Propylene Glycol: Propylene Glycol is a form of mineral oil, an alcohol produced by fermentation of yeast and carbohydrates. It is a synthetic ingredient, and therefore can be an irritant to the skin. Synthetic ingredients are known to trigger allergic reactions, breakouts, and adverse reactions for many people. PG has been used for a variety of uses. Industrial grade PG is used as an active ingredient in engine coolants and antifreeze; airplane de-icers. If you are looking for a natural, organically-derived Vitamin C Serum, avoid this ingredient. Look for a product that contains a Vegetable Glycerin instead.
Water: Vitamin C oxidizes reading in the presence of the air and water. When this happens, the Vitamin C Serum for an intents and purposes becomes useless. If water is among the top ingredients in your Serum, watch out - it may already be oxidized. Avoid this ingredient in the best Vitamin C Serum at all costs.