Head to Toe

A microbiologist explains why we smell bad—and what it means for our health

Photograph by NIAID
NIAID / CC BY 2.0 Streptococcus bacteria.
Varieties of Sweat

Body odour is caused by a combination of sweat and bacteria. Sweat keeps our skin moist and regulates our temperature, but it also contains fats, hormones, and proteins to strengthen the barrier provided by our skin. These molecules are also used as food by the bacteria there. When they are done eating, these species release waste products—their own fecal matter—right onto the skin. Many of these have an aroma; some are sweet and musky, but most are quite unpleasant. The determining factor isn’t the bugs, however, it’s the type of sweat they are given. Humans produce four varieties of sweat, each one serving a different purpose. Depending on their content, the potential for smell can range from minimal to downright awful.

The least troublesome types of perspiration are thermal and gustatory sweat. They are produced when our bodies get too warm from the weather or exertion. The latter, more commonly known as the “meat sweats,” is caused by increased blood flow to the stomach when we eat dense food. Unlike thermal sweat, this type of sweating is limited to the forehead. These varieties are not associated with significant smell, since they consist mainly of water, electrolytes, and small quantities of waste products, primarily urea and ammonia.

The other two forms of sweat, emotional and apocrine, are the ones that cause all the trouble. It’s because these provide social cues to others about our health and welfare, both biological and psychological. Emotional sweat usually occurs on the hands and in the armpits, the genital regions, and the feet. Apocrine sweat is similar to scent secretions from animals and is designed to keep enemies at a distance while inviting potential mates to get closer. (The word apocrine comes from the Greek krinein, “to separate”.)

Emotional and apocrine sweats are thicker than thermal sweat because they are rich in molecules such as fats and hormones, many of which have a mildly pleasant aroma. Some of these secretions will be used by the skin to maintain elasticity and bonding. But most will be taken up by bacteria, used as nutrients, and eventually shed as smelly by-products. Over time the strength of the smell will grow as the concentration of the bacteria and the by-products rises. Eventually, without intervention—washing—the odour will emanate and find those sensitive olfactory nerves.

How often you need to clean comes down to the relative concentrations of the four types of sweat. Thermal and gustatory sweat will have little impact on your scent even if the sweating becomes profuse. The lack of any significant nutrients will keep the microbes from causing any olfactory offences.

But even the slightest amount of apocrine or emotional sweat can eventually turn a person’s scent foul. The stench is essentially a call for help from a suffering body, and it signals to other people that an individual is experiencing abnormalities such as infection, fear, and even depression.

As to how often you can go between washes, ask someone you trust. Let that person get within a foot of you, inhale, and then decide. Whatever you do, don’t trust yourself. Our olfactory systems have a tendency to become desensitized to our own smells and will be unable to pick up on any offending scents until they are overbearing. If you can smell yourself, it’s already too late.

The armpit of our emotions

There are two easy ways to find out if someone is stressed. The first is to ask. The other is to sniff that person’s armpit.

The armpit is scientifically known as the axillary region and has been the focus of many a study on human psychological states. The decades of research have revealed that a trained nose can detect anxiety, fear, calmness, and even attraction.

Depending on the person, a wide variety of bacteria may call the armpit home. Some of these produce sickly-sweet aromas; others make woodier and muskier scents. Some release odours that smell like cheese. Then there are those that offer the earthy odour of a damp forest. The determining factor is what we feed them.

Depending on our emotional state, we may form different types of fats and hormones in different concentrations. The best example of this comes when we are stressed. The main biological signal of stress is adrenaline, and it sparks the entire body to go on the defensive. Being on the defensive produces higher amounts of sweat, a higher body temperature, and a greater number of chemicals (which will give off the smell of fear once broken down by the resident microbes). Within minutes to hours, our scent can change from pleasant to offensive. It may take days before the system calms down and we return to a more natural, pleasing aroma.


Which kind of underarm protection is best? Deodorants are specifically designed to target the bacteria on the skin. They don’t necessarily kill, but they can inhibit growth so these microbes cannot produce an overwhelming odour. Most also have some form of fragrance to mask any bacterial unpleasantness that may develop over time. Deodorants are quite efficient, although they don’t tend to last very long. They also don’t prevent sweating, so those who have an overactive sweat gland could still end up with wet marks on their shirts.

The chemicals in antiperspirants differ depending on the brand, but almost all contain metal aluminum. This element gets into the skin and blocks pores to prevent sweat from escaping. Instead, excess water is sent to other regions of the body for excretion. This is a highly effective way of keeping your armpits dry. There may be hazards, though. Some researchers have suggested aluminum in the blood increases the risk for some cancers and Alzheimer’s disease. But no concrete links have so far been found, meaning this may simply be a matter of coincidence, not causation.

The other issue with antiperspirant is that it cannot fend off certain types of very smelly bacteria. When antiperspirant is used constantly, some environmental types of bacteria can find a home by attaching to the chemical components of the cosmetic. One such example is a group of bacteria found in dirt. We pick these hitchhikers up on our hands and no doubt transfer them to the armpits during our daily checks. Once they have made a home, they can use the sweat to thrive. For us, this means a change in our odour to include the smell of, well, mud. This is reversible by quitting antiperspirants for a while and going with either a deodorant or your natural scent.


It happened to me soon after I turned forty. The person walking beside me noticed I had a particular smell.

I checked my armpits to be sure, but I couldn’t detect anything. So I asked what the aroma was like. She said, “Old-man smell.” For anyone who hasn’t had the chance to experience this odour, it’s a combination of stale air mixed with grass and grease. It’s not pleasant.

The chemical behind this particular aroma is a compound known as 2-nonenal. It’s essentially a fat molecule formed as a by-product of various cellular processes in humans and bacteria. The chemical is better known in the food world as a component of fermentation, to increase the grassy and oily odour of aged foods such as beer and wheat. The chemical is also made in larger amounts in people over forty years of age.

Unlike other human odours, which are mainly caused by bacteria, this one happens to be our own fault. It’s formed under the skin and released as a waste. You might be able to reduce the levels of the stuff by reducing your consumption of alcohol and tobacco, which tend to force the body to send out fats in much higher concentrations. You can also reduce your intake of animal fats and butter. These are high in omega-7 fatty acids and they can make for a rather stinky output once metabolized.

Showering and bathing temporarily removes 2-nonenal. A longer-lasting fix is to flush it out of the system with a lot of sweating. Getting a good workout can help keep the concentration of the smelly scrounger low enough to be undetectable to most people. It may take a few days to get there if you’ve been for the most part sedentary. Increased showering over this time is a must! Keep exercising regularly and you will soon begin to smell youthful and look it too.


Foot odour comes in four main varieties: sweaty, cheesy, vinegary, and cabbage-y. That’s because of chemicals produced by the bacteria down there. Methanethiol is a key component in the flavour of cheddar cheese. Acetic acid is a result of sugar fermentation—and is better known as vinegar. By-products associated with rot, such as propionic acid and butyric acid, can leave feet smelling like rancid cabbage. The most common foot-related chemical, isovaleric acid, is responsible for the smell we call “sweaty.” Our noses are up to two thousand times more sensitive to this chemical than the others, and many of us can recognize it even at the slightest concentration.

Only a few types of bacteria have learned to enjoy inhabiting the foot. Most of these are friends, despite their smell, and our lifelong partners. At any given time, we have hundreds of millions of them living happily on our feet, which they regard as the perfect environment: warm, moist, and offering an unending supply of nutrients in the form of dead skin cells. They adhere to us shortly after birth and stay with us for the rest of our lives. They are also a necessary part of keeping our feet healthy. The bacteria release oils that help keep skin soft and enzymes that break down dead skin and prevent dry, flaky areas, as well as calluses. Our foot friends also provide a barrier against microbial pathogens. Our bacteria are very territorial, and they have mechanisms to ward off disease-causing visitors. They produce a number of defensive molecules, called antimicrobial peptides, which seek out and kill any invaders. These molecules are similar to antibiotics, but pathogens cannot develop resistance to them. To have the healthiest feet, we need these good microbes working hard for us. It can be difficult to assess their presence with our eyes, but we can always perform a smell test to ascertain if our feet are in good microbial hands. When we have a smell that is familiar to us—even if it isn’t pleasant—we can be sure we’re maintaining the same microbial population.

If that smell changes, though, and becomes more bread-like, grape-like, or acrid instead of sour, it can be a warning sign. There are several infections, mainly fungal, which can take residence on the foot and start to attack. Unlike our microbial flora, which prefer to feed off dead skin cells, these intruders want to eat something fresh. Without proper treatment, these pathogens can cause rashes, breaks in the skin, and larger wounds. Should this happen, you may require medical attention.

While the smell of your feet is usually a sign of your overall health, it might not do wonders for your social life. Thankfully, there are ways to keep the friendly bacteria happy while still keeping scents to a minimum. One option is to use talcum powder or charcoal inner soles. They both absorb the smelly chemicals and prevent them from dispersing in the air. While they won’t make your feet smell nice, they can keep your shoes from accumulating noisome chemicals.

There are other naturally derived compounds—including citral, geraniol, and limolene—that are known to help improve that familiar foot smell. These chemicals shift the way the bacteria make by-products, inhibiting isovaleric acid from being produced in the first place. They can be found in several common foot-care products available in drugstores.

Excerpted from The Germ Files: The Surprising Ways Microbes Can Improve Your Health and Life (And How To Protect Yourself From the Bad Ones) by Jason Tetro. Copyright © 2016 Jason Tetro. Published in Canada by Doubleday Canada, a division of Penguin Random House of Canada Limited. All rights reserved.

Jason Tetro
Jason Tetro (@JATetro) is the author of The Germ Files.