What is your city
doing to your skin?

We have developed an objective and scientifically-based Skin Stress Index for comparing cities. The index combines data from five of the environmental stressors. The five metrics are each represented by a single city-wide dataset collected over the four seasons (Winter: Dec.-Feb., Spring: Mar.-May, Summer: Jun-Aug., Autumn: Sep-Nov.).

Each environmental index was normalized to a 1-100 stress scale, where 1 represents the lowest stress and 100 the highest. The relative humidity measure utilized a U-shaped formula that penalized deviations from the "optimal" range of 40-60%, following Engebretsen et al. (2016).

Seville secures first place due to the combination of four pressure points. First, Seville has a peak Summer UV Index of 2.62, which can burn an unprotected fair skin in about 10 minutes during midday. Second, Seville's temperature peaks at 36.5°C, making it one of the hottest cities in our data set. Third, the PM2.5 mean is 10.08 μg/m³. This is twice the WHO's annual guideline of 5 μg/m³.

Fourth, Seville experiences low humidity. While the optimal humidity range is 40-60%, Seville averages 27.91%. Such a condition accelerates transepidermal water loss (TEWL).

Denda et al. (1998) demonstrated that when skin was exposed to relative humidity below 30%, there was a greater-than-normal hyperproliferative response to minor irritants, as well as faster skin barrier disruption than occurs with minor irritants at relative humidity above 30%.

Murcia is in the driest part of continental Spain. High temperatures (34.8°C), a low humidity of 27.64%, and a peak UV Index of 2.76 in summer severely affect the skin. The heat is so severe in this dry environment, as it combines with a very low humidity, and the body is unable to retain moisture. The Natural Moisturizing Factor (NMF) is continuously functioning to keep the skin moistened. Physiologically, exposed skin is under stress during the six-month period.

Limassol has the highest UV peak during the summer months among all locations (2.82). Due to its location in the eastern Mediterranean, Cyprus is exposed to high amounts of ultraviolet radiation because of low cloud cover and a high sun angle. This results in an amount of ultraviolet radiation per square meter of skin. The research conducted by Watson et al. (2016) for Seminars in Oncology Nursing confirmed that intermittent high-intensity UV exposures carry a stronger melanoma risk than low-level chronic exposure. Additionally, the average PM2.5 in Limassol of 10.84 μg/m³ and the average summer humidity level of 35.82% increase the overall skin stress level.

UV is ranked the highest in the summer index for a single reason. It is the only metric that causes damage that can never be repaired at the cellular level.

Oxidative stress from PM2.5, trans-epidermal water loss from increased humidity, and disruption of the cell membrane due to cold all have an impact and can be eliminated with appropriate measures.

DNA strand breaks and cyclobutane pyrimidine dimers formed as a result of UVB damage will be removed by the skin's nucleotide excision repair mechanism. However, the removal process becomes less efficient with age, and the accumulation of these alterations contributes to carcinogenic activity decades before they become visible.

All eight of the cities with the highest UV index during the summer months are located south of the 44th parallel. The 44th parallel is an important geographic location that marks a point where the UV index drops significantly. Oulu, Finland, has a UV index of just 1.58 for its summer peak, while Limassol has a UV index of 2.82. This represents about 80% more UV radiation delivered to the skin at midday during the same month and on the same continent.

UV affects the skin the most during summer from a biological perspective. In addition, it is the only environmental metric that has an irreversible effect at the cellular level. Still, oxidative stress from PM2.5, transepidermal water loss from high humidity, and ceramide dysfunction from cold can be minimized by taking appropriate measures.

While Mediterranean cities cause skin stress through UV exposure and high temperatures, cities such as Bilbao (74.9%) and Liverpool (73.53%) experience high humidity. When the humidity is above ~70%, the stratum corneum becomes over-hydrated. This means that the rate at which dead skin cells naturally slough off slows down, and creates an environment where bacteria and fungi thrive.

Bilbao is among the top-15 most skin-stressed cities during summer with a final index of 62.93. It is primarily caused by excessive humidity and strong winds (3.5+ m/s for all Atlantic cities). According to Alexander et al. (2018), wind can increase the rate of transepidermal water loss both when it is dry and when it is humid, as it increases air movement on the skin and strips away the unstirred air layer, which limits evaporation.

Air quality is usually better in most European cities during the summer months than in winter, largely due to lower levels of fossil-fuel and wood-burning emissions. However, the Iberian Peninsula is an exception, and PM2.5 levels in urban centers remain much higher than the World Health Organization guidelines suggest. PM2.5 concentrations of 11.83 μg/m³ in Porto, 10.94 μg/m³ in Barcelona, and 10.92 μg/m³ in Lisbon are significantly affected by both Atlantic sea-salt aerosols and vehicle emissions. A study published in JCI Insight by Kim et al. (2021) showed that exposure to a constant PM2.5 concentration has a measurable negative impact on the function of the skin's barrier layer via the AhR–TNFα–filaggrin pathway.

Turin’s PM2.5 averages 49.0 μg/m³ daily during the winter, which is 10x the WHO guideline, and the 3rd highest fine particulate concentration in Europe. Humidity at 86.03% - high enough to affect stratum corneum integrity and reduce the rate of desquamation. The coldest average is 0.9°C, which is much warmer than in Northern Europe, but still a little below the optimal level. Almost every metric of Turin is above the optimal level, which makes it “earn” its first place.

High pollution and humidity; Milan’s situation is similar to Turin’s: PM2.5 averages 52.15 μg/m³ in winter, and humidity is around 84.19%. Milan falls behind by 1 point in the skin-stress ranking because it’s warmer and a little less humid. However, its fine particulate concentration is 3.4x higher than the European average and the 2nd-highest in the whole dataset.

Bucharest ranked 3rd due to the combination of high pollution and cold weather. Fine particulate matter averages 43.6 μg/m³, with peaks of 183.1 μg/m³, 4th-highest in the entire dataset. On average, the temperature drops to -1.3°C, and the humidity averages 75.47%, which is above healthy levels for skin but not as critical as pollution levels.

PM2.5 peaks in winter across most European regions, especially Poland and the Balkans. Kraków records the highest PM2.5 level with 54.23 μg/m³, followed by Milan (52.15), Turin (49), and Bucharest (43.6 μg/m³). While Krakow records the highest mean, it also has the highest peak PM2.5 level at 271 μg/m³, followed by Warsaw with 262.1 μg/m³ - around 18 times more than the WHO considers a safe level.

Coal and wood use for heating are the most common sources of PM2.5 pollution. Dijkhoff summarized the epidemiological and mechanistic evidence of the effects of long-term PM2.5 exposure on skin: premature aging, increased pigmentation spots, and a higher incidence of eczema/atopic dermatitis.

During the winter season, humidity in the Northern European regions can reach almost 95% on average. Thus, despite cities like Tartu having very clean air, a relative humidity of 94.58% is becoming the next biggest skin stressor (as well as temperature, of course). When relative humidity is above 80%, the stratum corneum absorbs excess moisture, causing it to swell and weaken the lipid matrix of the stratum corneum.