As seasons shift, we see our surroundings change.
With spring comes warmer weather, as the natural world revives and reinvigorates itself. By summer, we see longer days with more sun and higher temperatures. The mercury gradually decreases and days shorten as autumn arrives, taking us through to winter, where we experience the lowest temperatures year-round, with cold winds and low humidity.
Animals and plants around the world respond and adapt to these changing seasons. They migrate, hibernate and flower, in turn. But the impact of seasons on microbial life is much less-understood – and warrants more investigation, particularly in the face of climate change.
Seasonal skin impacts
Moving from season to season, our skin constantly adapts to changes in environmental conditions, such as temperature, humidity and UV rays.
In winter, for example, cold winds and indoor heating can lead to the loss of key structural lipids such as ceramides and fatty acids from the skin’s outer layer (the stratum corneum), reducing the flexibility and hydration of our skin and weaking our protective skin barrier [1,2,3].
In summer, high humidity and temperatures increase oil production  and transepidermal water loss (TEWL) as a result of increased sweating . The change in season effects the temperature of your skin surface – in summer, distal skin temperature decreases and proximal skin temperature increases relative to wintertime . More exposure to the sun and stronger UV rays can also increase the visible signs of aging by contributing to fine lines and wrinkles [5,6].
Skin conditions such as acne and psoriasis have also been found to vary across the year, with spikes in severity depending on season [7,8]. In the case of acne, low-humidity winters and summer temperatures can lead to flares as the dryer conditions and increased oil production make it more likely that individuals will suffer from clogged pores and breakouts. For psoriasis, dry air and low exposure to sunlight can worsen symptoms during autumn and winter.
The role of the skin microbiome – stable or seasonal?
Existing research has led to a general agreement among experts: the skin microbiome is fairly stable in terms of composition over time [9,10], with only minor variations year to year [11,12].
However, very few studies have investigated whether the bacterial composition of our skin is specifically influenced by the four seasons. So, it could be that a lack of research in this area is masking seasonal skin microbiome patterns.
We know that skin structure and properties and the skin microbiome are tightly linked – healthy skin is essential to support a healthy microbiome, and having a balanced skin microbiome also contributes to a healthy and stable skin environment (explore more in our Skin Health 101). Skin conditions, such as acne and psoriasis, have also been shown to be associated with, and potentially caused by, skin microbiome imbalances .
Such links allude to potential seasonal effects when it comes the skin microbiome. For example, those winter changes discussed above, where the skin’s outer layer loses key lipids such as ceramides and fatty acids, are consistent with other research that also found a decrease in skin hydration in winter along with a drop in pH  – effects that are known to disrupt the skin microbiome .
Similarly, in summer, as well as fine lines and wrinkles, increased UV can also disrupt the balance of skin microbes by damaging microbial DNA, suppressing the immune system and triggering the release of antimicrobial peptides [2,5,15,16].
The fungal community of our skin (the mycobiome) has been found to vary seasonally – in one of the few studies in Asian populations – with the fungal community composition shown to vary over the four seasons, but with less variation at oily, sebaceous sites .
So, it seems that skin microbiome could indeed have a seasonal component, but the question remains open as to whether this seasonal effect is significant for our skin health and its microbial communities over time.
The influence of seasonal effects on the skin microbiome and, in turn, skin health, remains controversial and under-researched. However, scientists are already evaluating whether the effects of the ongoing climate change, which impacts all seasons, can change skin microbiome interactions in future .
With clear evidence that temperature, humidity, UV rays and air pollution will all be changing climatic factors in our warming world, all of which have been shown to influence the skin microbiome, changes in the longer term may indeed be a reality.
For instance, when it comes to temperature, the optimal range for the survival and growth of skin bacteria (such as Corynebacterium and Acinetobacter) is 33.2–35.0° C, and hydration has been shown to influence the growth of these resident skin flora [16,18]. Research has also shown a spike in harmful Staphylococcus aureus skin infections in early September in connection to increased temperature and hydration conditions , indicating another potential negative impact for the skin microbiome and health in the face of rising temperatures.
Importantly, the climatic factors discussed do not exist independently and often influence the severity of one another, such as temperature increases causing increases in both humidity and UV intensity. We are also seeing shifting seasons and more extreme conditions due to climate change – which may have a knock-on effect for our health, as well as our microbiome and the wider global ecosystem .
Going forward, more skin-site-specific studies investigating the human skin microbiome over time will be needed to evaluate seasonal effects more thoroughly.
Gaining a better understanding of how the seasonal environment alters the human microbiome will inform efforts to support skin health and care. It holds promise for the treatment and prevention of many skin diseases, and will also help us understand potential future effects of climate change.
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