The Science Behind Sunscreens
The Science Behind Sunscreens
Sabrina Schrope
Cosmetic Chemist, Owner of Vitamin You
We’ve all been told how important it is to apply and reapply sunscreen during the day to prevent skin damage from ultraviolet (UV) radiation. But how does the sunscreen actually interact with UV rays?
To start, there are three types of UV rays:
UVA |
UVB |
UVC |
400-320nm |
320-290nm |
290-200nm |
Penetrates the deepest into the skin and causes free radical damage. Does not cause sunburn. |
Causes sunburn and direct DNA damage to skin cells. |
Currently does not pass through our ozone layer and is not included in sunscreen testing. |
The best sunscreens are “broad spectrum” meaning they protect against both UVA and UVB damage. Its effectiveness is rated by SPF, or sun protection factor, which can be calculated by measuring the MED ratio between skin with and without the sunscreen being tested. MED is the “minimum erythemal dose” or the minimum amount of UV radiation needed to cause visual reddening of the skin. However, the average sunscreen wearer only applies about a quarter of the actual thickness used when running SPF testing.1
There are two types of sunscreens:
2. Chemical Sunscreens
Also called “organic” sunscreens, common examples are aminobenzoates, cinnamates, and salicylates. These are claimed to work by “absorbing” UV rays.
Like the picture above, chemical sunscreens contain multiple double bonds close enough to allow for electron delocalization. The electromagnetic UV energy is “absorbed” by being converted into chemical energy. The compounds then “release” this energy through a lower magnitude, such as infrared energy.1, 2
2. Physical Sunscreens
Also called “inorganic” sunscreens, common examples are zinc oxide and titanium dioxide metals. These work by “reflecting” UV rays, but are more accurately described as scattering them.
When UV rays hit physical sunscreens, the rays are scattered among other sunscreen compounds based on their refractive index, and some is even absorbed. Better sunscreens are held within a medium that also has a high refractive index.3 Physical sunscreens are often coated with silicans to prevent interaction with the skin and agglomeration, or clumping together.1, 3
Applying sunscreen is only the beginning of a day out in the sun. Reapplications are needed to maintain an effective SPF, especially after sweating, prolonged UV exposure, and towel drying. Preventing premature aging and skin damage can easily be accomplished with sunscreen, and now you know how they work!
Citations:
1 Wolf, R. Matz, H. et al. Sunscreens - The Ultimate Cosmetic. Acta Dermatovenerol Croat. 2003. 11(3): 158-162. [Internet] Available at: https://hrcak.srce.hr/file/131453#page=17
2 Shaath, N. The Chemistry of Ultraviolet Filters. Principles and Practice of Photoprotection. 2016. 143-157. doi:10.1007/978-3-319-29382-0_9
3 Duarte, A. et al. Application of Titatium Dioxide. Janus, Magdalena (eds). InTech. 2017. 63-64. http://dx.doi.org/10.5772/intechopen.70121