Human skin is alive with a host of microorganisms. One recent study published in the journal Science found that at least 18 different phyla of bacteria dwell in 20 different skin habitats. The underarm is one of those habitats. There, in the dark, swampy environment, the bacterium Corynebacterium minutissimum makes a nice home. Normally, it goes unnoticed, but if your body chemistry changes -- if you sweat -- the bug can cause problems. The least serious problem is body odor, which results when the bacteria break down sweat. A more serious problem is erythrasma, a chronic infection in the deeper layers of the skin. Sometimes, the infection can lead to the formation of red, scaly patches. The border of the affected skin is well-defined, with a clear demarcation between the discolored patch and the surrounding normal skin. Over time, the pink or red skin fades to brown.
Doctors can diagnose erythrasma by shining a Wood's lamp on the affected area. A Wood's lamp produces ultraviolet light, which can be used to detect several conditions affecting the skin. If you have the infection, the patches will fluoresce in the presence of the UV light and glow a magenta color. The treatment, as with any bacterial infection, requires a course of either topical or oral antibiotics.
If you have underarm darkening and all other hormonal and bacterial causes have been eliminated, you might evaluate your shaving habits. Underarm hair tends to be dark and coarse, which means that stubble -- the hair left just beneath the surface of the skin after shaving -- is darker and coarser than stubble on your legs. This stubble can be visible under the surface of the skin, especially if there are ingrown hairs. Over time, your underarm skin can look dark and shadowy, much as a man's face may appear to have a "five o'clock shadow" just a few hours after shaving.
Waxing your armpits can help you avoid this problem. Waxing removes hair by the root and leaves no stubble. But the technique hurts, especially in the nerve-rich skin under your arm. It's also more expensive, especially if you go to a salon. A good compromise is to get your first waxing treatment done in a salon to watch how the professionals do it. After that, you can do your own hair removal with waxing kits available in drugstores.
Of course, always consult your doctor if your underarm darkening persists or seems to be related to other symptoms, such as itching or tenderness. Even the lowly armpit can be an effective warning sign of larger, more serious problems.
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