You’ve heard it, or read it; it’s all over the internet.  Makes me crazy (er).

  • “Skin absorbs everything you put on it”.
  • “Everything you put on your skin goes to your bloodstream in 29 seconds”.
  • “Your skin absorbs 60% of what you put on it”.

The above shows the lack of understanding of skin physiology or too much time on junk science sites perpetuating false fears.

cosmetic penetration of the skin


Please try this at home:  Sit in 8 gallons of water in a bath tub and see if you come out weighing 64 additional pounds from absorbing all that water.

With so many variables and thousands of cosmetic ingredients, the extent of skin absorption lies between 0 and 100%.  To be sure there is passive absorption of materials that get into the body.  No argument there.  Caffeine is found in urine a day after topical application, and there’s a lot of focus lately on the intrafollicular pathway for absorption of chemicals.  But don’t fret; the stratum corneum is a unique physiochemical barrier.  A very efficient barrier.  One tough integument.  If it didn’t keep bad things out and the good things in, we’d be in trouble.

Most substances applied to the skin simply sit there until it is rubbed or washed away.

Why?  Because certain criteria are generally required: the molecule < 500 daltons, oil-water partition coefficient of the substance favorable (lipophilic), pH, degree of ionization etc.  There is not a hard and fast rule for molecular size.  But for the most part it does obey the less than 500 Dalton rule based on the following:

1) virtually all common contact allergens are under 500 Dalton, larger molecules are not known as contact sensitizers.

2) the most commonly used pharmacological agents applied in topical dermatotherapy are all under 500 Daltons.

3) all known topical drugs used in transdermal drug-delivery systems are under 500 Daltons.

4) clinical experience with topical agents such as cyclosporine, tacrolimus and ascomycins gives further arguments for the reality of the 500 Dalton rule.

SourceExperimental DermatologyVolume 9, Issue 3, Version of Record online: 24 DEC 2001.

I find it amusing that the “skin absorbs everything” people point to transdermal drug delivery patches as proof for their argument.  There has been tremendous research, time and money spent to develop transdermal patches that work because the stratum corneum is so efficient in keeping things out of the body.  Not all therapeutic drugs are suited for transdermal patches.  

That is why we still have to inject drugs intramuscularly, subcutaneously or intravenously

If it were so easy to get chemicals to cross the dermal barrier, the pharmaceutical industry wouldn’t be developing adjunct methods to accomplish this- such as nano materials, lasers, microneedles, iontophoresis, electroporation and ultrasound AND we wouldn’t have the need for penetration enhancers.

*That’s why I snicker at the sellers of “magnesium oil”.  It’s ionic (charged molecule) and not likely to absorb through the skin-  even if you sit in a tub of Epsom salts all day.  There’s no good science to support it.  You get magnesium from food.  If your body is in need of it, your doctor will put you on supplements.

The one study that tried to show transdermal absorption was done by (drum roll) the Epsom Salts people lol.  Small study 19 subjects.  Duration of bath 12 minutes.  Measurements via blood and urine.  Without radioactive labeling of what is being measured, how can they prove the increase in Mg or SO4 is from a 12 minute bath?  The levels of these ionic species are always changing in the blood and urine.  They’re dynamic-and affected by the metabolic processes in the body.   BTW, if magnesium oil did work, it would be a drug- not a cosmetic.   There may be people that swear by it, but hey, placebos work pretty well.