Propylene Glycol: An Honest Conversation

Glycol antifreeze spilled on pavement

Propylene glycol is a common ingredient found in personal care items like shampoo, conditioner, and hair gels. It’s relatively cheap and versatile as a solvent, making it a popular choice as a solvent delivery vehicle in both cosmetics and food products. Best of all, it’s labeled by the FDA as GRAS—generally recognized as safe—meaning that propylene glycol has been found by experts to be safe when used as instructed at reasonable levels. Including propylene glycol in a formulation can fulfill several purposes, ranging from dissolving an active ingredient to improving product texture and feel, enhancing its reputation and usefulness in the eyes of cosmetic chemists.

Recently, however, propylene glycol has suffered a bit of a PR backlash. Because of propylene glycol’s use as a nontoxic alternative to ethylene glycol—a similar, more hazardous compound—in commercial antifreeze, the chemical itself has now become almost synonymous with antifreeze itself. Fearing any potential backlash, several manufacturers have decided to eliminate propylene glycol from their consumer products. Rather than improving safety or providing clarity, however, these knee-jerk reactions have only perpetuated misinformation about the ingredient’s supposed dangers. Despite being completely safe, nontoxic, and useful, propylene glycol has simply fallen prey to fearmongering marketing strategies—let’s break down how it happened. 

First, what exactly is propylene glycol? 

There are many different types of glycols

A glycol, by definition, is any compound in the alcohol family with two -OH groups. There are many common (and theoretically infinite) glycols that are used in our everyday lives:

  • Ethylene glycol - used to create PET plastic (like that used in water bottles), polyester, and as a constituent of antifreeze. It is generally regarded as toxic for humans to ingest. This type of glycol is what most people refer to when they say ‘glycol’ as a general term.
  • Diethylene glycol - used in printing inks, resins, and adhesives. Like ethylene glycol, it is also considered not safe for human ingestion.
  • Polyethylene glycol - a term used to describe glycol polymers that have many repeating chains and have many applications in our everyday lives, including cosmetics, food, and personal care product.
  • Propylene Glycol (1,2-propanediol)
  • 1,3-propanediol - similar to propylene glycol but with slightly different properties, used in cosmetics and can be derived from plant sources, like corn.
  • Butanediol - a versatile ingredient used in cosmetic products that has different properties based on its different structures
  • And that only scratches the surface… 

Propylene glycol is a specific type of glycol

Propylene glycol, also known by its formal name propane-1,2-diol, is a relatively small molecule and the safest of the glycol family of chemicals.  Propylene glycol contains two hydroxyl groups, which means it mixes well with water. It also dissolves many compounds that tend to not mix well with water, a valuable quality. It’s viscous, colorless, and odorless, meaning that the liquid is an ideal neutral vehicle for delivering a variety of different other molecules. 

Image: Wikipedia

 

In consumer goods, this means propylene glycol has a variety of beneficial properties:

  • As an effective humectant, it locks in moisture, making it a valuable component of hand creams and lotions.
  • As a versatile solvent, it can dissolve both hydrophilic (e.g. water) and hydrophobic (e.g. oil) molecules
  • As an emulsifier, it can stabilize processed foods or cosmetics to improve thickness, texture, and uniformity.
  • As an antifungal and antimicrobial agent, it can be used as a preservative.

Propylene glycol in food

Most significantly, however, propylene glycol has been classified by the FDA as Generally Recognized As Safe since 1973. This designation means that it's already in many of the foods and cosmetics we use on a daily basis. In ice cream, for example, it’s used to help inhibit the formation of those unwanted ice crystals you see after putting the carton back in the freezer. In hand creams, because propylene glycol draws in moisture, it can fight back against flaky, dry skin.

 
Inhibit freezer burn Fighting Dry Skin Anti-caking for baking mixes

 

So why does propylene glycol get a bad rap?

"Those sound like good things!" you might say. However, propylene glycol (PG for short) has been the subject of many high-profile news stories as of late, and not for good reason. Why? Because fear sells. We’re all predisposed to respond more strongly to negative information and fearmongering can be a profitable sales tactic that uses our human nature against us. Unnecessary fearmongering takes advantage of the fact that doing your own research is difficult and exhausting and preys upon the fact that everyday consumers don’t have the time or resources to become experts in chemistry. Take this quote for instance:

The science can be murky, but we avoid the clear offenders. Do you want antifreeze (propylene glycol) in your moisturizer? We’re going to guess no.

Creating a scapegoat

There’s a lot going on there! We mentioned antifreeze earlier, so now that we’ve gotten more background information on propylene glycol, let’s take the time to finally unpack that statement.

Creating a common enemy in propylene glycol generates an implicit contrast between mainstream consumer goods and positions products being pushed by other up-and-coming companies as outside alternatives, a strategy we’ve talked about before. Put simply, it creates a sense of distrust and fear for certain products and simultaneously attempts to build a trust-based rapport through a sense of mutual common goals. In reality, the responsibility and duty of companies should be to provide consumers with facts, data, and statistics and let them make the best decisions for themselves.

Skillful misrepresentation

The second line, however, is deceptively problematic. Propylene glycol is used in many products, so why mention antifreeze, completely out of context?

First, let’s clarify: Antifreeze is a functional name. It’s a mixture that’s named for the job it does: keeping things from freezing. Antifreeze is not a chemical in and of itself, but a role that a chemical or mixture fulfills. Originally, the goal of an antifreeze was to help transfer heat in an engine—which water is quite good at on its own!—while avoiding the tendency to freeze and expand when used in wintry temperatures the way water would. Antifreeze is also a component of deicing fluid that’s sprayed on airplanes during stormy conditions, and it works by lowering the temperature at which water freezes for any liquids on the wings. Salt does something similar when we scatter it on the sidewalk to melt away ice, yet we still use salt to bake cupcakes or season a steak. The point is: many chemicals can fill many different roles, but some are better than others for a certain task.

Perhaps antifreeze was mentioned for because we’re all familiar with the dangers of pets lapping up any antifreeze that’s leaked from the car, or the imagery of toddlers being drawn to its bright colors and mistaking it for a sugary drink. What makes antifreeze taste appealing to pets and kids alike? The primary ingredient, ethylene glycol. Historically, ethylene glycol—not propylene glycol—had been widely used as the main additive in antifreeze, and its slightly sweet taste made it attractive to animals. For this reason, a variety of laws have been passed within the last 20 years mandating that bittering agents be added to commercial antifreeze, with American manufacturers voluntarily adding the bittering agent as of 2012.

Ethylene glycol is also extremely toxic. In humans, ingesting 786 milligrams per kilogram of bodyweight of pure ethylene glycol can be lethal—that’s roughly 2 fluid ounces to kill the average adult man, slightly more than a single shot’s worth if you’re a cocktail aficionado. But the quote mentioned propylene glycol.

Propylene glycol vs polyethylene glycol

Speaking of relatives that quite often get confused for each other, propylene glycol and polyethylene glycol, are two glycols for which "P" sometimes gets confusing. Though the two chemicals sound quite similar, from a structural standpoint, they’re quite different! Propylene glycol, as we’ve discussed, is a small molecule with a variety of useful properties. Polyethylene glycol, or PEG, sounds similar enough to the toxic ethylene glycol to easily confuse the two; however, the key difference is in the poly- portion of the name. That prefix, poly-, signifies that the compound in question is what’s known as a polymer, or a massive chemical chain made up of repeating monomeric links—think paper chains or train cars. A monomer is the name for the single, repeating subunit of a polymer—in the case of PEG, the monomeric subunit is ethylene oxide. Starch and cellulose are examples of polymers (of sugar) that you may be more familiar with. Propylene glycol, for reference, is not a polymer.

Given their size and the types of bonds involved in their linkage, polymers can be difficult for humans to digest—in this case, polyethylene glycol is actually considered to be biologically inert, meaning that it won’t react with living tissue when it comes in contact with it. Among its useful properties is PEG’s ability to absorb water, making it commonly used as a laxative by drawing water into the gastrointestinal tract. You’ll also frequently see polyethylene glycol in medicines, too, usually as a nonreactive bulking or thickening agent alongside the active ingredients. 

Cinnamon-flavored whiskey gets an unfair rap

You may not drink it, but you’ve certainly heard of it: Fireball Whiskey. Besides a unique flavor profile, the cinnamon-flavored liquor contains a compound you should be familiar with by now: propylene glycol! In fact, up to 5% of the whiskey may be propylene glycol, and that amount falls well within the bounds set by the FDA for alcoholic beverages. In 2014, European regulators caused a bit of a stir by recalling quantities of Fireball Whiskey out of concern for its high propylene glycol content.

Was there contamination in a certain batch? Nope! As it turns out, European regulatory agencies just happen to set a lower threshold for the safe limits on propylene glycol in alcoholic beverages and products. By American standards, that batch of Fireball Whiskey was perfectly fine.

That hasn’t stopped the media and other outlets from capitalizing on this fear though. In fact, a few published articles—and both published and articles are really pushing the definitions of those terms— purportedly documented acute toxicity from consuming Fireball Whiskey. In this article, which is a unique case rather than a standalone publication or investigation, two middle-aged patients with a history of alcohol abuse reported to the emergency room with acute lactic acidosis. Both mentioned consuming disturbing amounts of Fireball Whiskey, and that's the underlying basis of the study. 

First, it’s highly dangerous to rely on an article with a sample size of two! In perspective, let's say I pulled two red M&Ms out of a bag and concluded that the bag contained only red M&Ms. 

Second, it’s worth considering a variety of confounding factors, including that the ethanol in Fireball—present in much higher concentrations than propylene glycol, in fact up to seven times higher—would likely have a stronger influence on any presentation of acute lactic acidosis.

What is lactic acid? It’s the byproduct that is produced when our cells can’t get enough oxygen and have to look to additional energy sources for fuel. Lactic acid can also be produced in the liver as both ethanol and propylene glycol are broken down. Lactic acidosis means a buildup of excess lactic acid, and it can lead to health complications. Alcoholism itself has been associated with lactic acidosis. This doesn’t mean that propylene glycol may not have exacerbated the situation, but a higher proof spirit would have likely accomplished the same result.

Consider that alcohol is a depressant of both the central nervous system and the respiratory system. When the respiratory system is depressed, our breathing becomes shallower, meaning we get progressively less and less oxygen—you might see how lactic acidosis could spiral out of control in certain cases. In the case of the latter patient, his blood had acidified to a pH of 6.94, well below the typical range around 7.4. His additional symptoms—and they were legion—included hypothermia, elevated heart rate, elevated respiratory rate, and a blood oxygen level of 54%.

Despite being numerous, these symptoms tell a coherent story: The past presentation of delirium tremens, or severe withdrawal from alcohol, indicates chronic use, and impaired liver functioning wouldn’t be out of the question. Impaired liver functioning with increased alcohol consumption would lead to lactic acid accumulation. Further still, because alcohol itself is a vasodilator, the blood vessels expand and release more body heat, lowering the overall core temperature. When this happens, the body restricts blood flow from the extremities in an attempt to preserve our vital organs and keep blood flowing throughout our core. When our extremities receive less blood, they get less oxygen, meaning that tissues in the extremities have to resort to producing additional lactic acid as a byproduct of using up an alternative emergency energy source. This exacerbates the original issue of acute lactic acidosis, and the patient’s condition can accelerate downhill.

Could propylene glycol have contributed? Perhaps! It certainly didn’t help. But in both cases presented by the medical professionals who submitted the case study for publication in 2021, it’s worth considering the following self-admission:

Although it is reported that Fireball has not contained propylene glycol in any of its products since 2018, the association with severe lactic acidosis needs further research.

Unless our patients were hoarding casks of vintage Fireball Whiskey, the cases presented are likely tragic stories of the toll that chronic addiction takes on its victims, not key pieces of evidence of propylene glycol toxicity. Always take scientific data into account when making critical life decisions but be vigilant and diligent in knowing that not all science is created equal — be critical of all research, even ours (please). 

Antifreeze? Fireball Whiskey? How did we even get here?

Exactly. Companies take advantage of the understandable negative connotations around a word like antifreeze without telling you this:

Propylene glycol is a similar compound to ethylene glycol, and its similarity to ethylene glycol helps it to fill a similar functional niche as a component of antifreeze. Referring to propylene glycol primarily as an antifreeze is a scare tactic. Where propylene glycol importantly differs from its chemical cousin is in its significantly lower toxicity: Studies have shown that when given an estimated 10 grams per kilogram of bodyweight per day for up to 2 years, no adverse effects were observed. That’s why propylene glycol has taken ethylene glycol’s place in many blends of consumer antifreeze, where it’s actually labeled as a nontoxic alternative.

What should consumers take away from this?

We realize it sounds a bit hypocritical to say trust us and not other companies. It would be even more hypocritical to follow that up with the advice to do your own research—a key point of our article here has been to break down why doing your own research is difficult and unreliable with even scientific publications presenting conclusions without data.

That non sequitur aside, what we do want to reinforce is that if you have to choose between trusting an upstart company or regulatory agency like the FDA, always lean towards the authoritative source. Rebuilding trust towards scientific outlets and agencies is important to us—basing results in sound science is the foundation of Revela, and we want to promote reliable data and trustworthy sources whenever possible.

So when it comes to propylene glycol, there’s a lot of smoke and mirrors, but no real fire. It’s useful, it’s safe, and it’s in a huge variety of products we all use on a daily basis. Other compounds might provide the same type of results in terms of texture or solubility, but those compounds should compete with propylene glycol on their merits, not on hyped-up controversies. Manufacturing a controversy around safety—particularly when the safety profile of propylene glycol is fairly settled—is a conscious strategy to distract from that and increase the value of alternative products in the eyes of consumers like you.

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