Sweating can cause acne.

More Info: Disputing popular belief, sweating does not cleanse the pores. In fact, sweating has the potential to cause an acne outbreak.   Exercise causes increased oil production that combined with the friction of clothing and sweat can cause acne to flare up, especially on the back and chest areas.

     Dispelling the Myths about Acne: A Fact Sheet for Patients." Otolaryngology University of Mississippi Medical Center.

Why Does the Body Sweat?

As warm blooded animals, humans need to regulate body temperature, and to cool the body, we sweat. As water changes from a liquid to a gas it absorbs energy, and in the case of sweat, as it evaporates, the source of heat is from the skin, thereby cooling the skin and the blood in the skin. Some conditions will cause a failure of sweating and that will lead to death from overheating, especially with heat stroke. Sweating is a vitally important function of the skin and without sweating humans would not be able to function in diverse and warm climates. 

What Are the Causes of Excessive Sweating?

Excessive sweating, also called hyperhidrosis, has a variety of causes. Localized sweating of hands, feet, or armpits runs in families; it can result from an exaggerated response to emotional stimuli. Generalized hyperhidrosis may signal a serious condition such as leukemia, lymphoma, heart attack, thyroid disorder, low blood sugar, or infection. It can also be caused by hormone changes or as a medication side effect. In the absence of another underlying cause, hyperhidrosis may be due to abnormal regeneration of sympathetic nerves or abnormality in the number or distribution sweat glands.