Preservatives

The decaying process is natural and happens with or without preservatives. Personal care products do not (and should not) last for ever. Just like food, all natural skin care products will eventually deteriorate and become unstable. Chemical preservatives are generally used because less expensive and extend the shelf life of the product more than natural alternatives.

Synthetic Preservatives:

Imidiazolidinyl Urea (Germall 115) and Diazolidinyl Urea (Germall II) - causes contact dermatitis. Germall 115 releases formaldehyde
DMDM Hydantoin - highly toxic, causes contact dermatitis, contains formaldehyde. Used in shampoos and deodorants.
Methyl, Propyl, Butyl and Ethyl Paraben - causes allergic reactions and skin rashes. Used in almost everything and linked to breast cancer
2-Bromo-2-Nitro-Propane-1, 3-diol. (Bronopol) - highly toxic, causes allergic contact dermatitis. Used in face creams, shampoos, mascaras and bath oils.
Benzalkonium Chloride - highly toxic, used in shampoos, conditioner and deodorants.
Quarternium-15 - toxic, causes skin rashes and allergic reactions
Chloromethylisothiazolinone and Isothiazolinone - causes contact dermatitis.
Methylisothiazolinone and Methylchloroisothiazolinone
Butylated Hydroxytoluene (BHT) and Butylated hudroxyanisole (BHA) - BHT is carcinogenic.

Natural Preservatives

Essential Oils (Tea Tree, Thyme, etc)
Grapefruit Seed Extract - there are conflicting opinions about its use. I believe it is a safe alternative.
D-Alpha Tocopherol Acetate (Vitamin E)