Penetration of sunscreen into fingernails?

[Kai]

 

In the summer, I sometimes used sunscreen to protect my skin from UV rays before long hikes. My son does not need such protection. On a UV image of both of our hands, I noticed that my fingernails - although I hadn't even protected myself that day - are clearly bluish. 
Hands are freshly washed with a surfactant!
[Canon 6D-FS, EF 50mm, 1:1.8, ZWB2 plus TSN575, f 4.5_1-30 s_3200 ASA, WB Teflon]

 

 

Anyone know anything about penetration of UV protectants into fingernails/keratin?
Are there already systematic studies? Dependence of penetration on degree of hydrophobicity, molecular mass, ...? Penetration kinetics?
 

Or does someone here simply have practical experience?

The product used contains the following UV protection agents:
- Ethylhexyl Salicilate
- Butyl Methoxydibenzoylmethane
- Ethylhexyl Methoxycrylene
- Phenylbenzimidazole Sulfonic Acid
- Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine

[JMC]

The remains of sunscreens will often hang around on the surface of skin for a long time, even after washing (very easy to contaminate filters if you handle them with sunscreen on your hands). I wouldn't be surprised to see traces the next day, not enough to protect you from the sun but enough to image.

 

Odd though. Organic sunscreens will typically show up as black in UV images as they absorb the light. To me the blue colour suggests it is something that is strongly reflecting the UV at around 380-400nm. Also, strange that it is only on one hand. I suppose a thin layer of something which absorbs well up to say 370nm, and they doesn't after that might then be imaged as blue if it allows the nails to reflect the longer wavelength UV while absorbing the shorter wavelength.

 

Does the product (or other products you've used) contain titanium dioxide or zinc oxide, often used as a whitening agent? That could be scattering and reflecting the light and could make it look blue. Not sure why it would be so strongly associate with the nails though.

 

Interesting observation and let us know if you find out what it is.

[Kai]

Hello Jonathan,
thank you for the fast reaction. That's right - skin and cosmetics are your specialty... :)
Yes, the bluing is perhaps less of a "faint black" and more of a selective reflection. But a pure "ad-on" reflection should result in lighter fingernails. The pure grayscale image shows that the blue fingernails are slightly darker than the untreated ones.
Zinc oxide (which looks very nice blue in the UV) is not included, titanium dioxide is the 20th place among the ingredients. Shouldn't that be washable too?

 

 

Of course, I don't have the pure substances and I don't have any standardized horn samples either. Otherwise the systematic investigation could start ;)

The product ist "Ladival Allergische Haut" (Stada). (Don't know if this is distributed internationally.)

[Stefano]

From my experience, thin layers of sunscreen look dark false blue-violet. Maybe very thin layers are mostly transparent above 380 nm while still blocking shorter wavelengths.

 

In this topic there are a couple of examples.

[Kai]

3 hours ago, Stefano said:

From my experience, thin layers of sunscreen look dark false blue-violet. Maybe very thin layers are mostly transparent above 380 nm while still blocking shorter wavelengths.

 

In this topic there are a couple of examples.

Thank you for your note Stephano. The more detailed question is which substance causes this effect and why this coloration persists after cleaning and which physico-chemical mechanism leads to this.

[JCDowdy]

Very interesting observation!

Having worked in the area some years I can say that some sunscreens are quite well known to penetrate the skin.  Having also once worked in antifungal research, particularly fungal nail, I can also say that while it is difficult to get topically applied agents through the nail into the nail bed it is not impossible.

What you are seeing is most likely some penetration into the outer surface of the nail and/or binding to the nail surface.  You could file the surface of one and see if you still observe staining below the surface.

Keep in mind that sunscreens have very high extinction coefficients in the UV and even scant residue can be detected.  I once discarded a set of ground quartz plates used in in-vitro sunscreen transmittance measurement because I could not get them clean enough to reuse. The residue visible spectroscopically resisted every solvent I had, ultrasonic cleaning and even soaking in Chromerge!

[Kai]

16 hours ago, JCDowdy said:

Very interesting observation!

Having worked in the area some years I can say that some sunscreens are quite well known to penetrate the skin.  Having also once worked in antifungal research, particularly fungal nail, I can also say that while it is difficult to get topically applied agents through the nail into the nail bed it is not impossible.

What you are seeing is most likely some penetration into the outer surface of the nail and/or binding to the nail surface.  You could file the surface of one and see if you still observe staining below the surface.

Keep in mind that sunscreens have very high extinction coefficients in the UV and even scant residue can be detected.  I once discarded a set of ground quartz plates used in in-vitro sunscreen transmittance measurement because I could not get them clean enough to reuse. The residue visible spectroscopically resisted every solvent I had, ultrasonic cleaning and even soaking in Chromerge!

That all sounds exciting! Thanks for the information. Yes, I could probably do my own research here. Will try to get the sunscreen ingredients individually. Perhaps a depth profile (by filing or scraping) can be created. If anything comes of it, I'll be sure to report it here : )