How does tanning oil increase UV skin damage? I see many websites claiming oils amplify UV waves.  This would only
seem possible if thicknesses caused no reflection as in a quarter-wave
coating, but that seems hard to believe.
So, is it a myth that oil increases UV absorption by skin?
If not, please explain the chemical/physical mechanism in detail.
I suspect that total UV exposure is unchanged, but the oil has some chemical
reaction with the skin, reducing scattering at the skin's surface.
 A: There is no physical mechanism by which tanning oil increases the intensity of UV light. the true problem is as follows:
"Tanning oils" generally do NOT contain any UV-absorbing component to protect your skin from burning. They are there to make you look all moist & tasty while roasting under the sun's rays and to prevent heat and tropical breezes from drying out your skin. However, many people who use these products do so in the belief that they do offer some UV protection and they then tend to stay out in the sun too long for their own good and then get burned badly by the end of the day. 
To them, it appears as though the tanning oil caused them to burn, when in fact it had nothing to do with the burn except for the nonphysical effect noted above. 
A: Artists put a final coat of varnish on their paintings which provides a surface of higher index of refraction material. This makes the colours appear brighter mainly by increasing the absorption of the incident light as the incident light is actually made of light from many angles.  The higher index material accepts light from more angles where normally some of it would be reflected. 
Oil on the body likely has a similar effect.
A: As far as I am aware there is no study in a reputable peer-reviewed dermatology journal that makes the claims of tanning oils evidence based. It should be noted that:

*

*Tanning oils generally do have a low SPF (in the range of 3 to 10) typically. In theory this can be effective but application of sunscreen is never uniform nor thick enough (think mass per unit area) for protection equivalent to the SPF advertised.


*Tanning oils may have UVA protection but no UVB protection, thus still allowing for a burn to easily occur, as well of course as tanning, which is just skin injury after all.


*Chemical sunscreens like avobenzone really have to be stabilised and not all brands do this, especially in tanning oil products, which means even if there is SPF it may wane.
These factors mean tanning oils don't provide adequate sun protection even if brands manage to justify sticking an SPF label on the product and thus there is a perception by the consumer one is somehow protecting the skin yet still - in contradiction - allowing for a tan.
