Acne is caused by the excess sebum produced by the sebaceous glands. This oil-based substance accompanied with dead skin cells clogs the pores, with bacteria getting trapped inside the pores. These bacteria multiply and cause swelling and redness. Perspiration and hot weather conditions could cause the neck that is the most vulnerable area to be prone to acne, with touching and friction aggravating acne conditions.
The dirt and oil from your hands could aggravate bacteria conditions and acne breakouts. In addition, people who wear helmets, tight collars or frequently carry packs with shoulder straps may develop acne on the neck because of the friction these items create against the skin.