During my teen years I struggled with back acne, and my sister even more so. It wasn't until a few years ago that I realized my conditioner may be the culprit. I believe that conditioners containing an ingredient called panthenol often cause cause acne and blemishes along the hairline and the back area where conditioner residue resides. Panthenol is commonly found in conditioner to provide strength to the hair, but in order for it to attach to the hair follicle and smooth the cuticle, it needs to be processed and combined with heavy oil-soluble ingredients, since hair is somewhat water-repellent. To this day if I use a shampoo or conditioner with panthenol, my skin will breakout. I believe that it’s a combination of panthenol when paired with other ingredients that may cause the pores to get clogged. So if you get blemishes along your hairline and your back, I encourage you to experiment with different types of hair conditioners to see if you can find one that isn't causing your skin to breakout. At the very least, after rinsing out your hair conditioner in the shower, put your hair up in a clip (if it’s long), and use your shower gel to wash your back to cut any oil residue that rinsing alone cannot accomplish. This will ensure that hair conditioners won’t cause any unnecessary trouble. As for preventing breakouts on the hairline, be sure to wash your face AFTER you have rinsed out your hair conditioner to remove the residue.
To happy, healthy skin!