24/06/2026
☀️Let’s talk SPF for a minute … this is the science behind your SPF number!!
Scientifically, the difference between SPF25 and SPF50 comes down to how much UVB radiation they block and how long they theoretically protect your skin before burning.
Contrary to what the numbers suggest, SPF50 is not “twice as strong” as SPF25. The relationship between the SPF number and protection is linear for time, but logarithmic for the percentage of UV rays blocked.
Let’s dive a little deeper … 🤿
☀️Percentage of UV Rays Blocked
SPF (Sun Protection Factor) measures a sunscreen’s ability to filter out Ultraviolet B (UVB) rays, which are the chief cause of sunburn and skin cancer.
💛SPF25 filters out approximately 96% of UVB rays (allowing 4% to pe*****te).
🧡SPF50 filters out approximately 98% of UVB rays (allowing 2% to pe*****te).
🟰The Mathematical Equation (Time to Burn)
The scientific definition of SPF is based on the Minimal Erythemal Dose (MED), which is the amount of UV radiation required to cause sunburn on protected skin versus unprotected skin.
In a controlled lab setting, this translates to a time multiplier. If your unprotected skin begins to burn after 10 minutes in the sun:
💛SPF25 theoretically multiplies that time by 25: 10 \times 25 = 250 minutes (about 4 hours) of protection.
🧡SPF50 theoretically multiplies that time by 50: 10 \times 50 = 500 minutes (about 8 hours) of protection.
🌍 While the math works out perfectly in a laboratory, the real world is messy. In reality, no sunscreen remains effective for 4 to 8 hours. Sunscreen breaks down under direct sunlight, rubs off on clothes and washes away with sweat or water. Because of this, dermatologists recommend reapplying any SPF every two hours, regardless of whether it is SPF 25, 50, or 100.
I found it really interesting so I am sure loads of you will too!
Hope you are all coping in this heat 🫠
