Preservatives.
They are in aqueous products (water based) and are necessary to prevent bacterial growth and also contribute to a longer shelf life. While most people do not experience any negative consequences to common preservatives, there are a small percentage of people who do. Allergenic reactions can be mild or even severe and may include one or more of the following symptoms: Hives, burning, stinging, swelling, rashes. Common preservatives are parabens, DMDM hydantoin, formaldehyde and methylchloroisothiazolinone (to name just a few). In the case of methylchloroisothiazolinone, it is also used in many "natural" products as well, so familiarize yourself with what ingredients are in your products regardless of their natural or synthetic status.