Luckily, the information you need is always included in the skin care product labels. Certain chemicals can cause an immediate reaction, such as overuse of skin care products (like acne cream) containing salicylic acid. It can be difficult to determine what exactly you are reacting to with synthetic products; the list of chemicals used can be overwhelming. Natural solutions will often contain essential oils, however, which can trigger an allergic reaction. The advantage to these natural solutions is that you will likely know exactly what has irritated you, making it easier to avoid in the future.
Other chemicals present more long term problems and pose risks to your future health and well-being. Though safe for occasional skin care, the buildup of chemicals like homosalate (found in sunscreen), resorcinol (found in acne facials), and parabens (found in shampoos, conditioners, and shower cleansers) can lead to significant long-term damage.
There are many resources available online that will list offending chemicals and the types of products they are used in. By checking skin care product labels in advance, you could be saving yourself from irritation or rashes in mild cases. But you could also be avoiding endocrine disruption or cancerous growths through long-term exposure to harmful chemicals. Having healthy skin is no substitute for overall health, so be sure your products are safe to use.