Interestingly, sweating does not cause acne. However, sweating can make the environment on the skin more hospitable to the development of acne. So warm,moist areas really do help bacteria thrive.
If you're not rinsing off after you work out, that warm, moist area does allow bacteria to increase, and acne then becomes more common. So we recommend that after workouts, patients lightly rinse off and apply moisturizer to prevent those warm, moist areas of the skin from becoming perfect petri dishes for the bacteria that cause acne.