How edible are the ingredients in your skincare routine?
And why does that matter – especially when we're talking about soap, that is usually washed off the skin within seconds of lathering up?
Because skin is your body's largest organ – and your skin can "eat" too, distributing what it takes in throughout your whole body.1,2
Consider the range of transdermal drugs that undergo clinical trials for use in conventional medical treatments:
Nitroglycerin patches for preventing chest pain;
Scopolamine patches for treating motion sickness;
Steroid creams for treating local skin conditions;
Nicotine patches for assisting with smoking cessation;
Fentanyl and buprenorphine patches for treating chronic pain;
Testosterone patches for treating hypogonadal men;
Rivastigmine patches for treating Alzheimer's disease;
Transdermal vitamins and minerals currently under investigation for post-bariatric surgery patients....3-6
Conversely, consider some of the known toxicities your body is also vulnerable to through skin exposure:
Pesticide toxicity in farmers through skin absorption;
Nicotine toxicity in tobacco workers who handle leaves and also in those who refill e-cigarettes;
Oxidative DNA damage and toxicity from nano -sized zinc oxide, titanium dioxide, and other nanoparticle metals – ingredients commonly found in commercial sunscreens and cosmetics, among other things; and
Glyceryltrinitrate toxicity in workers who handle dynamite.7-13
What factors affect skin absorption?
There is not a "one-size fits all" model when it comes to assessing levels of skin absorption for various skin products and their chemical components, and the pharmacokinetics of transdermal drug delivery are quite multifaceted.3 To give you an idea, several of the many factors evaluated include:
What size and type of molecules are being applied to the skin? What is the potency or the active ingredient concentration of the product? Where on the body is the product being applied? How long is the product maintaining contact with the skin? How quickly does the product degrade in sunlight? Etc....
In addition to all this, an interesting article by Singh and Morris (2011) discusses biological components that contribute to variability of skin absorption from person to person – some of which also interact with each other, including:
1. Age: Babies up to 5 months have higher skin permeability since their skin barrier isn't yet fully developed – so their skin absorbs more from a topically applied product compared to adult skin.14 Consequently, skincare ingredient lists matter all the more for babies compared to adults.
Some evidence also suggests that older adult skin seems to absorb less from hydrophilic ("water-loving" / able-to-bind-with-water) compounds compared to young-to-middle aged adults – in theory, due to lower water content in the skin barrier. Older adults do, however, appear to still be able to absorb lipophilic ("fat-loving" / able-to-bind-with-fats) compounds just as well as their younger counterparts.14
2. Ethnicity: Several studies have indicated varying levels of percutaneous skin absorption depending on ethnicity – a finding that is speculated to be due to differences in skin structure (such as skin water content, numbers of keratinocyte layers, and levels of cellular cohesion between these keratinocytes and higher lipid content in the outermost layer of skin).
Some research has pointed to highest skin permeability levels – and highest absorption rates – in Hispanics, followed by Caucasians, then Asians; with Afro-Caribbeans exhibiting the lowest skin permeability levels among these groups.14
3. Skin hydration: Skin with higher water content appears to better absorb certain compounds than skin with lower water content – a dynamic mentioned earlier, in reference to both older adults and certain ethnicities. Researchers who develop transdermal drug products even try to seize upon this dynamic by lining transdermal patches with non-porous surfaces so as to preserve and maximize the hydration of the skin underneath, improving drug absorption.14
Amazingly, it has been found that if skin is soaked in water for even a short amount of time, the outer layer of the skin can absorb 20 times more water than it might normally hold – thus increasing absorption potential of hydrophilic compounds topically applied to the skin.14
Want to absorb more from your skincare product? Apply the product when your skin is saturated with water!
----> Health-promoting soap, anyone...??
4. Skin temperature & emotional state: Higher skin temperatures have been found to also increase skin absorption through a fluidization process in the outer skin layer's lipid (fat) region. This finding is one reason that insulating wound bandages are sometimes applied on top of treated injuries – to maximize absorption of the skin treatment applied.14
And – perhaps surprisingly – this same temperature dynamic occurs with certain heightened emotional and physiological states where the sympathetic nervous system ("fight or flight") is triggered, increasing body heat.14
So... next time you find yourself really *peeved* off or charged – maybe think about seizing the moment and applying your favorite health-promoting skincare products before you cool yourself down :-D
5. Skin barrier condition: If the integrity of your skin barrier is damaged, such as in any of the following ways, you can expect your skin to have higher permeability and absorb topical drugs and product ingredients at higher rates:
Wounded skin;
Eczema;
Psoriasis;
Dermatitis; and
Other skin conditions that affect your skin barrier.14
This finding is highlighted in a Soroka University Medical Center study where penetration of electrolytes was measured in both healthy volunteers and in psoriasis patients after 4 weeks of daily bathing in the mineral-rich Dead Sea.
What was found? The only significant serum levels of calcium, bromine, rubidium, and zinc were discovered in psoriasis patients; while serum levels of these minerals among healthy volunteers were not found to be significant.15
This principle helps makes sense of why people with eczema, for example, often have a hard time with many commercial skincare products that can contain all kinds of synthetic and toxic ingredients. Their skin – similar to baby skin in this way – possesses a kind of megaphone that amplifies the absorption of whatever ingredients are in the skincare product.
Anatomical location & skin absorption
For topical treatments intended to treat localized conditions, these treatments are frequently applied directly at the site of the condition needing treatment – such as a pain-relieving patch that is applied to a specific site of the body that's in pain.14
But what about maximizing systemic effects of a topically applied product? Are there certain sites of the body with higher skin absorption rates than other parts of the body?
You might think that skin absorption rates decrease as thickness of the skin increases – but that is not necessarily always the case. And scientists who design clinical drug patches often advise applying patches to the upper back, chest, or arm regions – but the logic behind selecting these locations is not solely based on skin absorption rates. It is also influenced by choosing areas of skin that tend to be less hairy and don't move as much during movement so that the adhesive patch will stay on.5,14
And while the findings that point to certain body sites for drug patch application are well-founded – when there's no need to worry about the mechanics of applying a sticky patch, there is another approach that can help you maximize your skin absorption: by taking advantage of the glandular skin regions.
A recent 2021 study showed that:
skin absorption is exceptionally high in body regions that are rich with sebaceous and apocrine glands.16
"Sebaceous glands": These glands are part of the hair follicle unit and can be found wherever hair follicles are – everywhere but the palms of the hands and soles of the feet. However, they are most prominently located on the face and on the scalp, followed by the upper chest (see diagram above). The glands are controlled by androgens and secrete an oily substance called "sebum." Their function is not yet fully understood by scientists.17
"Apocrine glands": These are low-secretion sweat glands that don't appear to have any role in regulating body temperature. As shown in the diagram above, apocrine glands are mainly found in the axillary (underarm), anal, and genital regions – but they are also found on the eyelid, within the areola, and in the external ear (called "ceruminous glands" – believed to help clean and protect the external ear from infections). In animals, these glands play a role in producing scent and thus also in attracting potential mates.18
What does all this mean for you?
You'll want to especially avoid toxic ingredients – and rather seek out health-promoting ingredients – in these gland-rich regions.
Here are several typical skincare and cosmetic products we tend to apply to these regions – where product ingredients matter all the more:
Face & eyelids: Soap, toners, moisturizers, shaving cream (men), makeup, sunscreen
Scalp: Shampoo, conditioner, hair gels/hairsprays, hair dyes
Upper chest: Perfume, makeup, bug spray
Underarms: Deodorant, antiperspirant
Genital & anal: Soap, shaving cream, lubricants, deodorizing sprays
Areolas: Nipple cream (nursing mothers)
External ear: Soap, run-off from shampoos and conditioners, sunscreen
Common ingredients to avoid in soaps & skincare products
There's a lot that could be added to the list, but let's review a few key ingredients you may wish to avoid in soaps & skincare products, including:
Synthetic "fragrance"
Sulfates, parabens, silicones
Triclosan
Nano-sized zinc oxide and titanium dioxide
1. Synthetic "fragrance": It may be the reason your strong-smelling soaps and skincare smell so good, but the FDA permits skincare companies to simply slap this term on the label for all but a handful of fragrances – without disclosing what slew of chemicals are creating this fragrance (all in the name of protecting trade secrets).
"Fragrance" may emit volatile, air-polluting compounds that can trigger a range of adverse health effects on humans who are exposed:
Contact dermatitis
Respiratory problems
Asthma attacks
Migraine headaches
Mucosal symptoms19
Longer term human health effects of are not very well studied, although some common skincare fragrance chemicals are classified in the International Nomenclature of Cosmetic Ingredients (INCI list) as mutagens, carcinogens, and substances toxic to reproduction.20
Do you consider your skin to be sensitive or prone to inflammation? Then all the more, you will want to be wary of "fragrance" in your skincare products. According to a 2021 study published in an allergy journal,
"Fragrances are the most frequent chemicals causing contact dermatitis."20
Here are a few of the most common offending chemicals, when it comes to contact dermatitis and eczema sufferers:
Limonene (and oxidized limonene)
Geraniol (and oxidized geraniol)
Hydroxyisohexyl 3-cyclohexene carboxaldehyde (HICC)
(Oxidized) linalool
(Oxidized) linalyl acetate20
Skincare products that frequently carry these "fragrances" include liquid soaps, scented lotions, deodorants, shampoos, perfumes, aftershaves, and lipsticks.
But also be watchful for chemicals in hygiene products that are applied to moist, occluded regions, which can present high risk of sensitization: M. pereirae in diaper products that can be bought over-the-counter, for example, as well as chemicals that can be present in tampons, sanitary napkins, and panty liners (common chemicals include linalool, limonene, geraniol, citronellal, hydroxycitronellal, benzyl salicylate, and hexyl cinnamal).20
Where it gets dicey for the dermatitis-prone individual, however, is that even certain "natural" foods and ingredients can contain some of these compounds. Case in point:
"Peeling an orange releases 75 times [more] limonene than spraying a scented cleaning product."20
And even certain essential oils may naturally contain certain terpenes and organic compounds – sometimes which are also the very same source of that essential oil's therapeutic value (or believed therapeutic value).
So what's a sensitive skin individual to do? Probably the most conservative route would be to look for ingredient lists that don't contain either "fragrance" OR essential oils.
2. Sulfates, parabens, silicones
There is a lot of controversy surrounding the health safety of these ingredients. And there is also limited research, even though these ingredients are very common ingredients across the industry.
Sulfates
Sulfates are cleansing detergents that help produce lather, with one of the more common sulfates being sodium laureth sulfate (SLS). Many commercial "soaps" and shampoos on the market are actually synthetic detergents because they rely on sulfates and other chemicals for their cleansing properties. They can provide companies with an easy, inexpensive route for producing and selling a cleansing product to the masses.
According to the FDA regulatory rules, true "soaps" are made with fats or oils that have been "saponified" through a chemical reaction with lye (an alkali) – and after the chemical reaction is complete, the product no longer contains any lye and qualifies to be labeled as a "soap." 21
One of the known effects of sulfates and other surfactants is that they can strip the hair of its oils – and they have been associated with increased hair fragility, scalp dryness, and increase in frizz.22 And when used in traditional soaps, shampoos, and conditioners – there is also potential for aerosol inhalation of these chemicals, and therefore potential longer term inhalation toxicity.23
Silicones
Silicones are often added to conditioners or sulfate-based shampoos for their smooth, lubricating effects on hair that can counterbalance sulfate's stripping of hair oils. Although immediate after-effects of silicones can make hair feel softer and silkier, silicones and other cationic conditioning polymers can combine with anionic sulfates commonly found in commercial shampoos and deposit insoluble complexes that ultimately can damage hair.22
Parabens
Parabens are frequently used as preservatives in soaps, personal care products, and cosmetics. They have been known for posing health risks as endocrine-disrupting chemicals that mimic estrogen molecules in the body – potentially leading to health problems provoked by excessive estrogen, such as endometrial cancer or breast cancer.24,25
Research as recently as 2020 seems to state that no hazard is present with any single paraben exposure or use; but that repeated use, excessive use, or use through multiple products simultaneously should be avoided.26
Just curious – if you buy a shampoo or lipstick with parabens in it, do you plan to use it only one time and then throw it out?
Yeah. Didn't think so.
As for me and my household...I think we'd rather avoid that risk.
3. Triclosan
Triclosan is often found in soap claiming to be "antibacterial" but can actually promote the development bacteria with higher resistance. Triclosan also contains a known endocrine-disrupting carcinogen ("dioxin") that has been observed to be able to pass through breast milk to babies.27,28
4. Nano-sized zinc oxide and titanium dioxide
Zinc oxide and titanium dioxide have been commonly included in commercial sunscreen formulations in their small, nanoparticle-size (nano-size) form. They are also used in many cosmetics, soaps, shampoos, and toothpastes – in both their smaller "nano-size" form as well as their larger "non-nano" form.
The smaller nano-size form of zinc oxide and titanium dioxide have been shown, however, to cause light (UVB)-induced DNA damage by producing reactive oxygen species (ROS). And such DNA damage can lead to the genomic instability that many types of cancer exhibit. The skin-penetrating nature of the nanoparticle-size form can also provoke a more immediate response in allergy-prone skin by causing systemic body production of IgE antibodies. These findings do not apply, however, to larger non-nano sized molecules ("non-nano") that remain on the surface of the skin.10,29,30,31
In short, if your skincare or cosmetic products contain either zinc oxide or titanium dioxide, you'll want to make sure they contain "non-nano zinc oxide" and/or "non-nano titanium dioxide" – and not the nanoparticle form ("nano") of either mineral. There are mineral sunscreens available to choose from, for example, that do not use the nanoparticle form of these minerals but that instead use the safer "non-nano" versions.
Conclusion
Do you HAVE to be able to eat all your soap ingredients for your soap to be safe and healthy for you to use?
Probably not.
But if you'd rather stay on the safe side and try to minimize both known and unknown health risks, it might be a good idea to ask yourself: are you feeding your hungry skin with the same health-conscious intentionality that you feed your belly?
References
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