When you sweat, your skin becomes moist. This moist skin attracts dust and dirt that start sticking to the pores. When this happens, the pores start clogging and the oil secreted by the skin makes sure the dirt sticks to your pores. Once it evaporates, it leaves behind all the dirt, dust as well as bacteria on your skin surface. If you do not wash off your face with a face soap and water, the pores remain clogged. And finally, the next day, you observe acne greeting you.