Before I leave tomorrow (for a 5-day vacation, yahoo!), let me just post this list of synthetic/chemical preservatives I found from the web:
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Benzalkonium Chloride:
Benzyl Alcohol:
Bronopol (scientifically, 2-bromo-2-nitropropane-1, 3-diol)
Chlorhexidine Gluconate
Chloroxylenol (or Ottasept)
Dehydroacetic Acid
Diazolidinyl Urea
DMDM Hydantoin
Ethyl Alcohol
Formaldehyde
Glutral (also called Ucarcide)
Imidazolidinyl Urea
Kathon CG (otherwise known as methyl, chloromethyl isothiazolinone, and methyl isothiazolinone)
Parabens (combination of several esters of methyl, ethyl, propyl, and butyl, or benzoic acid)
Phenoxyethanol
Phenyl Ethyl Alcohol
Phenyl Mercuric Acetate
Potassium Sorbate
Quaternium 15
Salicylic Acid
Sodium Benzoate
Sorbic Acid
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They are the most commonly used synthetic preservatives for both personal care and cleaning products. You can check on them here to see their “hazard score”. This is just a partial list. There are more out there!
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Here are also a few more but with very helpful explanations:
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Alcohol
Works as a preservative when present in concentrations of over 20%.

Used in such high proportions it can strip away natural oils and be unsuitable for sensitive skin or Rosacea sufferers.

If Alcohol is listed near the top of the ingredients list, then a large proportion of the product will be alcohol and completely unsuitable for use in a moisturising product.

Benzoic Acid
A mild skin irritant thought to exacerbate Eczema.

Benzyl Alcohol
Considered a skin and eye irritant and has been linked to Contact Dermatitis and Urticaria.

Formaldehyde
Still used in some “natural” skin care products, Formaldehyde is a known carcinogen and skin irritant.

Japanese Honeysuckle (listed as Lonicera Japonica)
While this might sound innocuous, this is a naturally occurring Paraben.

Are natural Parabens better than the synthetic chemical type? A Paraben is a Paraben – whatever its source.

Parabens – Butyl, Ethyl, Propyl, Isopropyl, Methyl, Benzyl
The debate rages on about these six preservatives. Recent studies have raised concerns over the gradual build-up of these chemicals in the body and linked them to hormone disuption and cancer.

More clinical trials need to be done but, at Pai, we exercise a precautionary approach. If we don’t know enough about an ingredient, we don’t use it.

Parfum
This term was traditionally used to hide the precise formulae of commercial perfumes.

However it is now a common fixture on beauty products’ ingredient lists to hide a multitude of sins – from synthetic fragrances to chemical fixatives and preservatives.

200 different chemicals can be legally covered by the word “parfum”.

Phenoxyethanol
Used by the majority of ‘natural’ and ‘organic’ skincare brands, this petroleum based preservative is a skin irritant. It is banned in Japan and some European countries.

Sodium Benzoate
Sodium benzoate is derived from benzoic acid which can occur naturally in berries. This common preservative was at the centre of a food health scare a few years ago. In high doses it has been linked to DNA damage and the onset of degenerative diseases such as Parkinson’s. In cosmetics it can cause skin and eye irritation.

Sodium Hydroxymethylglycinate
Sodium Hydroxymethylglycinate is a common paraben replacement but is a harsh skin irritant and is thought to break down into formaldehyde in the body.