06 — The Environment

The skin exists at the boundary between the internal biological environment of the human body and the constantly changing external environment. Every day, skin encounters ultraviolet radiation, visible light, temperature fluctuations, humidity, wind, air pollutants, particulate matter, microorganisms, chemicals, mechanical friction, water, and changes in atmospheric conditions. At the same time, it responds to internal influences including hormones, circulation, immune activity, metabolism, hydration, nutrition, and the nervous system. Skin biology is therefore shaped by continuous interaction between intrinsic physiology and environmental exposure. The ability of skin to maintain homeostasis depends on its capacity to sense these changes, activate protective mechanisms, repair damage, modify barrier function, regulate inflammation, and adapt its cellular activity.

Environmental Effects on Skin encompass the broad range of biological changes produced or modified by external conditions. Some environmental influences are beneficial or physiologically necessary, while others can become damaging when exposure is excessive, prolonged, or poorly compensated by the skin's protective systems. The same environmental factor may produce different effects depending on its intensity, duration, wavelength, temperature, humidity, anatomical location, age, genetics, barrier condition, and previous exposure history.

Solar radiation is among the most biologically significant environmental influences on skin. The solar spectrum reaching the Earth's surface contains ultraviolet, visible, and infrared radiation. Ultraviolet radiation is conventionally divided into UVA and UVB, according to wavelength. UVA penetrates relatively deeply into the dermis and can contribute substantially to oxidative stress and photoaging. UVB is absorbed more strongly by the epidermis and is particularly important in direct DNA photodamage and erythema. Both wavelengths can influence cellular signaling, immune function, pigmentation, extracellular-matrix remodeling, and carcinogenesis.

The skin has evolved several mechanisms for responding to solar radiation. Melanin absorbs and dissipates portions of ultraviolet energy, reducing the amount reaching sensitive cellular structures. Melanin-containing melanosomes are transferred from melanocytes to keratinocytes, where they can form a protective supranuclear distribution sometimes described as a melanin cap. DNA-repair pathways correct many forms of ultraviolet-induced damage, while antioxidant systems neutralize reactive species generated by radiation. The epidermal barrier and physical architecture of the skin provide additional protection.

These defenses are effective but not absolute. Repeated ultraviolet exposure can produce cumulative molecular damage. UVB can generate direct DNA photoproducts, while UVA can produce substantial reactive oxygen species. These reactive molecules can oxidize lipids, proteins, and DNA and can activate intracellular signaling pathways associated with inflammation and extracellular-matrix degradation. Chronic exposure can therefore alter both the function and structure of skin.

Ultraviolet radiation also modifies immune behavior. Solar radiation can suppress or alter aspects of cutaneous immune surveillance, affecting antigen-presenting cells, cytokine signaling, and other components of the immune system. This immunomodulatory effect has physiological consequences but can also facilitate the persistence of damaged cells and contribute to the development of skin cancers.

Visible light occupies wavelengths between ultraviolet and infrared radiation and can also interact biologically with skin. One of the best-established effects is pigmentation. Visible light, particularly portions of the blue-violet spectrum, can stimulate pigmentation pathways through mechanisms that differ from conventional UVB-induced pigmentation. The response is particularly relevant in individuals with more highly pigmented skin, where visible-light-induced pigmentation can be more persistent.

Infrared radiation contributes primarily to thermal exposure. Excessive heat can influence blood-vessel dilation, inflammatory signaling, oxidative stress, and cellular responses. Chronic environmental heat may therefore interact with other environmental stressors, although the biological effects of infrared exposure are distinct from those of ultraviolet radiation.

Air pollution represents another important component of the modern skin exposome. Particulate matter, polycyclic aromatic hydrocarbons, nitrogen oxides, ozone, and other pollutants can interact with the skin directly or indirectly. Some pollutants can generate oxidative stress at the skin surface, alter barrier function, promote inflammatory signaling, and increase the formation of reactive chemical species.

Particulate matter can accumulate on the skin surface and interact with sebum, sweat, and other components of the surface environment. Certain pollutants can penetrate into superficial skin layers or enter through hair follicles. Pollutant exposure has been associated with oxidative stress, inflammation, pigmentation changes, barrier disruption, and processes relevant to premature skin aging.

Ozone presents an interesting example because it is highly reactive and generally does not need to penetrate deeply into skin to produce biological effects. It can react with surface lipids and generate secondary oxidation products. These products can subsequently participate in inflammatory and oxidative pathways. The skin's surface chemistry therefore becomes an important mediator between atmospheric pollution and cellular responses.

Climate and weather also influence skin physiology. Temperature affects blood flow, sweat production, lipid behavior, enzymatic processes, and water loss. In hot environments, increased sweating and vasodilation contribute to thermoregulation. In cold conditions, cutaneous blood vessels constrict to reduce heat loss. Prolonged exposure to cold can reduce skin perfusion and may contribute to dryness and barrier stress.

Humidity has a particularly direct relationship with epidermal barrier function. Low environmental humidity increases the water gradient between the skin and atmosphere and can increase transepidermal water loss. Prolonged exposure to dry air can therefore contribute to dehydration of the stratum corneum, changes in corneocyte organization, altered enzyme activity, scaling, and increased sensitivity.

High humidity produces a different physiological environment. Increased hydration of the stratum corneum can alter its mechanical properties and the activity of enzymes involved in desquamation. Heat and humidity also increase sweating and can modify the microbial and chemical environment of the skin surface. Environmental humidity therefore does not simply determine whether skin feels “dry” or “oily”; it changes the physical and biochemical conditions under which the barrier operates.

Water exposure itself can influence the stratum corneum. Repeated cycles of wetting and drying can alter corneocyte swelling and barrier organization. Prolonged exposure to water can increase skin hydration temporarily but does not necessarily improve long-term barrier function. Repeated exposure to detergents, solvents, and surfactants can remove or redistribute surface lipids and increase barrier disruption.

Chemical exposure is another major environmental influence. Surfactants, solvents, acids, alkalis, oxidizing agents, preservatives, fragrances, and occupational chemicals can interact with the skin depending on concentration, exposure time, molecular properties, and barrier integrity. Some substances remain primarily on the surface, while others can penetrate through the stratum corneum or enter through appendageal structures.

The skin responds to chemical irritation through coordinated barrier, neural, immune, and inflammatory mechanisms. Keratinocytes can detect chemical stress and release signaling molecules that influence neighboring cells and immune pathways. Sensory nerves can detect irritation and generate sensations such as burning, stinging, or itch. This demonstrates again that the skin is not a passive membrane but a biologically responsive interface.

Mechanical forces also influence skin. Friction, pressure, stretching, compression, scratching, shaving, and repeated movement can produce microscopic injury or stimulate adaptive changes. The skin responds through altered keratinocyte behavior, inflammation, extracellular-matrix remodeling, and changes in tissue thickness. Chronic mechanical stress can therefore contribute to conditions such as callus formation or localized barrier disruption.

The environmental microbiological landscape also interacts with skin. Skin is continuously exposed to microorganisms from air, water, clothing, surfaces, animals, and other humans. The resident microbiome helps maintain ecological balance, while the skin's physical barrier and immune system regulate microbial colonization. Environmental conditions such as humidity, temperature, ultraviolet exposure, sweating, and cleansing can alter the composition and activity of these microbial communities.

The skin's response to the environment is therefore highly adaptive. Barrier function can change, melanogenesis can increase, immune surveillance can be modified, blood flow can change, sweating can increase or decrease, and cellular repair mechanisms can be activated. These responses are not necessarily signs of damage. Many represent normal physiological adaptation. Biological stress becomes more consequential when exposure exceeds the capacity of the tissue to restore homeostasis.

Environmental exposure also accumulates over time. The concept of the exposome describes the totality of environmental exposures experienced by an individual across the lifespan and the biological responses they produce. Skin is particularly useful for understanding the exposome because it directly interfaces with many environmental factors. Ultraviolet radiation, pollution, climate, lifestyle, occupational exposure, cosmetics, cleansing practices, and other environmental variables can interact over years to influence the condition of the tissue.

This cumulative exposure helps explain why two individuals of the same chronological age may have very different skin characteristics. Their histories of ultraviolet exposure, climate, occupation, pollution, barrier disruption, hormonal environment, and other factors may be substantially different. Skin biology therefore cannot be understood exclusively through chronological age or inherited characteristics.

The second major dimension of environmental skin biology concerns variation in pigmentation and other biological characteristics among populations and individuals. Skin Biology Across Different Skin Tones must be understood scientifically without treating skin tone as a simple biological classification of people. Human skin pigmentation exists along a continuous spectrum, produced principally by differences in the quantity, type, distribution, size, persistence, and processing of melanin-containing organelles called melanosomes. Skin tone is therefore a phenotype arising from complex interactions among genetics, melanocyte biology, melanin chemistry, environmental exposure, hormones, inflammation, and other factors.

All humans possess melanocytes in the basal epidermis, and melanocytes generally occur at broadly similar densities across many body sites and across populations. Major differences in visible pigmentation arise primarily from how melanocytes produce, package, distribute, and maintain melanin rather than from simply having dramatically different numbers of melanocytes.

Melanin exists principally in two broad chemical forms: eumelanin, which is brown to black and provides relatively strong photoprotective properties, and pheomelanin, which is yellow to reddish and has different photochemical characteristics. The relative production of these pigments, together with their quantity and distribution, contributes substantially to visible skin coloration.

Melanin synthesis, or melanogenesis, begins within specialized organelles called melanosomes. Enzymes including tyrosinase participate in the biochemical conversion of the amino acid tyrosine through a series of reactions that ultimately produce melanin. Melanosomes mature through distinct developmental stages as pigment is synthesized and deposited within them.

Once formed, melanosomes are transferred from melanocytes into surrounding keratinocytes. The resulting distribution of melanin within the epidermis provides a major component of the skin's visible pigmentation and photoprotective capacity. Differences in melanosome size, maturation, distribution, persistence, and degradation help explain why pigmentation biology differs across skin tones.

In more highly pigmented skin, melanosomes tend to be larger, more individually dispersed within keratinocytes, and more resistant to degradation. In less pigmented skin, melanosomes are generally smaller, more likely to occur in clusters, and are degraded more rapidly within keratinocytes. These differences influence both the optical properties of the skin and the amount of melanin available to absorb and scatter ultraviolet radiation.

Melanin provides meaningful photoprotection, but it is not equivalent to sunscreen. It absorbs ultraviolet radiation and can dissipate much of the absorbed energy as heat, thereby reducing molecular damage. However, even highly pigmented skin remains susceptible to ultraviolet-induced DNA damage, photoaging, and skin cancer. The degree of protection is substantial but incomplete.

Differences in pigmentation also influence the visible consequences of inflammation and injury. One particularly important phenomenon is post-inflammatory hyperpigmentation, in which inflammatory or traumatic events lead to increased or uneven pigmentation after the original process has resolved. This response can occur in all skin tones but is often more persistent and clinically prominent in more highly pigmented skin.

Inflammation can influence melanocyte activity through cytokines, prostaglandins, growth factors, and other signaling molecules. Keratinocytes and immune cells can communicate with melanocytes during inflammatory responses, linking immune activity with pigmentation. Damage to the epidermal basement membrane can also allow melanin-containing material to enter the dermis, where it may be taken up by macrophages and persist for prolonged periods. This process, known as melanin incontinence, can contribute to longer-lasting pigmentation.

Pigmentation also affects the visual assessment of inflammation. Erythema, which results largely from increased cutaneous blood flow and vascular dilation, can appear differently across skin tones. In highly pigmented skin, inflammation may be less visibly red and may instead appear as violaceous, brown, gray, or deeper tonal changes. This does not necessarily indicate a weaker inflammatory response. It can simply reflect the optical interaction between vascular changes, melanin, tissue scattering, and baseline pigmentation.

Similarly, some dermatological conditions can present differently across skin tones. The biological disease process may be similar, while its visible expression differs because melanin modifies the optical appearance of the tissue. Scientific and clinical evaluation therefore benefits from understanding both the underlying biology and the visual properties of different pigmentation levels.

The optical properties of skin are determined by multiple variables, including melanin concentration and distribution, hemoglobin within blood vessels, collagen, water content, tissue thickness, and the way light is absorbed and scattered within the skin. Skin color is consequently not produced by melanin alone. It is an emergent optical characteristic of multiple biological components.

The protective role of melanin is particularly important in environments with high ultraviolet exposure. Evolutionary biology provides evidence that human pigmentation has been shaped in part by historical patterns of ultraviolet radiation. However, modern human populations live across diverse environments and frequently move between geographic regions, making pigmentation biology an important example of adaptation interacting with contemporary environmental exposure.

The relationship between pigmentation and vitamin D biology is also complex. UVB radiation contributes to the cutaneous synthesis of vitamin D, while melanin reduces the penetration of UVB. This does not mean that individuals with darker skin necessarily develop vitamin D deficiency, because vitamin D status is influenced by many factors including latitude, season, clothing, dietary intake, supplementation, age, body composition, and actual sun exposure. Skin pigmentation is one variable within a much larger physiological system.

Environmental effects on skin therefore cannot be separated completely from pigmentation biology. Ultraviolet radiation can stimulate melanogenesis; inflammation can alter pigmentation; pollution can contribute to oxidative stress; climate can alter barrier function; and barrier disruption can modify susceptibility to irritants. The response of skin is produced through the interaction of multiple biological systems rather than a single environmental pathway.

Importantly, variation in skin tone should not be interpreted as a hierarchy of skin quality, health, or biological complexity. Every human skin tone represents a functioning biological phenotype with particular optical, photoprotective, inflammatory, and pigmentary characteristics. Differences are best understood as variations in physiology rather than as differences in biological worth or complexity.

There is also considerable variation within every broad skin-tone category. Two individuals who appear to have similar pigmentation may differ substantially in melanogenesis, tanning response, inflammatory pigmentation, barrier function, sensitivity, aging patterns, and response to environmental exposures. Skin tone therefore provides useful biological information but cannot predict every aspect of an individual's skin behavior.

The most scientifically accurate approach is to treat pigmentation as one component of a continuous biological spectrum and to consider environmental exposure alongside individual genetics, age, anatomical site, hormonal state, barrier condition, immune response, and personal exposure history.

The living skin is consequently an environmental sensor, a protective barrier, a pigmentary organ, an immune interface, and a continuously adapting tissue. Its biology reflects both what occurs inside the body and what the body encounters outside it. Ultraviolet radiation, pollution, climate, humidity, chemicals, mechanical forces, microorganisms, and visible light can all influence cutaneous physiology. At the same time, melanin biology, barrier architecture, immune activity, sensory signaling, and tissue repair determine how skin responds to those environmental pressures.

Understanding environmental skin biology therefore requires moving beyond the idea that the environment simply “damages” skin. The environment is also a source of physiological information to which skin continuously adapts. The biological outcome depends on exposure, dose, duration, susceptibility, protective mechanisms, previous exposure, and the ability of tissue to recover.

Skin tone forms an essential part of this scientific picture because pigmentation changes how skin interacts with light and how certain biological processes become visible. Yet pigmentation is only one component of the extraordinarily complex human skin phenotype. Across the full spectrum of human skin tones, the fundamental architecture of the skin remains shared, while differences in melanin biology, optical properties, inflammatory visibility, pigmentation responses, and environmental adaptation contribute to the remarkable diversity of human skin.

The central principle of Skin & Environment is therefore one of interaction. Skin is shaped by its environment, but it is not passively shaped by it. It senses, adapts, protects, repairs, communicates, and remembers through biological processes operating at molecular, cellular, tissue, and systemic levels. The skin we see is consequently the visible expression of an ongoing dialogue between the organism and the world around it.

References

Passeron, T., Krutmann, J., Andersen, M. L., Katta, R. & Zouboulis, C. C. Clinical and biological impact of the exposome on the skin. Journal of the European Academy of Dermatology and Venereology, 34(S6), 4–10, 2020. 

Krutmann, J., Bouloc, A., Sore, G., Bernard, B. A. & Passeron, T. The skin aging exposome. Journal of Dermatological Science, 85(3), 152–161, 2017.

McDaniel, D., Farris, P. & Valacchi, G. Atmospheric skin aging—contributors and inhibitors. Journal of Cosmetic Dermatology, 18(2), 319–328, 2019.

Moutraji, R. & Taylor, S. C. Skin aging exposome in skin of color populations: Review of the literature. Dermatologic Surgery, 49(3), 272–277, 2023. 

Ultraviolet Radiation, Visible Light & Environmental Photobiology

Sklar, L. R., Almutawa, F., Lim, H. W. & Hamzavi, I. Effects of ultraviolet radiation, visible light, and infrared radiation on erythema and pigmentation: a review. Photochemical & Photobiological Sciences, 12(1), 54–64, 2013. 

Narla, S., Kohli, I., Hamzavi, I. H. & Lim, H. W. Visible light in photodermatology. Photochemical & Photobiological Sciences, 19, 99–104, 2020. 

Ezekwe, N., Maghfour, J. & Kohli, I. Visible light and the skin. Photochemistry and Photobiology, 98(6), 1264–1269, 2022. 

Lim, H. W., Arellano-Mendoza, M. I. & Stengel, F. Current challenges in photoprotection. Journal of the American Academy of Dermatology, 76(3S1), S91–S99, 2017.

Mahmoud, B. H., Ruvolo, E., Hexsel, C. L., et al. Impact of long-wavelength UVA and visible light on melanocompetent skin. Journal of Investigative Dermatology, 130(8), 2092–2097, 2010.

Air Pollution & the Skin

Araviiskaia, E., Berardesca, E., Bieber, T., et al. The impact of airborne pollution on skin: a review. Acta Dermato-Venereologica, 99(2), 123–131, 2019.

Krutmann, J., Liu, W., Li, L. & Pan, X. Pollution and skin: from epidemiological and mechanistic studies to clinical implications. Journal of Dermatological Science, 76(3), 163–168, 2014.

Vierkötter, A. & Krutmann, J. Environmental influences on skin aging and ethnic-specific manifestations. Dermato-Endocrinology, 4(3), 227–231, 2012.

Climate, Temperature & Environmental Stress

Ugwueke, G., Alomary, S. A., Baker, N. J., Taylor, S. C. & Elbuluk, N. The dermatologic effects of climate change on skin of color populations: a comprehensive review. JAAD Reviews, 8, 112–119, 2026. 

The review identifies rising temperatures, worsening air pollution, increased ultraviolet exposure, and extreme weather as important environmental pressures on skin, while emphasizing the limited amount of skin-of-color-specific climate research currently available. 

Human Skin Pigmentation & Melanosome Biology

Quevedo, W. C. Jr., Fitzpatrick, T. B. & Jimbow, K. Human skin color: origin, variation and significance. Journal of Human Evolution, 14(1), 43–56, 1985. 

Jimbow, K., Quevedo, W. C. Jr., Fitzpatrick, T. B. & Szabó, G. Some aspects of melanin biology: 1950–1975. Journal of Investigative Dermatology, 67(1), 72–89, 1976.

Yamaguchi, Y. & Hearing, V. J. Melanocytes and their diseases. Cold Spring Harbor Perspectives in Medicine, 4(5), a017046, 2014.

D'Mello, S. A. N., Finlay, G. J., Baguley, B. C. & Askarian-Amiri, M. E. Signaling pathways in melanogenesis. International Journal of Molecular Sciences, 17(7), 1144, 2016.

Skin Biology Across Different Skin Tones

Taylor, S. C. Skin of color: biology, structure, function, and implications for dermatologic disease. Journal of the American Academy of Dermatology, foundational literature on cutaneous variation and dermatologic manifestations in diverse pigmentation phenotypes.

Bradshaw, M., Taboas, J. M., et al. Reinforcing photoprotection for skin of color: a narrative review. Journal of Drugs in Dermatology, 2023. This review summarizes differences in pigmentation, melanosome biology, ultraviolet responses, visible-light-induced pigmentation, and photoprotection across skin tones. 

Visible Light & Pigmentation in Skin of Color

Narla, S., Kohli, I., Hamzavi, I. H. & Lim, H. W. Visible light in photodermatology. Photochemical & Photobiological Sciences, 19, 99–104, 2020. 

Mahmoud, B. H., Hexsel, C. L., Hamzavi, I. H. & Lim, H. W. Effects of visible light on the skin. Photochemical & Photobiological Sciences, foundational work demonstrating stronger and more persistent visible-light-induced pigmentation in darker skin types. 

Ezekwe, N., Maghfour, J. & Kohli, I. Visible light and the skin. Photochemistry and Photobiology, 98(6), 1264–1269, 2022. 

Visser, W. I., Moola, H., Kannenberg, S., Tod, B. & Lim, H. W. Beyond tint: active ingredient strategies for post-inflammatory hyperpigmentation due to visible light in skin of color. Photodermatology, Photoimmunology & Photomedicine, 42(4), e70112, 2026. 

Photoprotection in Skin of Color

Taylor, S. C. Skin of color: biology, structure, function, and implications for dermatologic disease. Journal of the American Academy of Dermatology.

Al-Niaimi, F. & Chiang, N. Y. Topical antioxidants and photoprotection in darker skin types: current evidence and considerations.

Lim, H. W., Kohli, I., Granger, E., et al. Photoprotection for skin of color: current evidence and clinical considerations.

The contemporary literature emphasizes that greater constitutive pigmentation provides meaningful protection against ultraviolet-induced erythema and some forms of DNA and photodamage, but does not eliminate the need for photoprotection. It also demonstrates that visible light can produce particularly persistent pigmentation in darker skin types. 

Environmental Exposome & Skin of Color

Moutraji, R. & Taylor, S. C. Skin aging exposome in skin of color populations: review of the literature. Dermatologic Surgery, 49(3), 272–277, 2023. 

Passeron, T., Krutmann, J., Andersen, M. L., Katta, R. & Zouboulis, C. C. Clinical and biological impact of the exposome on the skin. Journal of the European Academy of Dermatology and Venereology, 34(S6), 4–10, 2020. 

The exposome framework is particularly valuable for skin biology because it integrates cumulative solar radiation, air pollution, climate, hormones, nutrition, lifestyle, and other environmental influences over the life course rather than examining each exposure in isolation. 

Contemporary Perspective

Recent literature is moving toward a more inclusive environmental model of skin biology. Climate change, increasing heat, air pollution, ultraviolet exposure, and changing environmental conditions may have different dermatological consequences depending on pigmentation phenotype, baseline barrier function, geography, occupation, and cumulative exposure. Importantly, the 2026 literature identifies a significant research gap: people with darker skin tones remain underrepresented in environmental and climate-related dermatology research. 

This makes 06 — Skin & Environment an important concluding article for the foundational Skin Biology sequence: the skin is not biologically isolated from its surroundings, and neither environmental exposure nor pigmentation can be understood adequately without considering the other.