Sweat is about 99% water, with the remaining 1% being a surprisingly complex mix of salts, waste products, trace metals, amino acids, and even natural antibiotics. That small fraction tells a big story about what’s happening inside your body, from how hydrated you are to how well your kidneys are working.
The Main Ingredients
Sodium and chloride, the same two elements that make up table salt, are the most abundant solutes in sweat. Their concentrations range from 10 to 90 millimoles per liter, which explains why sweat tastes salty and can leave white streaks on dark clothing. Potassium shows up in smaller amounts, roughly 2 to 8 mmol/L, similar to its concentration in blood.
Beyond electrolytes, sweat contains lactate (5 to 40 mmol/L) and urea (typically 20 to 25 mmol/L in healthy people). Both are normal byproducts of metabolism. Lactate comes partly from the sweat glands themselves, which are metabolically active little organs, while urea is a waste product your body generates when it breaks down protein. Ammonia is present too, though in smaller quantities.
Sweat also carries free amino acids, including serine, histidine, alanine, glycine, and lysine, at concentrations several times higher than what’s found in blood plasma. The reasons for these elevated levels aren’t fully understood, but they may play a role in how sweat glands reabsorb sodium and chloride before sweat reaches the skin’s surface.
Two Types of Sweat Glands, Two Different Recipes
Your body has two main types of sweat glands, and they produce noticeably different fluids. Eccrine glands cover most of your body and are responsible for temperature regulation. Their output is the watery, salty sweat you’re most familiar with. It contains water, minerals, and metabolic waste products like urea and lactate. What ends up in eccrine sweat depends on your diet, hydration, metabolic rate, and even medications you’re taking.
Apocrine glands are concentrated in your armpits and groin. They produce an oily, initially odorless substance rich in proteins, lipids, and steroids alongside the same minerals and metabolites found in eccrine sweat. Apocrine glands secrete by pinching off parts of their outer cells, which is why they can expel fatty, water-resistant substances that eccrine glands cannot. The smell people associate with sweat doesn’t come from either secretion directly. It comes from bacteria on your skin breaking down those apocrine proteins and lipids.
During exercise, apocrine glands actually excrete higher concentrations of urea and potassium than eccrine glands do. Sodium output from apocrine glands also increases significantly during physical activity compared to passive sweating, like sitting in a sauna.
A Built-In Antibiotic
One of the more surprising ingredients in sweat is dermcidin, a natural antimicrobial peptide secreted by sweat glands. Dermcidin remains active across a broad pH range and in the high-salt conditions of human sweat, which means it keeps working even as sweat dries on your skin. It helps regulate the bacteria living on your skin’s surface and may limit infection by harmful microbes in the first hours after they land on you. Sweat is slightly acidic, with an average pH of about 6.3 (more acidic than blood), which further discourages some bacterial growth.
Heavy Metals and Trace Elements
Sweat serves as a meaningful excretion route for heavy metals. Nickel, lead, copper, arsenic, and mercury have all been measured in sweat, and for some of these metals, concentrations in sweat are 10 to 30 times higher than in blood or urine. One study found copper levels around 652 micrograms per liter and lead around 26 micrograms per liter in sweat collected during sauna sessions.
Zinc and iron also show up, with research suggesting sweat is an important excretory pathway for zinc and copper specifically. Women tend to have lower zinc and iron levels in their sweat than men, possibly because their bodies compensate for losses through menstruation. Exercise-induced sweating appears to be more effective at flushing heavy metals than passive heat exposure: concentrations of nickel, lead, copper, and arsenic were all significantly higher during dynamic exercise than during sauna sitting.
Drugs and Their Metabolites
Substances you put into your body can come out in your sweat. Cocaine, opiates, methamphetamine, and MDMA (ecstasy) have all been detected in sweat after use. This is the basis for sweat patch testing, a monitoring tool used in drug courts and rehabilitation programs. A patch worn on the skin for days or weeks can accumulate drug residues as they migrate from blood into sweat. Methamphetamine, for example, appears as the primary substance in sweat after oral dosing, with its metabolite amphetamine consistently detected alongside it.
Prescription and over-the-counter medications can also be excreted through eccrine sweat. This is part of why sweat composition is considered highly individual: two people exercising at the same intensity can produce chemically different sweat based on what they’ve recently eaten, drunk, or taken.
What Sweat Can Reveal About Health
Because sweat reflects what’s circulating in your blood, it has real diagnostic value. The best-known example is the sweat chloride test for cystic fibrosis. People with CF have a defect in the protein that reabsorbs chloride from sweat, so their sweat chloride levels are abnormally high. A sweat chloride concentration of 60 mmol/L or above is consistent with a CF diagnosis, while below 30 mmol/L makes CF unlikely. The intermediate range of 30 to 59 mmol/L requires further testing.
Sweat urea levels can also flag kidney problems. Healthy individuals typically have sweat urea around 20 to 25 millimoles per liter. In people with kidney dysfunction, that number climbs, and concentrations up to 50 mmol/L are considered a clear sign of uremia (waste buildup in the blood). Patients with kidney failure can show sweat urea levels of 65 to 100 mmol/L.
Why Sweat Composition Varies So Much
No two sweat sessions are chemically identical. Exercise intensity is a major factor: during moderate exercise, whole-body sweat sodium concentration averaged about 53 mmol/L compared to roughly 33 mmol/L during low-intensity exercise in one controlled study. Total sodium loss jumped from around 659 milligrams to 1,565 milligrams as intensity increased, and chloride losses more than doubled.
Fitness level matters too. Well-trained athletes tend to start sweating sooner and produce more dilute sweat over time because their sweat glands become better at reabsorbing sodium. Diet, hydration status, the ambient temperature, humidity, and even which body region you’re measuring all shift the numbers. Forearm sweat sodium alone can range from 19 to 59 mmol/L depending on how hard someone is working.
Your sweat is, in a very real sense, a liquid snapshot of your body’s internal environment, filtered through millions of tiny glands and shaped by everything from your fitness to your last meal.