Shampoo, hair gel, toothpaste, foot balm, body lotion, cleaning products, laundry detergent, washing up liquid; every chemical is a suspect. And even if you think you are using a natural organic product that is great for someone else, it doesn't mean it suits you. Some people (myself included) are so sensitive that they need to vary the skin products they use from week to week so they don't become intolerant.
Candida overgrowth is another big factor in acne and other skin complaints. If it is growing on the inside, herbs are generally the best way to sort it. If it is simply Candida on the skin, then tea tree oil or the Australian Bush remedy Green essence among others may clear it. Seeing a practitioner who specialises in Candida can be incredibly useful.
TVNL Comment: In an effort to find the cause of my newly severe asthma I eliminated wheat from my diet. Within 2 days the acne that plagued my back for the past few years vanished. Just an FYI.