Sweating is the body’s response to increased temperature, as sweat evaporating from the skin cools the body down. However, for those with eczema, sweating can lead to even more itching than normal. The “presence of itch when sweating” is actually one of the criteria used to diagnose eczema. Common triggers of sweating (e.g. physical activities, hot weather, and emotional stress) can worsen the itch in eczema.[2-6] In warmer climates, sweating may be the leading cause of increased itchiness, often associated with the sensation of heat and pain in the afflicted areas.
Why Does Sweat Cause Itching in Eczema?
The exact reasons why sweating worsens itchiness are still unclear. Researchers have found that people with eczema are overly sensitive to their own sweat, and their bodies release histamine in response, which can worsen itching. Those with eczema have areas of thickened skin due to chronic scratching, producing more sweat than regular skin.
Another theory suggests that because sweat is composed primarily of water and salt, the salt left on the skin after the water in sweat evaporates directly activates nerves that transmit itch. Researchers have found that the same neural chemical (acetylcholine) that stimulates sweating is involved in transmitting itch and burning sensations in the skin. The chemicals that control this itch sensation surround nerves located near the skin’s sweat glands, possibly explaining why people with eczema experience itching during and after sweating, thereby beginning the itch-scratch cycle.[10,11] The itch-scratch cycle is when itching becomes worse after sweating, people scratch intensely, making the rash worse by causing more damage to the skin’s protective barrier, thus leading to even more inflammation and itching.
Yet another theory proposes that the sweat secreted through the sweat duct is rapidly absorbed by the thirsty stratum corneum, resulting in rapid swelling around the openings of the sweat duct at the microscopic level—eventually causing its closure. Instead of being secreted onto the skin surface, sweat then leaks into the surrounding skin, causing irritation and itching.[2,13]
On the other hand, the sweat of those without eczema contains proteins that fight off skin infections, lowering the amount of bacteria on their skin after sweating.  People with eczema lack these anti-microbial proteins, which explains their frequent bacterial colonization and infection.[14,15] Hypersensitivity to the bacteria living on their skin tends to result in more eczema outbreaks and itching.[15,16]
Prevent Itching After Sweating
Wearing loose and sweat-absorbing clothes will prevent sweat residue build-up and skin irritation. A quick shower with lukewarm water will rinse off sweat and minimize skin irritation. After showering, apply moisturizer to keep the skin hydrated. Cooling the skin with cold showers or menthol-containing lotions may be helpful for fighting itchiness.