Air Pollution: Harmful To Skin And Overall Health

Air pollution has been shown to have a significant impact on skin health, with a range of consequences from irritation and premature ageing to organ damage and, in the worst cases, cancer. The skin is one of the first and major targets of air pollutants, which can enter the body through the skin itself or through inhalation or ingestion.

The main air pollutants affecting the skin include ultraviolet radiation, polycyclic aromatic hydrocarbons, volatile organic compounds, oxides, particulate matter, ozone and cigarette smoke. These pollutants can induce oxidative stress in the skin, leading to inflammation, disruption of the skin barrier function, and damage to DNA and proteins.

Prolonged or repeated exposure to high levels of air pollutants may result in profound negative effects on the skin, including skin ageing, inflammatory or allergic skin conditions such as atopic dermatitis, eczema, psoriasis or acne, and skin cancer.

Additionally, air pollution has been linked to increased sebum production, which can contribute to acne, and altered skin microbiota, which can impact skin health. Overall, the impact of air pollution on the skin is a growing area of concern, with potential consequences for both skin health and appearance.

Skin ageing

The skin is the first defensive barrier of our body, and all the cells and layers of the skin are involved in the barrier defence. However, some environmental factors could make the skin vulnerable. It is well-documented that sunlight can produce oxidative stress, photoaging, and photocarcinogenesis and more recently, research is focused on the assessment of injury that environmental contaminants may cause to the skin.

The skin ageing exposome falls into the following major categories: sun radiation (ultraviolet radiation, visible light, and infra-red radiation), air pollution, tobacco smoke, nutrition, some less well-studied factors, and cosmetic products.

Air pollutions are of more significant interest for also having a possible synergistic effect with solar radiation. As new data reported by the World Health Organization (WHO) reported in 2018, nine out of 10 people breathe high levels of air pollutants. The deaths per year due to ambient (outdoor) air pollution reached 4.2 million in 2016.

The increase in air pollution has harmful effects on human skin. The skin is exposed to environmental air pollutants, such PAHs, oxides, volatile organic compounds (VOCs), PM, and O3. Although human skin acts as a biological barrier against pro-oxidative air pollutants, prolonged exposure to their high levels can induce alterations in skin homeostasis and has been associated with ageing as well as inflammatory skin conditions (AD and eczema). The risk of skin cancer induced by air pollution is one of the central health fundamental problems from environmental studies.

Three mechanisms seem to be involved in the adverse effects of ambient air pollutants on skin health: a) the generation of free radicals; b) the induction of an inflammatory cascade; and c) the impairment of the skin barrier.

Skin cancer

Ultraviolet radiation (UVR) is a physical pollutant that is the most harmful environmental factor affecting skin biology. It contributes to photodamage on chronically sun-exposed areas, accelerating the physiological ageing process and increasing skin cancer risk. UVR influences the composition of the cutaneous microbiome and is the primary etiological factor responsible for skin cancer development.

Airborne polycyclic aromatic hydrocarbons (PAHs) are among the most widespread and dangerous organic pollutants. PAHs are highly lipophilic, which allows them to easily penetrate the epidermal skin barrier. PAHs have been implicated in the development of skin cancer. They can lead to acneiform eruptions and are found in cigarette smoke.

Particulate matter (PM) is among the most harmful pollutants. It is a heterogeneous mixture of very small particles and liquid droplets composed of organic chemicals, acids, metals, and soil or dust particles. PM can penetrate the skin either through hair follicles or transdermally, and exerts its detrimental effects through the generation of oxidative stress, which contributes to extrinsic skin ageing.

Ozone (O3) is a highly reactive pollution oxidant formed by the interaction of volatile organic compounds (VOCs) and gaseous nitrogen compounds upon UV-photoactivation. O3 exposure increases lipid peroxidation byproducts and antioxidants depletion. O3 exposure has been associated with urticaria, eczema, contact dermatitis, and other nonspecific eruptions.

Cigarette smoke contains thousands of chemical substances, including reactive oxygen species, carbon monoxide, reactive nitrogen species, and electrophilic aldehydes. It causes oxidative stress, induces transepidermal water loss, and upregulates matrix metalloproteinases, which degrade collagen and elastic fibres. Smoking has been associated with skin ageing, skin cancer, psoriasis, acne, and skin malignancy.

Nitrogen oxides (NOx) are emitted mainly from mobile and stationary combustion sources. They react with O3-forming nitrogen dioxide (NO2). NO2 causes oxidative damage, leading to the formation of free radicals that oxidise amino acids in tissue proteins and initiate lipid peroxidation of polyunsaturated fatty acids.

Volatile organic compounds (VOCs) are an important indoor source of air pollutants. They have been linked to an increased prevalence of atopic dermatitis and can induce precancerous skin lesions.

Other air pollutants, including carbon monoxide (CO), sulfur dioxide (SO2), and heavy metals, have also been implicated in skin ageing, inflammatory skin conditions, and skin cancer.

Skin irritation

Air pollution can cause skin irritation, which can range from mild discomfort to more severe conditions. The skin is the body's first line of defence against external factors, including air pollution. However, prolonged or repeated exposure to high levels of air pollutants can have detrimental effects on the skin.

Types of Air Pollutants

Air pollution consists of various gaseous compounds and particles, including nitrogen dioxide, sulphur dioxide, carbon monoxide, particulate matter, and heavy metals. These pollutants can be emitted from sources such as vehicle emissions, industrial facilities, and fossil fuel combustion.

Impact on Skin

The skin acts as a biological shield against pro-oxidative chemicals and physical air pollutants. However, exposure to air pollution can lead to oxidative stress, which is an imbalance between the production of destructive reactive oxygen species (ROS) and the body's ability to neutralise them. This can result in damage to the skin and other organs.

Particulate Matter

Particulate matter, or tiny floating particles in the air, can be particularly harmful to the skin. These particles can enter the pores and cause irritation and inflammation. They can also strip the skin of essential vitamins, leading to collagen damage and premature ageing.

Ozone

Ozone, a component of smog, is another air pollutant that can irritate the skin. It can deplete the skin of vitamins C and E, leading to increased lipid peroxidation and oxidative stress.

Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are air pollutants found in vehicle emissions, cigarette smoke, and grilled or charred meats. They can induce oxidative stress and are associated with skin ageing, pigmentation issues, and acne.

Volatile Organic Compounds

Volatile organic compounds (VOCs) are emitted from sources such as paints, varnishes, and vehicle refinishing products. They can contribute to the formation of ground-level ozone and have been linked to the development of inflammatory skin conditions like atopic dermatitis and eczema.

Strategies for Protection

To protect the skin from the harmful effects of air pollution, it is essential to use skincare products that contain antioxidants, such as vitamins C, E, and B3. Cleansing the skin thoroughly and maintaining proper skin hydration are also crucial. Additionally, limiting exposure to air pollution by reducing time spent outdoors during high-pollution periods can help minimise skin irritation.

Skin pigmentation

One proposed mechanism for this is that organic compounds present on the surface of particulate matter may penetrate the skin and directly affect viable skin cells, such as keratinocytes and melanocytes. Additionally, both UV radiation and particulate matter-loaded particles activate the aryl hydrocarbon receptor (AhR) in human skin, which contributes to skin pigmentation.

The aryl hydrocarbon receptor is a ligand-activated transcription factor that regulates the expression of genes, including those encoding the cytochrome P450 monooxygenases, which are important enzymes in the metabolism of xenobiotics. Activation of the AhR pathway has also been implicated in the pathology of atopic dermatitis.

Melasma, a benign hyperpigmentation disorder, may also be influenced by air pollution. Airborne particulate matter and polycyclic aromatic hydrocarbons present in polluted air enter the skin and generate quinones, which then trigger high metalloproteinase levels that lead to skin pigmentation.

Skin barrier function

The skin acts as a biological shield and plays a vital protective role, forming an 'inside-outside' barrier that regulates water loss and an 'outside-inside' barrier that protects the body from external harm, including mechanical, chemical, and microbial threats.

The outermost layer of the epidermis, the stratum corneum, is composed of dead corneocytes in a "bricks-and-mortar" arrangement. This layer controls the diffusion of molecules across the skin, including water loss and the entry of chemicals. The rate of diffusion across the stratum corneum is influenced by the unique lamellar organisation of the lipid matrix and its interaction with protein components of keratinocytes, including tight junctions and scaffolding proteins, as well as the diffusion path length, which is determined by the thickness of the stratum corneum, the number of layers of corneocytes, their size, and their cohesion.

The skin's protective ability is not unlimited, and problems can arise when abnormal exposure to environmental stressors exceeds the skin's normal defensive potential. Alterations that disturb the skin barrier function, such as those in the stratum corneum lipid metabolism or protein components of the corneocytes, can lead to the development of various skin diseases.

The stratum corneum contains a high level of unsaturated fatty acids and lipids, which are targets of oxidative damage caused by air pollutants. This damage can lead to barrier perturbations, the production of lipid ozonation products, and inflammation. The generation of oxidative stress by pollutants contributes to extrinsic skin ageing and can also cause pigmentation spots on the face, nasolabial folds, and wrinkles.

Overall, the skin barrier function is critical in maintaining the integrity and health of the skin, protecting it from external threats and regulating vital processes such as water loss.

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