We’re often asked if it’s OK to immediately layer products with differing pH levels, such as applying a moisturizer over an AHA exfoliant. People worry that doing so causes one or both products not to work because the different pH levels conflict with each other. Concern about this interaction means many people wait 20 minutes or more before applying another skin care product. Surprise: You don't have to wait! he reason you don’t have to wait is because once the pH range of a water-based product is established, it will hold within that range even when applied at the same time as products outside that pH range. That’s because the ingredients chemists use to establish a product’s pH range are strong enough to keep it within the desired range no matter what you apply at the same time.