|Grade||Level of Evidence|
|A||Multiple double-blind, controlled clinical trials.|
|B||1 double-blind, controlled clinical trial.|
|C||At least 1 controlled or comparative clinical trial.|
|D||Uncontrolled, observational, animal or in-vitro studies only.|
|Grade||Effect||Size of Effect||Comments|
Seems to improves seborrheic dermatitis of the scalp by clearing M. furfur. Also effective in relieving the symptoms of atopic dermatitis.
Functions as both a humectant and occlusive, drawing water to the stratum corneum and preventing it from being lost through evaporation.
Increased the protective efficacy of a mixture of chemical sunscreens against UVB radiation in one study.
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Table of contents:
- 1. Sources
- 2. Bioavailability
- 3. Effects on the skin
- 4. Safety
1.1 In cosmetics
Propylene glycol or 1,2-propane diol is an alcohol that functions as a skin conditioner, viscosity-decreasing agent, solvent and fragrance ingredient in cosmetics. Its frequency and concentration of use has increased over the years. According to information submitted to the FDA, in 1994 propylene glycol was used in 5,676 products at concentrations up to 50%, but this had risen to 9,747 products at concentrations of up to 73% by 2009. In bath oils, tablets or salts it is used in even higher concentrations of up to 99%.
Overall, propylene glycol was reported to be used in 26% of all cosmetic formulations and 54% of all deodorants. It is also quite frequently used in moisturizers, as discovered by an analysis of 276 moisturizers that detected it in 20% of the products.
1.2 In food
Propylene glycol is generally recognized as safe (GRAS) by the US FDA as a direct food additive. It is permitted at maximum levels of 5% for alcoholic beverages, 24% for confections and frostings, 2.5% for frozen dairy products, 97% for seasonings and flavourings, 5% for nuts and nut products, and 2% for all other food categories. According to the Joint FAO/WHO Expert Committee on Food Additives (JECFA), the acceptable daily intake (ADI) of propylene glycol is 25 mg/kg/bw/day.
1.3 In pharmaceuticals
Propylene glycol is used as an active ingredient in a number of FDA-approved drug products. It has been approved at concentrations up to 98.09% for topical drugs, 94.925% in auricular solutions and 92% in oral solutions.
The cumulative penetration of propylene glycol into excised hairless mouse skin from a ternary cosolvent containing 84% propylene glycol, 10% oleic acid and 6% dimethyl isosorbide was 57% of the applied dose, over a 24-hour period. The propylene glycol did not seem to reach the dermis.
A smaller percentage (23% of the applied dose) has been estimated for human skin, based on the measured absorption of propylene glycol across excised human abdominal skin. Another experiment observed that propylene glycol failed to cross the stratum corneum even after 2 hours, supporting the notion that propylene glycol is poorly absorbed by the skin.
In an in vivo study, propylene glycol applied to the fingertip of a human subject reached a depth of 6 µm after 32 minutes. The highest concentration at this depth was only 0.2%. After more than 2.5 hours, the maximum concentration of propylene glycol was found at a depth of 3-4 µm Hence, it was suggested that propylene glycol diffuses only to a depth of 6-7 µm in human skin. Because the thickness of the stratum corneum of the human fingertip is >10 µm, propylene glycol therefore does not reach the dermis.
3. Effects on the skin
3.1 Moisturizing effect
Propylene glycol is a humectant. It dries on the skin to a hygroscopic film that retains moisture by forming a barrier against the evaporation of water, and is also thought to function by an osmotic mechanism that attracts water to the stratum corneum, which accounts for the observation that it leads to a transient swelling of the stratum corneum. It also helped enhance the occlusive properties of nanostructured lipid carrier (NLC) formulations by reducing transepidermal water loss (TEWL) from the skin.
Propylene glycol does not hydrate the skin as well as glycerin, probably because glycerin's superior hygroscopic character enables it to better modulate water fluxes in the stratum corneum and preserve the skin barrier more effectively.
However, one study found that propylene glycol had no significant hydrating effect, at least when applied as an oil-in-water emulsion. Another noted that propylene glycol dehydrates rather than hydrates the skin, as shown by a decrease in bound water content, due to alterations in the protein domains of the stratum corneum.
3.2 Antimicrobial effect
Propylene glycol is both an antibacterial and antifungal agent. At high concentrations, it is effective against the bacteria S. aureus, S. epidermidis, S. pyogenes, S. mitis and E. coli, as well as the yeasts C. albicans and M. furfur. As a result, it is effective at treating skin conditions caused by these pathogens, including Tinea versicolor, Malassezia folliculitis and seborrheic dermatitis.
3.3 Treatment of dermatitis
In a double-blind, controlled study of 39 patients, 16 out of 18 (89%) patients treated with a solution containing 15% propylene glycol, 50% ethanol and 35% water showed healing, compared to 6 out of 19 (32%) patients treated with 50% ethanol and 50% water. The number of M. furfur organisms was reduced significantly after treatment with the solution containing propylene glycol, but not with the other solution, indicating that propylene glycol's antifungal effect was responsible for the healing. 2 other double-blind and placebo-controlled studies likewise found that topical solutions of urea, lactic acid and propylene glycol led to clinically meaningful improvements in seborrheic dermatitis endpoints.
Propylene glycol's moisturizing properties also make it useful in improving atopic dermatitis, which is characterized by dry skin. Propyless, a commercially available lotion based on propylene glycol, was at least as effective as Fenuril, a commercial cream based on urea and sodium chloride, in alleviating the cutaneous symptoms of atopic dermatitis in 55 patients enrolled in a randomized, single-blind comparative study.
3.4 Treatment of ichthyosis
Mixtures of propylene glycol and water are very effective in treating ichthyosis when applied under occlusive dressings, and is even more effective at removing its keratolytic scales with the addition of salicylic acid to the formulation. The commercial cream Locobase Repair, which contains 20% propylene glycol + 5% lactic acid, was superior to 3 other cream formulations in improving lamellar ichthyosis in 20 patients. It reduced xerosis, scaling and skin roughness, and improved skin hydration. It also produced the most rapid effect, with a response rate of 63% after 4 weeks.
3.5 Other effects and uses
One study has suggested that the addition of propylene glycol can increase the capacity of chemical sunscreening agents to protect against UVB irradiation in both non-photosensitive subjects and patients suffering from a variety of photodermatoses.
Another study revealed that propylene glycol inhibited the activity of stratum corneum aspartic proteinases, which may serve as a marker for skin aging or for certain skin disorders.
Propylene glycol is also used to stabilize formulations. It helps prevent the aggregation of titanium dioxide nanoparticles, maintaining them in a monodispersed state, enhances the physical stability of nanostructured lipid carriers (NLC) formulations, and inhibited the decomposition of deoxyarbutin when used together with glycerin as the inner hydrophilic phase of anhydrous emulsions.
Propylene glycol and polypropylene glycols, the polymers of propylene glycol were originally reviewed by the Cosmetic Ingredient Review Expert Panel in 1994 and found to be safe for use in cosmetic products at concentrations up to 50%. In 2012, a re-review determined that concentrations up to 73% are safe and do not present a sensitization risk.
Propylene glycol is moderately cytotoxic to human fibroblasts and keratinocytes. Its IC₅₀ value for proliferation inhibition was 280 mM for fibroblasts and 85 mM for keratinocytes. It also inhibited collagen contraction by fibroblasts with an IC₅₀ of 180 mM, caused keratinocytes and fibroblasts to undergo changes in cell shape, and induced irreversible cell damage in both cultures at a concentration of 660 mM.
4.2 Adverse skin reactions
Propylene glycol is capable of producing both primary irritant skin reactions and allergic sensitization when applied topically, though it appears to be only slightly irritating and the risk of sensitization is very low for uncompromised skin. Some researchers have proposed classifying skin reactions to propylene glycol into 4 groups: irritant contact dermatitis, allergic contact dermatitis, non-immunologic contact urticaria, and subjective or sensory irritation. Propylene glycol does not seem to be a photoallergen however, as it did not produce any photoallergic responses in a provocative photopatch test on 82 subjects with photoallergic contact dermatitis.
Prospective studies and retrospective analyses of patch test data have found widely varying positive reaction rates, ranging from 0% to as high as 12.5%. In North America, the positive reaction rate seems to have hovered around 3.5%, though patch testing results from Mount Sinai Medical Center identified propylene glycol as an allergen in 7.8% of cases, making it one of the most frequent contact allergens. In Europe, positive reactions rates to propylene glycol was 1% in Austria, 0.4% in the Czech Republic, 0.8% in Denmark and 0% in Turkey. Propylene glycol was also one of the top allergens in German children and adolescents. At least part of the observed variance is probably due to differences in test protocols and concentrations.
There are numerous case reports of adverse skin reactions to propylene glycol. Personal care products are known to be a common source of propylene glycol exposure leading to allergic or irritant contact dermatitis, but contact allergy and contact dermatitis to propylene glycol can also arise due to its inclusion in topical medicines ultrasonic gels and ECG electrode gels. There have also been isolated reports of contact sensitivity to propylene glycol in a lubricant jelly and in an antiperspirant. Avoidance of propylgene glycol-containing products can lead to improvement of contact dermatitis.
Further, the addition of propylene glycol to an isopropanol vehicle enhanced the irritant reaction of benzoic acid in a non-occlusive test of 15 human subjects. In rare cases, overdoses on propylene glycol can cause intoxication in young children, leading to central nervous system depression and metabolic acidosis.
4.3 Enhanced penetration
Propylene glycol acts as a permeation enhancer, possibly by altering the barrier function of the skin or by increasing the solution capacity within the stratum corneum. It has been demonstrated to increase the skin penetration of the model substance pyrene butyric acid, the flavonoid hesperetin, the sunscreen oxybenzone as well as many drugs including topical glucocorticoids such as hydrocortisone.
4.4 Little evidence of genotoxicity
Propylene glycol was not mutagenic in Ames tests with or without metabolic activation, up to a concentration of 10,000 µg/plate. It was also not mutagenic not in mitotic recombination or base pair substitution assays, in a micronucleus test or in a hamster embryo cell transformation assay. Sister chromatid exchange assays using Chinese hamster cell lines showed inconsistent results, but propylene glycol did not induce chromosomal aberrations in separate studies on human cultured fibroblasts and human embryonic cells.
4.5 No evidence of carcinogenicity
Propylene glycol has not been found to be carcinogenic whether the method of administration is oral, dermal, or via subcutaneous injection. For instance, dermal application of undiluted propylene glycol to Swiss mice in a lifetime study produced no significant carcinogenic effects.
4.6 Reproductive and developmental effects
The National Toxicology Program has evaluated the potential human reproductive and developmental effects of propylene glycol and concluded that there is no cause for concern. High oral doses of propylene glycol did not affect fertility in mice or produce developmental toxicity in their offspring, and the pharmacokinetics of propylene glycol indicates that the lack of adverse effects observed in these laboratory animals is relevant to humans.
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