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Sweat smells are primarily caused by bacteria that live on the surface of the skin. Sweat is made up of urea, salt, traces of fat, and water. When these compounds interact with bacteria on the surface of the skin, the interaction produces an odor. The severity of the odor generally is dependent on the amount of bacteria present on the skin. Other factors such as diet and hormone levels can mildly influence sweat smells.
Immediately after bathing, the surface of the skin is free of almost all bacteria, but within just a few minutes, bacteria begin to reform. Most of the time, the bacteria that live on the surface of the skin are actually a health benefit. This bacterium helps protect the skin from surface inflammation, and recent studies indicate that some types of bacteria may even protect against infection. The amount of bacteria present on the skin when the body begins to sweat is the most important factor in determining how odorous the sweat will be.
Sweat smells that occur several hours after bathing are usually much stronger and heavier, and are the reason why body odor is usually worse after a full day of work or play. This is because the bacteria have had more time to populate the skin surface. People who go days without bathing have an inordinate amount of bacterial buildup and, in most cases, have a much stronger body odor. Even though their bodies could look clean, they will still be playing host to much more bacteria than those who bath regularly.
The severity of sweat smells is also determined by the type of gland that is producing the sweat. Eccrine glands are scattered all over the body, though they are concentrated on the soles of the feet and within the palms of the hands. These glands are responsible for the majority of human sweating, and typically coat the entire body in a thin layer of perspiration. During vigorous workouts, the eccrine glands produce more sweat in an attempt to cool down the temperature on the surface of the skin. The sweat smells that are produced by eccrine glands are usually mild, because sweat produced by these glands is made of up mostly salt and water.
Sweat smells that are produced by the apocrine glands are usually much more odorous than those produced by eccrine glands. Apocrine glands are located where body hair is abundant, such as on the head, the groin, and under the arms. Apocrine sweat is fat-based, and because it is secreted from parts of the body where bacteria are heaviest, apocrine sweat smells are much stronger.
To a lesser extent, the types of foods consumed and hormone levels can sometimes slightly influence the strength of body odor. Alcohol, caffeine, and spicy foods can sometimes contribute to heavy sweating and stronger body odor. High hormone levels such as those that occur during pregnancy and puberty can also produce odorous perspiration.