The effects of extreme heat on the skin.

Heat rash, also called “miliaria,” is one of the skin eruptions commonly seen during the summer. It appears as small vesicles or papules, often grouped together, which 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 occur in various areas exposed 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 about 36.5°C (97.7°F). However, in conditions of intense heat and humidity, this sweat can accumulate and may not be eliminated properly. It then mixes with sebum and the impurities present on the surface of the skin, promoting blockage of the sweat ducts. Contrary to the common belief that sweat clogs pores, it is this combination that is actually responsible.

This obstruction leads to the retention of sweat beneath the epidermis, which causes a localized inflammatory reaction and the appearance of these characteristic small bumps, sometimes associated with heat-related skin redness. Preventing heat rash in adults and children is based on simple measures aimed at limiting the buildup 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 adaptive mechanisms designed to regulate body temperature, particularly via sweating and vasodilation. While these processes are essential, they are not without consequences for skin balance. In fact, heat promotes an increase in water loss, which can paradoxically lead to dehydrated skin in summer, despite a sensation of sometimes oilier skin associated with sweating. This water loss weakens the barrier function, making the skin more vulnerable to external aggressions and potentially intensifying feelings 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 skin parameters, including transepidermal water loss. The results show a significant increase in this water loss. This change does not appear to be directly related to the increase in cutaneous blood flow, but rather to a greater 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 to an alteration of the skin barrier.

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

During periods of intense heat, the skin becomes more prone to skin irritations, which generally appear 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 as an aggravating factor in this context by weakening the skin barrier and increasing skin reactivity, especially in individuals with heat-sensitive skin.

These irritations are explained by a combination of factors: excessive sweating, a buildup of heat on the skin’s surface, and repeated friction, which gradually damages the stratum corneum. The moisture from sweat causes the skin to macerate, while textiles—especially those that are not very breathable—intensify micro-mechanical aggressions. This context promotes the onset of local inflammation and heat-related skin irritation. To reduce these issues, it is recommended to opt for loose, breathable clothing made from natural fibers such as cotton or linen, in order to minimize friction and facilitate the evaporation of sweat.

Advice : To soothe this irritation, you can use daily treatments 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 in the ankles, feet, and hands. This phenomenon corresponds to an edema, that is, an accumulation of fluid in the tissues. It is often temporary and occurs more easily in people who are not acclimated to heat. The skin may then appear tight and leave an indentation 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 can lead to a blood flow increase up to three times higher than normal, via the activation of thermosensitive mechanisms at the level of the microcirculation. This increase in perfusion is accompanied by an enhanced movement of fluid from the capillaries into the tissues, thereby promoting the development of edema.

Although this vasodilation can lead to visible effects such as swelling, it nevertheless plays an essential physiological role. By improving blood circulation and tissue oxygenation, it contributes to the body’s thermoregulation and can promote a feeling of muscle relaxation. However, when this fluid accumulation becomes uncomfortable, a few simple measures such as elevating the legs, limiting heat exposure, or being more physically active can help restore venous return and reduce swelling and sensations of heavy legs.

Beyond its immediate effects, heat can also influence the behavior of certain skin conditions. For example, pityriasis versicolor, an infection linked to the fungusMalassezia furfur, is favored by a warm and humid environment. It presents as scaly spots of varying color (white, pink, brown), located mainly on the trunk, neck, or shoulders. Heat and sweating create an environment that promotes the proliferation of this microorganism, which explains the high rate of recurrence in summer.

Heat can also worsen certain chronic inflammatory diseases such as eczema, psoriasis, rosacea, hives, or even 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 skin permeability, which facilitates the entry of allergens and irritating substances.

In addition, extreme heat can also influence how certain skin conditions are perceived, such as acne. Many people notice an apparent improvement in their acne during the summer, mainly due 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 the fall, once sun exposure is reduced, a rebound effect is frequently observed, with the lesions sometimes reappearing more severely, linked to increased keratinization and stimulation of sebum production.

Note : While heat itself is not directly responsible for skin cancers or for sunburns, it is often associated with increased exposure to UV radiation, the main risk factor.

Indeed, in summer, the 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 systematically protect your skin 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).