What does SPF on a sunscreen mean?
Understanding sunscreen protection and the math behind it!
Every sunscreen I’ve seen in my life has an SPF value printed in big numbers. I know that SPF 70 is strong, and that SPF 15 is weak. BUT THERE WAS MORE TO IT??
Shoutout to my friend Doosh whose friend Rosa who casually mentioned that SPF actually means something legit. And that there’s math involved.
SPF stands for Sun Protection Factor. Knowing the full form doesn’t make us immediately smarter but it’s one step closer.
Sunscreen ingredients, such as zinc oxide or titanium dioxide, usually work by either absorbing UV rays or reflecting them away from the skin. The efficacy of a sunscreen is contingent on its ability to create a barrier, preventing UV radiation from penetrating the skin.
SPF Explained (no math)
SPF is a number that tells us about the sunscreen’s ability to shield the skin from the harmful effects of ultraviolet B (UVB) rays. UVB rays are the culprits behind sunburn, and prolonged exposure can contribute to skin aging and increase the risk of skin cancer.
A simple way of explaining SPF would be: how many times longer you can stay out in the sun without getting burnt, as compared to without any sunscreen before getting burnt.
Therefore applying an SPF 15 sunscreen would mean you can (theoretically) stay in the sun for 15x longer before getting burnt.
An SPF 50 sunscreen would mean you can (theoretically) stay in the sun for 50x longer before getting burnt.
I say theoretically because this might be affected by multiple factors including, but not limited to, skin, water, and whether the sunscreen was applied properly or not.
SPF Explained (yes math)
The SPF number correlates with the level of protection a sunscreen offers against UVB rays. This is a rough approximation of how much protection against UVB the SPF value corresponds to:
Note the difference between SPF30 and SPF50. While it sounds like SPF50 should provide twice the protection as SPF30, the difference in protection is just ~1.3%. A higher SPF gives only an incremental percentage increase in UV protection, but could make a significant difference for an individual. More of that to be explored a little later in the article.
I really need to figure out what the formula for SPF is. The simplest formula I came across was:
SPF = 1 / (The % of UV-B rays that are not blocked)
Therefore an SPF of 50 will allow 1/50 = 2% of UV-B rays to be transmitted, thereby blocking 100% -2% = 98% of UV-B rays.
However, Wikipedia clearly says this isn’t really the case:
“SPF is not simply the inverse of the transmittance in the UVB region. If that were true, then applying two layers of SPF 5 sunscreen would always be equivalent to SPF 25 (5 times 5). The actual combined SPF may be lower than the square of the single-layer SPF.
Instead, Wiki says the formula is this:
“where E(λ) is the solar irradiance spectrum, A(λ) is the erythemal action spectrum, and MPF(λ) is the monochromatic protection factor, all functions of the wavelength λ.”
Yes, I will do a hand wave now — swisssh — and we can move on from this equation now.
UV-A vs UV-B
While SPF primarily measures protection against UV-B rays, there’s also another band in the spectrum called UV-A rays.
UV-B rays have both beneficial and harmful effects on the skin, and mostly affect the skin’s outer layers. Outside of its harmful effects such as sunburns and skin-cancer, UV-B radiation is needed by the skin for synthesizing vitamin D. Vitamin D3, which is produced, helps improve bone health, muscle movement, and immune system functioning. So take UV-B rays in balance, and don’t be exposed for too long.
UV-A rays have quite a few harmful effects on the skin too, and can affect the skin’s deeper layers. Exposure to UV-A leads to premature aging (also known as Photoaging), hyperpigmentation, and an increased risk of skin cancer too.
When selecting a sunscreen, try choosing one offers protection against both UV-A and UV-B rays (this is often labeled as ‘broad-spectrum’ on the product)!
To leave you, here’s Drake helping you with sunscreen logistics:
