Why do the armpits/underarms get darker?

There is no consensus among experts but several reasons can be attributed to this:

• Constant friction which happens if you are overweight or if you wear tight blouses and tops that brush against the skin
• Excessive sweating that is common in warm places (How to stop excessive sweating), particularly when women use skintight blouses and tops (Related articles:  Tips on picking the right tops and Hyperhidrosis)
• Improper hygiene leading to infections.
• Shaving of underarm hair

How to lighten the skin in that area?

• Start with proper hygiene.
• Switch to sugaring instead of shaving or waxing to remove unwanted hair.
• Use a skin lightening cream.
• You can also scrub the area with lemon peel powder while bathing.
• Skin darkening is also treated sometimes with bleaching agents such as hydroquinone and other mild acids such as glycolic and retinoic acid.  

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