The effects of extreme heat on the skin.

Heat rash (also known as "miliaria") is one of the skin eruptions commonly seen in the summer months. It presents as small vesicles or papules, often clustered together, and may be associated with itching and a stinging or prickling sensation in the skin when exposed to heat. In both adults and children, it can appear in various areas prone to sweating, particularly the face, forehead and cheeks, where the skin is especially affected by heat.

Heat rash is directly linked to a dysfunction of the sweat glands.

When the outside temperature rises, the skin produces more sweat to regulate its own temperature and maintain it at around 36.5°C. However, in conditions of intense heat and humidity, this sweat can accumulate and may not be eliminated properly. It then mixes with sebum and impurities present on the surface of the skin, promoting blockage of the sweat ducts. Contrary to the common belief that sweat itself clogs the pores, it is this combination that is responsible.

This obstruction leads to the retention of sweat beneath the epidermis, which causes a localised inflammation and the appearance of these characteristic small spots, sometimes associated with heat-related skin redness. Preventing heat rash in adults and children relies on simple measures aimed at limiting the accumulation of sweat, such as wearing breathable clothing, maintaining good body hygiene, and using light, non-occlusive skincare products.

When temperatures rise, the skin is subjected to adaptation mechanisms designed to regulate body temperature, in particular via sweating and vasodilation. Although these processes are essential, they are not without consequences for the skin’s balance. Indeed, heat promotes an increase in water loss, which can paradoxically lead to dehydrated skin in summer, despite the feeling that the skin is sometimes oilier due to sweating. This water loss weakens the barrier function, making the skin more vulnerable to external aggressions and potentially intensifying sensations of discomfort.

These observations were confirmed by a study conducted in 30 young adults, in which the skin of the forearm was exposed to a local increase in temperature (from 35°C to 40–42°C) for 12 minutes. The researchers measured several cutaneous parameters, including transepidermal water loss. The results showed a significant increase in this water loss. This change does not appear to be directly linked to the rise in cutaneous blood flow, but rather to an increased activation of the sweat glands. In other words, heat stimulates sweating, which leads to greater evaporation of water at the skin surface, contributing to progressive dehydration and impairment of the skin barrier.

Hence the importance, during the summer months, of maintaining good skin hydration, by favouring serums rich in humectants, such as hyaluronic acid, combined with a moisturising cream, ideally with a lighter texture to adapt to high temperatures, while still supporting the skin’s barrier function.

During periods of extreme heat, the skin becomes more prone to skin irritations, which generally present as redness, itching, and a sensation of skin that feels hot or stinging. These reactions are particularly common in areas where the skin is covered or subjected to friction, such as skin folds or areas in contact with clothing. Heat acts here as an aggravating factor, weakening the skin barrier and increasing skin reactivity, particularly in individuals with sensitive skin to heat.

These irritations can be explained by a combination of factors: excessive sweating, a build-up of heat on the skin’s surface, and repeated friction, which progressively damages the stratum corneum. Moisture from sweat causes the skin to macerate, while textiles, especially if they are not very breathable, intensify these micro-mechanical aggressions. This situation promotes the onset of local inflammation and heat-related skin irritation. To minimise these problems, it is advisable to opt for loose, breathable clothing made from natural fibres such as cotton or linen, in order to reduce friction and facilitate the evaporation of sweat.

Advice : To soothe this irritation, you can use daily skincare products formulated with soothing and repairing active ingredients, such as panthenol, allantoin or Centella asiatica).

The heatwave is also associated with swelling of the extremities, particularly around the ankles, feet and hands. This phenomenon corresponds to oedema, meaning an accumulation of fluid within the tissues. It is often temporary and occurs more readily in people who are not acclimatised to the heat. The skin may then appear tight and may pit when pressed, a characteristic sign of this fluid retention.

This phenomenon is largely explained by heat-induced vasodilation. To dissipate body heat, blood vessels dilate, which increases blood flow to the skin and the extremities. A study conducted in healthy adults showed that a local increase in limb temperature could lead to a threefold increase in blood flow compared with normal, via the activation of thermosensitive mechanisms within the microcirculation. This increase in perfusion is accompanied by a greater movement of fluid from the capillaries into the tissues, thereby promoting the development of oedema.

While this vasodilation can lead to visible effects such as swelling, it nonetheless plays a key physiological role. By improving blood circulation and tissue oxygenation, it contributes to the body’s thermoregulation and can promote a feeling of muscular relaxation. However, when this fluid build-up becomes uncomfortable, certain simple measures such as elevating the legs, limiting exposure to heat, or increasing physical activity can help to stimulate venous return and reduce swelling and sensations of heavy legs.

Beyond its immediate effects, heat can also influence the behaviour of certain skin conditions. For example, pityriasis versicolor, an infection caused by the fungus Malassezia furfur, is promoted by a warm and humid environment. It presents as scaly patches of variable colour (white, pinkish, brown), located mainly on the trunk, neck or shoulders. Heat and sweating create an environment favourable to the proliferation of this micro-organism, which explains the high rate of recurrence in summer.

Heat can also exacerbate certain chronic inflammatory conditions such as eczema, psoriasis, rosacea, urticaria and lichen planus. In the case of eczema, for example, heat promotes sweating, which can irritate the skin and trigger flare-ups. Furthermore, by weakening the skin barrier, heat increases the permeability of the skin, which facilitates the entry of allergens and irritant substances.

In addition, high temperatures can also influence how certain dermatoses are perceived, such as acne. Many people notice an apparent improvement in their acne in summer, mainly linked to sun exposure, which can dry out lesions and mask imperfections through a temporary anti-inflammatory effect. However, this effect is often short-lived: in autumn, once exposure is reduced, a rebound effect is frequently observed, with the sometimes more marked reappearance of lesions, associated with hyperkeratinisation and stimulation of sebum production.

Note : While heat itself is not directly responsible for skin cancers and sunburn, it is often associated with an increase in exposure to UV radiation, the main risk factor.

Indeed, in summer, the amount of time spent outdoors increases, as does the intensity of solar radiation, which is often correlated with the sensation of heat. It is therefore more than necessary to protect one’s skin systematically with a sunscreen during the summer period, and to reapply it in the event of prolonged exposure, in order to prevent both short- and long-term damage.

• KISSIN M. Edema in hot weather. JAMA (1975).

• PRAUSNITZ M. R. & al. The effect of heat on skin permeability. International Journal of Pharmaceutics (2008).

• AGGARWAL G. & al. Miliaria – An update. Research Journal of Pharmaceutical, Biological and Chemical Sciences (2017).

• BUCKLEY T. J. & al. Assessment of temperature and ultraviolet radiation effects on sunburn incidence at an inland U.S. Beach: A cohort study. Environmental Research (2018).

• MAYROVITZ H. N. Effects of local forearm skin heating on skin properties. Clinical Physiology and Functional Imaging (2020).

• O-HAGAN J. & al. Public health implications of solar UV exposure during extreme cold and hot weather episodes in 2018 in Chilton, South East England. Journal of Environmental and Public Health (2020).

• GONZALEZ-ALONSO J. & al. Regional thermal hyperemia in the human leg: Evidence of the importance of thermosensitive mechanisms in the control of the peripheral circulation. Physiological Reports (2021).

• KIM B. E. & al. The impact of temperature on the skin barrier and atopic dermatitis. Annals of Allergy, Asthma and Immunology (2023).

• NIU Y. & al. Acute effects of temperature fluctuations on skin health: A randomized, crossover trial. Environmental Research (2026).