Sun Protection Factor
Updated:
Sunscreens are primarily measured based Sun Protection Factor (SPF), which is a rating of what percentage of ultraviolet (UV) rays it blocks.
The formula for SPF is pretty straight-forward but everyone has overcomplicated things and introduced way too much confusion. Statements such as “it will take 10 times longer to burn” or “SPF 60 is 4 times the protection of SPF 15”.
It shouldn’t be used to determine how much longer you can stay out for (it’s not like an oxygen tank for divers). And it’s not “4 times the protection”, it’s actually only 5% more UV rays blocked.
Calculation
SPF is a simple formula:
% of UV exposure = 1 / SPF
For example, if SPF rating is 10, then you’ll experience 10% of UV ray exposure.
With SPF 50, 2% exposure.
Because users are more interested in how much UV ray they are not getting, a more common formula is to find the complementary ratio:
% of UV blocked = 1 - 1/SPF
| SPF | % blocked |
|---|---|
| 10 | 90 |
| 15 | 93 |
| 50 | 98 |
SPF follows a reciprocal relationship, which makes larger values have smaller incremental change.
Consumer Protection
The reciprocal relationship and focus on percent of UV blocked (as opposed to allowed) is what causes a lot of confusion. This is what allows the advertisers to play on fears and use distortion of perception to advertise that their higher SPF products warrant higher prices. In fact, there needed to be consumer protection in some countries to outlaw this deceptive practice.
It allowed marketers to take advantage of consumers. It also gave consumers false sense of confidence, believing that higher SPF would reduce the need to reapply as frequently or carefully.
Takeaways
Having broken down this magical number to its actual mathematical formula was great. Now we can see and understand why SPF 30 (96.7%) is probably just as good as SPF 50 (98%). We’d be splitting hairs at the difference.
As a wise man once said:
90% sun protection with 100% coverage is better than 99% sun protection with 80% coverage
It’s way more important to focus on the correct application: thickness, coverage, reapplication over time due to sweat or water.