How Sunscreen Is Tested in a Lab (the SPF Number's Origin Story)
Where that '50' on your bottle actually comes from.
Film-forming polymers help maintain a more uniform sunscreen layer through sweat and water.
Modern 'SPF boosters' and film formers help spread filters into a more even, protective layer.
Frequently asked questions
What is the difference between nano and non-nano zinc?
Nano zinc has smaller particles, so it rubs in more clearly with less white cast; non-nano uses larger particles that scatter more visible light but are preferred by some for environmental or personal reasons. Current evidence indicates nanoparticles in sunscreen don't penetrate healthy skin.
How do sunscreens become water-resistant?
Water resistance comes from film-forming polymers that help the sunscreen layer cling through water and sweat. US labels may claim '40 minutes' or '80 minutes' of water resistance — never 'waterproof.' Reapply after swimming, heavy sweating or towel drying to keep protection intact.
Does sunscreen expire?
Yes. Filters degrade over time, so expired product gives unreliable protection. The FDA requires sunscreens to remain stable for at least three years; check the expiration date and discard any that's past date or has changed color, smell or texture — heat exposure speeds breakdown.
r/AsianBeauty: 'Should I layer vitamin C under my sunscreen?'
Sources & citations
- Hanson KM et al., radical sun protection factor (RSF) study
- incidecoder.com ↗