which component of forest exposure produced the observed changes. See Li, Q., et al. (2007), “Forest bathing enhances human natural killer activity and expression of anti-cancer proteins,” International Journal of Immunopathology and Pharmacology, 20(S2), 3–8; and Li, Q., et al. (2008), “Visiting a forest, but not a city, increases human natural killer activity and expression of anti-cancer proteins,” International Journal of Immunopathology and Pharmacology, 21(1), 117–127.[Return]
9. Bringing Forest Volatiles Indoors. Li, Q., et al. (2009), “Effect of phytoncide from trees on human natural killer function,” International Journal of Immunopathology and Pharmacology, 22(4), 951–959. In this experiment, healthy male subjects stayed in urban hotel rooms while vaporized hinoki cypress stem oil was introduced with a humidifier. Researchers detected phytoncides including α-pinene in room air and reported changes in NK-cell activity, cytolytic proteins, and urinary stress hormones.[Return]
10. The Biodiversity Hypothesis. Hanski, I., et al. (2012), “Environmental biodiversity, human microbiota, and allergy are interrelated,” Proceedings of the National Academy of Sciences, 109(21), 8334–8339. The study found associations among surrounding environmental biodiversity, skin microbiota, and allergic sensitization in adolescents. It contributed to the development of the biodiversity hypothesis: that reduced contact with natural environmental biodiversity may alter human microbial communities and immune regulation. The hypothesis does not imply that greater exposure to microorganisms is always beneficial; rather, it proposes that ecological relationships between environmental and human microbiota deserve consideration in understanding immune health.[Return]
11. Air Pollution and Long-Term Health. The adverse health consequences of chronic exposure to air pollution provide strong evidence that the composition of inhaled air can affect health over long periods. Fine particulate matter (PM₂.₅), ozone, nitrogen dioxide, and other pollutants have been associated with cardiovascular and respiratory morbidity and mortality, among other outcomes. The World Health Organization substantially tightened its global air-quality guidelines in 2021 after reviewing this evidence. FE2006 uses this established relationship as a starting point for a complementary and much less established question: if harmful characteristics of air can progressively damage health, might other characteristics of air positively contribute to maintaining health? See World Health Organization (2021), WHO Global Air Quality Guidelines: Particulate Matter (PM₂.₅ and PM₁₀), Ozone, Nitrogen Dioxide, Sulfur Dioxide and Carbon Monoxide.[Return]