Thanks to Manda Ferguson, cosmetic chemist.
I am a cosmetic chemist and I’d like to give a few tips on safety. Anyone can react to anything at any time, if someone else doesn’t, it doesn’t mean you won’t.
Keep cosmetics in a cool dry area. Don’t keep cosmetics in an area that gets hot or humid, like a bathroom.
Keep an eye on your products. Are there any changes in smell, look, feel, or color of the product? If so, the product is becoming unstable and/or the preservation system is breaking down, so discard the product.
Check your product's expiration date, and period after opening date (the little open pot symbol with a time one it, for example 12m means 12 months). Beyond these times, the preservation system is breaking down, so discard the product.
You might heard that preservatives are bad for you. In fact, preservatives are tested, and they protect your from microbe contamination, which may cause sickness, blindness, or death. You cannot see bacteria in a product because they are microscopic.
Water-based products:
If a water-based product has clearly-split where a cream has an oil and water not mixed, then discard it. You can not save it by remixing it.
Never buy any water-based product from markets or hobbyists UNLESS they can prove their products have been professionally tested for stability and microbial contamination or manufactured in a professional lab.
Never buy any water-based product that does not contain a preservative (or preservative system which would be hard for general public to know). If alcohol is in the top few ingredients, that is probably used as a preservative, but it could be drying.
If someone says a water-based product is preserved with vitamin E, grapefruit extract, or rosemary extract, then do not use it. These are not preservatives; these are antioxidants which stop oils going rancid, and these don’t do anything for water-based products.