Polyethylene is a plastic compound and in fact is one of the most common plastic compounds made. The symbol you often will find is 'PE' plastic. Many bottles and lids are made from this plastic. It is a polymer compound made of repeating ethylene units that make it a strong and sturdy plastic bond.
So, why is PE used in cosmetics and is it safe? It is used to help emulsions stay together (keeps oils and waters bound together) and can help with thickening some products (1). On another site, it is listed as "an emulsion stabilizer, viscosity increasing agent, and binding agent"(2). That is the simplified version. It also helps put a thin 'coating' on the skin to keep moisture in. It is also used in dental products to help exfoliate. It is also the main ingredient in Polyethylene Glycols (PEG's).
So, what's the big deal? Is it safe? Is it something to be concerned about? Many items the FDA (Food & Drug Administration) deem to be 'GRAS' or 'Generally Recognized as Safe' really are not that safe. This is one of those items and in fact just recently Crest toothpaste is withdrawing this item from their toothpaste after a very prudent dental hygienist noticed a 'blue line' in the gums of her patients. This 'blue line' were the polyethylene beads let from the toothpaste after repeated use (3, 4). So, if it's in our mouths after repeated use, why are we also putting it on our skin? What does it do?
According to the website 'Truth in Aging', there are many issues with PE and PEG's.
"Polyethylene is considered a low hazard ingredient by the Cosmetics Database, for cancer. But a moderate hazard as regards skin irritation. There is moderate evidence that Polyethylene is an immune system toxicant as well, according to the National Library of Medicine's study entitled "Occupational Exposure to Hazardous Agents."
Polyethylene is also considered a potential carcinogenic that has shown tumor formation at moderate doses when given to animals, and blood tumor formation and tumors at the site of application, although many of these findings resulted from Polyethylene being directly implanted into the body.
Products and formulas containing Polyethylene should not be used on broken or irritated skin. The contamination concerns regarding the use of Polyethylene in cosmetics and skin care formulas is somewhat countered by the CIR's finding that the large molecules in Polyethylene are unable to penetrate skin, and therefore do not enter the body. Studies specifically focusing on PEGs, in which Polyethylene (combined with Glycol) is a primarily component, show that although PEGs are considered safe for use topically on healthy skin, studies showed that patients suffering from severe burns were treated with PEG-based antimicrobial cream; this treatment resulted in kidney toxicity. "The PEG content of the antimicrobial cream was determined to be the causative agent. However, no evidence of systemic toxicity occurred in studies with intact skin. Because of the observation of kidney effects in burn patients, the CIR Expert Panel qualified their conclusion on the safety of the PEG ingredients to state that cosmetic formulations containing these ingredients should not be used on damaged skin" (CosmeticsInfo.org)." (3)
So, there is the information. What you do with it is now up to you. I for one no longer use any cosmetics that contain these chemicals. It is a good plastic for container use, yes. But no thank you for use in my cosmetics or toothpaste!