PEGs can be synthetic or claimed as being from natural origin ; nevertheless, the "natural" is quite far, due to their manufacturing process, which includes ethoxylation.
This chemical reaction involves a grafting of Ethylene Oxide molecules on the basic raw material at very high temperatures and under high pressure .

Why they are used

The derivatives of Polyethylene glycol are widely used  in cosmetic products , mainly as humectants to "retain" water and prevent drying, in the product itself as on the skin.

Their name, formed from their acronym followed by their molecular weight, gives a clue as per their origin and aspect. A molecular weight below 500: it is a liquid. Above 500: oil or wax. PEG-8 is thus a liquid humectant .

Their esters are used as emulsifiers or surfactants (which allow for an homogenous mixture of non miscible raw materials as oil and water ). Therefore, they can be found in almost any kind of cosmetics supplied as an emulsion , from shower gel to care creams, shaving creams or toothpastes.

Why they are criticized

According to most scientists and health authorities, these compounds do not in themselves involve a real health hazard, although they may possibly serve as a support to other chemicals, some carcinogenic, mainly present as impurities at vey low doses.

Most are fairly well tolerated by skin, but some have an irritating potential if used in large quantities. Making skin more permeable, some have also the property to increase the absorption by skin of other ingredients found in cosmetics.

They are mainly criticized because of their manufacturing process, one of the most polluting processes for the environment used in the cosmetic industry, and proof is established they are not easily biodegradable.

Easy to find them in a list of ingredients: look for "PEG" in capital letters.