Perspiration: what is the origin of unpleasant odours?

Contrary to a very widespread misconception, sweat is not in itself malodorous.

When it has just been secreted, it is generally odourless. Unpleasant smells mainly appear later, on the surface of the skin, when certain micro-organisms in the skin microbiota break down the compounds present in the secretions. Body odour therefore results less from perspiration alone than from the interaction between sweat, the skin and the bacteria that live there.

Not all sweat glands contribute to this phenomenon in the same way. Eccrine glands, distributed over almost the entire surface of the body, secrete a clear, watery sweat, primarily intended for thermoregulation. This sweat consists mainly of water, mineral salts and a few small molecules, and it contributes little to unpleasant odours. Apocrine glands, by contrast, play a much more significant role in unpleasant body odour. Located mainly in the armpits, the areola of the breast and the ano‑genital region, they become active at puberty under the influence of androgens and produce a more turbid secretion, richer in lipids and proteins. It is precisely these organic compounds that provide skin bacteria with a favourable substrate for the formation of odours.

Note : The sebaceous glands may also be involved, but in a more secondary way. By secreting sebum into the hair follicle, they supply lipids which can themselves be transformed by certain bacteria into odorous molecules. Nevertheless, in terms of their contribution to body odour, apocrine secretions clearly come first, followed by eccrine secretions, while sebum appears to play a more modest role.

The role of the microbiota is central in the development of unpleasant odours.

The bacteria most often involved mainly belong to the genera Corynebacterium and Staphylococcus, for example Staphylococcus hominis or Staphylococcus epidermidis. From odourless precursors present in sweat, they produce volatile molecules responsible for different olfactory notes. Isovaleric acid, derived in particular from the metabolism of leucine, is classically associated with a smell of perspiration or “changing room”. Other compounds, such as certain volatile fatty acids, give sour, rancid or cheesy notes. Finally, sulphur-containing molecules, especially thioalcohols, are involved in the strongest and most persistent axillary odours.

It should be noted that not all areas of the body smell in the same way, because they do not have the same glands, the same skin environment, or the same microbiota. The armpits are considered the reference area when discussing unpleasant body odours, as they combine several favourable factors: an abundance of apocrine glands, the presence of hair, a warm and humid environment, and a high bacterial density. The feet are another area frequently affected, but here the odour depends more on eccrine sweat, maceration, and bacterial proliferation in an often confined environment. The scalp or certain genital areas can also develop specific odours, linked to their own combination of secretions and micro-organisms.

There are also major individual variations. Sex, age and genetics influence the composition of skin secretions. For example, men on average have larger apocrine glands, which may contribute to a more intense odour. Age also plays a role: before puberty, apocrine activity is low, which explains why the typical unpleasant body odours of the armpits mainly appear during adolescence. Genetics can also modulate body odour, particularly in the axillary region, notably via the ABCC11 gene. Certain variants of this gene are associated with a much lower production of odour precursors in the armpits, and therefore with a markedly reduced body odour. This genetic variation is particularly common in East Asia.

Unpleasant odours result not so much from the amount of sweat produced as from its composition.

Furthermore, emotions can influence body odour. Under the effect of stress, fear or anxiety, the apocrine glands are more strongly stimulated, in association with adrenaline and activation of the sympathetic nervous system. This phenomenon explains why emotional sweating, particularly in the underarm area, is often perceived as stronger or more unpleasant than sweating caused by heat.

Diet can also modify body odour. Alcohol, certain spices and garlic can influence the composition of secretions or alter the skin microbiota. Hygiene also plays an obvious role: the longer secretions accumulate on the skin surface, the more time bacteria have to break them down. The presence of underarm hair can also increase the retention of odorous secretions. Finally, certain metabolic or infectious diseases can characteristically modify body odour, although these are more specific situations.

Limiting odours linked to perspiration is primarily based on a simple principle: reducing the build-up of sweat and restricting the bacterial transformation of secretions.

Maintaining good hygiene is therefore the first step. Washing regularly, particularly after physical activity, helps to remove both sweat and some of the micro-organisms present on the surface of the skin. It is also important to dry yourself thoroughly, as residual moisture promotes bacterial growth.

The choice of clothing can also help. Synthetic materials tend to trap heat and moisture, creating an environment that favours odour formation. In contrast, natural fibres such as cotton, linen or wool allow better evaporation of sweat and limit maceration. The same principle applies to the feet: choosing breathable shoes, avoiding overly tight designs and alternating between pairs helps to reduce moisture and therefore bacterial proliferation.

Beyond these measures, deodorants and antiperspirants can be a solution.

Their mode of action differs. Deodorants act mainly by limiting the growth of the bacteria responsible for forming odorous compounds or by neutralising these molecules. Antiperspirants, on the other hand, aim to reduce sweat production by partially blocking the sweat ducts, most often through the use of aluminium salts. By decreasing the amount of sweat available, they indirectly reduce the substrate used by the bacteria. The use of aluminium salts is, however, a matter of discussion. Their ability to block sweat glands is well documented, but their long-term safety remains debated, particularly because of their potential to accumulate in the body. To date, the available data do not allow a definitive conclusion to be drawn regarding a proven risk at the doses used in cosmetics, but this uncertainty is driving the search for alternative solutions.

Thus, more recently, some scientists have sought not to act by eliminating sweat, but by modulating the skin microbiota. A study therefore evaluated antiperspirant formulations enriched with lysates of Lactobacillus ferment, combined with plant extracts such as hops (Humulus lupulus) and sage (Salvia officinalis). In an axillary microbiota model, the addition of these compounds made it possible to significantly reduce the micro-organisms involved in the formation of odours, while promoting a more stable microbial balance. Formulations containing these postbiotics proved more effective than those without them, suggesting that targeting the bacterial ecosystem could be an interesting alternative to aluminium salts.

Finally, certain lifestyle adjustments can complement these measures. Limiting the consumption of alcohol or highly spiced foods, which can alter the composition of secretions, as well as learning to manage stress more effectively, which can increase apocrine sweating, may help to reduce odours on a daily basis.

• LAZAREVIC V. & al. Mapping axillary microbiota responsible for body odours using a culture-independent approach. Microbiome (2015).

• NAGARAJAN N. & al. Understanding the microbial basis of body odor in pre-pubescent children and teenagers. Microbiome (2018).

• SMEETS M. & al. Intrinsic and extrinsic factors affecting axillary odor variation: A comprehensive review. Physiology and Behavior (2023).

• CELEBI L. & al. Postbiotics cosmetic formulation: In vitro efficacy studies on a microbiome friendly antiperspirant. Journal of Research in Pharmacy (2023).