Yesterday, a friend of mine trotted out that old urban legend about using WD-40 to ease the pain of arthritis. I tried to tell him it was a myth, but he didn’t believe me. His argument: fish oil is known to help relieve arthritis pain, and WD-40 contains fish oil. Oh, boy.
First, the manufacturer of WD-40 recommends against putting their product on your skin. In addition, WD-40 does not contain fish oil, as you can see by reading their Material Safety Data Sheet. It’s mostly petroleum distillates, and I certainly wouldn’t want those passing through my skin and into the joints. Now there’s a thought. Can they? We’ll get back to that.
So what about fish oil, anyway? It turns out that there is some evidence that fish oil can reduce the swelling associated with rheumatoid arthritis. That is, fish oil that’s taken as a dietary supplement. There is no evidence that rubbing fish oil on your skin will have any measurable effect other than that provided by the act of massaging. The fish oil can’t pass through the skin in sufficient quantity to have any effect.
Along the same lines, there are countless sites pushing “natural” skin care products that warn of the dangers your cosmetics pose. A popular myth seems to be that a woman will absorb between 5 and 20 pounds of skin care chemicals through her skin per year. If you believe those claims, a woman’s body is a veritable toxic waste dump. That claim is more absurd than the colon cleansing sites’ claim that I have 5 to 20 pounds of stuff stuck to my colon, “like spackle or paste.” But I digress.
Back to the point. Is it even possible for WD-40 to pass through the skin?
There are some chemicals that do pass through the skin very easily. Probably the best known is DMSO. Although not toxic itself, DMSO is a very powerful solvent that can carry through the skin the things that it dissolves. Unfortunately, I’ve not found a list of other chemicals that are as easily absorbed.
The primary ingredients in WD-40 are petroleum distillates, specifically alphatic hydrocarbons and petroleum based oil. Everything I’ve been able to find shows that the danger of absorbing these chemicals through the skin is very low, provided you don’t have any cuts or open sores. I suppose if you bathed in it for an hour every day you might get some under the skin. It’s unlikely, however, that the effects would be good. It almost certainly wouldn’t relieve the aching joints.
I’ve found it rather difficult to find good information about the permeability of skin to different substances. What I’d really like to see is a list of chemicals (including common names, where applicable) that gives an indication of the danger of skin absorption. It’d be difficult to do that in a single list, though. Some chemicals will pass through the skin readily, but pose no real health hazard. Others might have more difficulty passing through the skin, but pose an extreme hazard if they get into the bloodstream.
I did run across a couple of interesting links having to do with the dangers of absorbing jet fuel and gasoline. The article Assessment of Skin Absorption and Penetration of JP-8 Jet Fuel and Its Components, published in Toxicology Sciences, says in its abstract:
These results suggest: (1) that JP-8 penetration will not cause systemic toxicity because of low fluxes of all the components; and (2) the absorption of aliphatic components into the skin may be a cause of skin irritation.
In other words, you might absorb a bit of it and it might irritate your skin, but it’s unlikely to cause major problems.
According to the Canadian Centre for Occupational Health and Safety’s Health Effects of Gasoline:
When gasoline is NOT trapped against the skin and can freely evaporate, it is probably only mildly irritating or not irritating. However, case reports indicate that when gasoline is trapped against the skin (clothing is soaked in gasoline, skin is in contact with a puddle) for a long period (probably more than 30 minutes), serious burns and skin loss may occur. Absorption through the skin occurs, but is normally not significant.
All the research I’ve found about the dangers of gasoline and diesel indicates that the real dangers are in inhaling, ingesting, or getting it into the body through a tear in the skin. I wouldn’t soak my hand in gasoline, but I wouldn’t worry too much about a few splashes on my arm.
Some friends who work in auto repair report sometimes using brake fluid as a hand cleaner. This is probably a bad idea, but not disastrous. Most brake fluids are glycol-ether based. I’m not certain, but it looks like all of these solvents are quite toxic if ingested. There is some evidence of skin absorption through lesions, but I wasn’t able to find any solid information on absorption through intact skin. Again, I doubt that there’s much danger if you’re not soaking in it.
An interesting resource for the more technically minded is the Skin Permeation Calculator available from the National Institute for Occupational Safety and Health. You’ll need to know the CAS Number for a particular chemical, or have its molecular weight and a number called the LOGKOW. Given those two numbers, the calculator will give you some numbers that indicate how easily the chemical will pass through the skin. I don’t yet know enough to make good use of that information, though.
Update 2012/08/10
I got a note from the person who wrote the original Skin Permeation Calculator, linked above. He says:
There is a newer much more sophisticated version of the skin permeation calculator at http://www.cdc.gov/niosh/topics/skin/finiteskinpermcalc.html,
though it needs several more parameters to enter. This version can actually predict how much of the stuff will get through or stay in different skin layers. It has four example buttons at the bottom, to help guide through the program a bit.
You can find the CAS Number for lots of different chemicals from the NIOSH Pocket Guide to Chemical Hazards. Unfortunately, the Skin Permeation Calculator doesn’t recognize all of the CAS Numbers and the NIOSH data lists the molecular weight, but not the log KOW. So you end up having to find the CAS and use it to search the LOGKOW database.
I’d be interested in hearing about any list like the one I described above: substances listed by danger of absorbing them through the skin. Anybody have a link? I’d hate to have to download those databases and run the numbers myself.