Tonya McKay

Propylene glycol is an ingredient found in many personal-care products, including shampoos, hair conditioners, and leave-in styling products. It is widely used because of its relatively low cost and versatile nature. Its inclusion in a formula can fulfill a variety of purposes, which makes it a popular choice by the cosmetics chemist. However, some manufacturers have recently made the decision to no longer include propylene glycol in their products.

My speculation is that this is possibly due to misinformation and propaganda circulated on the Internet, and elsewhere in the interest of marketing "natural" products. I am an avid supporter of using natural products whenever possible. and of avoiding putting toxins into our bodies whenever possible or practical. But I get frustrated by the dissemination of inaccurate and incomplete information in an attempt to frighten consumers into using different products. In this article, I seek to clear up some misconceptions about this chemical.

The Chemical Facts About Propylene Glycol:

Propylene glycol (also known as 1,2 propanediol) is a relatively small molecule with two alcohol (hydroxyl) groups (-OH). It is a colorless and odorless liquid and is completely water-soluble. PG is a synthetic product obtained from the hydration of propylene oxide, which is derived from petroleum products. I do not personally consider a petroleum-sourced product to be a bad thing in and of itself, as I consider the final structure and its properties to be more relevant than the source, unless residual contamination is a legitimate concern.

model of propylene glycolMolecular model of propylene glycol

The FDA has categorized propylene glycol as "Generally Recognized as Safe." There is little to no skin irritation or sensitization even with prolonged direct exposure to the undiluted chemical. Irritation to the eye or respiratory system in the event of direct contact is mild and transitory, meaning it subsides quickly once the area is flushed. The MSDS recommends avoiding direct handling due to potential irritation, which is a smart recommendation for any chemical. But is not necessarily an indication of level of toxicity. It is important to remember that in the chemical industry, where a worker is exposed to continuous and possibly large quantities of a chemical in its concentrated form, it is imperative to use the strongest safety precautions possible. Although many sites would have you believe differently, this simply is not relevant to consumers using a product by the teaspoonful that "might" contain a few percent by weight of propylene glycol (if that much).

An interesting fact about propylene glycol is that it is non-toxic when ingested even in reasonably large amounts. Unlike its dangerous and frequently lethal cousin, ethylene glycol, PG is easily metabolized by the liver into normal products of the citric acid metabolic cycle, which are completely nontoxic to the body. Approximately 45 percent of any ingested PG is excreted directly from the body and never even comes into contact with the liver. The elimination half-life for propylene glycol is approximately four hours, and there is no bioaccumulation (buildup in the body over time). A few rare incidents have occurred where a person ingested a large quantity of propylene glycol and suffered some liver and neurological effects as a result, but these were short-lived and subsided once the material was metabolized and excreted.

Metabolic Cycle for Propylene Glycol:

Propylene glycol → lactic acid → pyruvic acid → CO2 + water

Both experimental and anecdotal evidence to date indicate PG to be completely non-carcinogenic, despite its "petroleum-based" origin. In a very interesting study, some unfortunate rats were fed propylene glycol at amounts equal to 5% of all of their food intake every day for two years, which is a pretty huge volume over a large portion of their lifetime. There were no observable effects on their health or behavior.

Another frequently cited concern about propylene glycol is its use as a component of antifreeze, especially in applications such as the airline industry for de-icing wings and runways. Many of the articles I have found online make a point to highlight the fact that PG is "antifreeze," a term which has many negative connotations in the minds of people who have heard of the significant and potentially fatal dangers of "antifreeze." This reputation is well-deserved for ethylene glycol, an extremely toxic material that was commonly found in automotive and other antifreeze solutions. However, propylene glycol has become a common replacement for ethylene glycol in many commercially available antifreeze formulations because it is much safer than ethylene glycol.

Propylene Glycol

Another relevant point is that the word “antifreeze,” frequently used to alarm consumers, is simply a scientific term used to describe a completely innocuous process — the lowering or depression of the freezing point of a liquid. An example of this process is the application of salt to roads and walkways in a snow storm. This helps melt snow and ice and prevent development of dangerous icy conditions. The salt accomplishes this by lowering the freezing point of water. This is an example of a "safe" chemical being used as antifreeze. My point here is to not be alarmed by the term antifreeze or by the chemical, propylene glycol. While few chemicals are entirely without risk, propylene glycol is considered to be safe at the low concentrations used in personal care products and even food products.

What is propylene glycol used for in personal care products?

  • It is a very effective humectant
  • It is a solvent or carrier agent for fragrances and preservatives.
  • It can be used as an emulsifier or co-surfactant.
  • It is used as a solvent for pigments in cosmetics.
  • It can be used as a preservative due to its anti-fungal and anti-microbial properties.
  • It is frequently used in deodorants and antiperspirants.
  • It is found in hand cleansers and disinfecting gels.
  • It is a common additive in shaving creams and gels.

I think for those of us with curly hair, propylene glycol's main benefit is the fact that it is a humectant, and a pretty effective one at that.

How will propylene glycol affect my hair?

The application of propylene glycol that is most relevant to those of us with curly hair is as a humectant. All the usual cautions apply with regard to its capabilities to attract water to the hair from the environment or to draw water from the hair to itself. In other words, unless you have the perfect atmospheric conditions, you may experience problems with this ingredient. Propylene glycol is a completely water-soluble material that will not build up on the hair, regardless of whether a low shampoo or shampoo free routine is used. It is also important to note that it is a diol with low volatility, so it will not evaporate easily and cause dry hair in the manner of low molecular weight alcohols such as SD alcohol and isopropyl alcohol.

Overall, after reviewing the available information from diverse sources, my opinion is that propylene glycol should not be a feared ingredient. If product manufacturers are finding replacement ingredients to fulfill the same purposes in their formulas served by propylene glycol, I see no harm in that. However, I hesitate to support marketing materials that use this as a selling point.

As a curly, it would be wise to be aware if you are using products that contain this ingredient, just in case you observe increased frizz or dryness. Always be certain you use plenty of moisturizing products to help lock moisture into your hair shaft, which can help prevent any potential problems caused by a humectant.


References

"Case Studies in Environmental Medicine (CSEM), Ethylene Glycol and Propylene Glycol Toxicity"
What is Propylene Glycol?
Gaunt, IF, Carpanini, FMB, Grasso, P and Lansdown, ABG, "Long-term toxicity of propylene glycol in rats, Food and Cosmetics Toxicology," April 1972, 10(2), pages 151-162.
Dow
McKay, T. "Humidity, Humectants and Hair", online publication, Aug. 2007


Email your questions to Tonya.