REACTIVE OXYGEN SPECIES & POLLUTION

•  Reactive Oxygen Species (ROS) are a natural occurrence in the body and are essential for cellular health at low concentrations. However, pollution, along with other environmental factors such as UV, can increase these levels causing these species to do more harm than good.
• Unstable by nature, ROS can oxidize important molecules present in the skin such as lipids and proteins, depleting skin barrier function. 
• ROS can also be responsible for cellular damage. Furthermore, the natural defenses of cells in the form of antioxidant enzymes are often reduced by pollution, as well as the actual antioxidants, including glutathione and ascorbic acid.

SKIN MICROBIOME, POLLUTION & ALTITUDE

• In the uppermost layer of the skin called the stratum corneum, there are colonies of good bacteria that facilitate proper skin function known as the microbiome.
• The composition of this microbiome is dependent on factors such as age, location on the body, and lifestyle, and it constitutes an integral part of the human immune system.
• When excessive pollution comes in contact with the microbiome, it creates an environment which favors the growth of harmful bacteria, or pathogens such as Staphylococcus and Streptococcus varieties. These can then lead to skin conditions such as cellulitis and acne.
• Air pollution can also decrease the size of the microbiome, exemplified by a 50% decrease observed in a study of ozone exposure.
• High altitudes are associated with skin microbiome that has lower diversity. This development can be the result of selective evolution for typical environmental characteristics such as lower oxygen concentrations, lower temperatures, and greater ultra-violet radiation exposure.

INFLAMMATION & POLLUTION

• Some pollutants can penetrate deeper layers of the skin triggering inflammation by activating the Aryl Hydrocarbon Receptor (AhR) pathway. These receptors are a natural defense against toxins and pollution, as they prompt the production of cytokines, including certain interleukins, which, when activated for extended periods, can cause skin damage.
• Skin lesions and inflammatory skin conditions such as psoriasis and atopic dermatitis are all associated with increased activity of these receptors.
• Other inflammatory markers activated by pollution include transforming growth factor (TNF-alpha) and prostaglandins. Extended exposure to these markers is associated with skin aging and skin cancer.
• Air pollutants are also able to sit on the top of the skin, clogging pores and preventing oxygen permeation, creating a favorable environment for acne flares.

SKIN BARRIER FUNCTION, HUMIDITY & TEMPERATURE

•  Filaggrin, an essential protein in skin barrier function, is more available in low humidity environments, which leads to increased NMF levels.
• The skin adapts to long term exposure to dry environments by developing a thick stratum corneum to decrease Transepidermal Water Loss (TWL) and increase skin barrier function.

SKIN HYDRATION, HUMIDITY & TEMPERATURE

• When exposed to low humidity, skin hydration decreases, which consequently causes more sebum production to combat the loss of moisture. Such changes can be observed after 7 hours in a 30% relative humidity environment.
• The capacity for the skin to hold water is not dependent on age according to a study of Chinese women aged 20 to 70 years. Instead, low humidity and cold temperatures were the primary causes of skin dryness observed in their Beijing sample compared to the Guangzhou sample, which lived in a hotter, more humid climate.
• Women between 20 and 30 years old exhibited considerable variation in sebum production in the winter and summer, but this variation decreased significantly for those more than 40 years.
• Inflammatory skin conditions such as dermatitis are associated with low levels of Natural Moisturizing Factor (NMF), which often decrease in the winter. However, these levels are generally higher in older people after taking into account the environment.
•  Females also tend to have higher levels of NMF than males, after taking into account the general increase in NMF in the summer.

TROPOSPHERIC OZONE

• The increasing ambient concentrations of Tropospheric Ozone, the most strong oxidant ever known, is a rising global concern.
• Short term exposure to ambient tropospheric Ozone causes cellular level reactions in the upper as well as deeper skin layers.
•  The changes that happen at cellular level include oxidation of bio molecules, generation of radical species, depletion of antioxidant defense, cytotoxicity and cellular stress.
• It is hypothesized that long term exposure will contribute to skin aging.
•  O3 rapidly oxidizes lipids and proteins in the skin, thus creates radical species, like hydroxyl radical, and triggers oxidative stress.
• O3 depletes antioxidants from the stratum corneum in murine skin.
• O3 induces the expression of P450 genes in the skin which in turn may cause atopic dermatitis.

TRAFFIC SOMOKE, SOOT AND PARTICULATE MATTER

•  Nitrogen Dioxide, released from road traffic and burning of fossil fuels, causes skin pigment spot formation, say studies conducted in Germany and China.
• A 2019 study suggests that exposure of human skin ex vivo or in vivo to non-toxic concentrations of a standardized diesel exhaust mixture increased skin pigmentation by inducing melanin de novo synthesis via an oxidative stress response.
• PM, or coarse particles are invisible solid and liquid particles with diameters of either less than 10μm8 (PM10), or 2,5μm (PM2.5).
• An increase in soot and particulate matter in air from road traffic and other sources causes 20% more pigment spots on forehead and cheeks.
• Women having certain genetic variants of the aryl hydrocarbon receptor (AHR) signaling pathway are more vulnerable to developing facial pigment spots caused by exposure to particulate matter.

VOLATILE AND NON-VOLATILE ORGANIC COMPOUNDS

LOW HUMIDITY AND HIGH WIND VELOCITY

• Low relative humidity and temperature coupled with high wind velocity in high altitudes may lead to Xerosis which may further develop into pruritus, cracks, fissuring and oozing.
•  Chilblains also is an inflammatory disorder of skin exposed to nonfreezing temperatures for a long duration of time.
•  Chronic oxygen deprivation in high altitudes causes increased erythropoiesis, which is the increased production of red blood cells and subsequent depletion of iron inside the cells, and results in thinning of the nail plate.

POLLEN

•  Plant pollen in the air can cause allergic reactions on skin.
•  Hazelnut and birch pollen in the air destroy tear film components in the eye and damage ocular surface cells.
• Different pollen species cause degradation of tear proteins.
• Plant pollen can also destroy the epithelial cells in the eyes.
• Pollen from the plant, Parthenium, causes itchy scaly plaques on the exposed sites involving the folds in airborne contact dermatitis.
•  Cedar pollen has caused airborne contact dermatitis in Japan.