Human skin is acidic, but only slightly, with a pH value of 5.5. Too much acidity or alkalinity can make the skin lose its health, resulting in cell-deterioration and dead-looking skin. Some deodorants can increase the skin's acidity due to their alkalinity. It's because alkaline substances neutralize acidity and any increase in the pH level of skin only compensates for loss. On the other hand, the use of a deodorant with a low pH value (a higher level of acidity) will result in more alkaline underarms. Anything below or higher than the normal pH level of skin can be damaging.
What we put on our skin can change its pH level to a slightly higher or lower one. Most generally make skin more acidic and citrus products can make it more alkaline. While skin products like deodorants are essential, the ingredients are a big factor in what they can do to the skin. Thus, if a deodorant is labeled "pH Balanced," then it should approximate the mean acidity level of the skin. Such deodorants can generally be safely used by most people and will not likely cause any reactions or effects that will make the skin darker, drier or flakier.
The pH-balancing compounds that are usually used in deodorants are organic acids from plants like hydroxy and citric acids. Consequently, eating fruits rich in these acids, specifically the alpha-hydroxy and citric ones, also serves to keep skin pH at healthy levels. You can find these acids in lemons, cranberries, apples, and grapes. Citrus fruits are generally good for the skin whether eaten or massaged on.
Do get a pH-balanced deodorant for your underarms the next time you buy.
More on skin care in Everything Nu.