SPF numbers get most of the attention on a sunscreen label, but they only tell half the story. Understanding what SPF and PA rating each actually measure, and what water resistance really requires, makes it much easier to choose sun protection that genuinely fits your needs.
What SPF Actually Measures
SPF, or Sun Protection Factor, measures how long a sunscreen protects skin from UVB rays, the ones responsible for sunburn, compared to unprotected skin. SPF 30 blocks about 97 percent of UVB rays, SPF 50 blocks about 98 percent, and SPF 100 blocks about 99 percent. The jump looks small on paper, but it matters more in consistently high UV climates.
What PA Rating Measures, and Why It's Separate
PA rating measures UVA protection, the rays responsible for deeper skin damage and premature aging, using a scale from PA+ to PA++++. This comes from a lab test called the PPD test, which measures how long a sunscreen delays UVA-induced skin darkening compared to unprotected skin. A high SPF does not automatically mean high PA, since the two measure protection against different types of UV rays, and a sunscreen needs ingredients specifically effective against UVA, like zinc oxide or avobenzone, to earn a strong PA rating.
Why Both Numbers Matter Together
SPF alone protects against burning, PA alone protects against aging and deeper damage. A sunscreen listing only SPF, with no PA rating, may be leaving a real gap in protection, particularly relevant in high UV, high sun exposure climates.
What Water Resistance Actually Requires
Water resistance is not a texture claim, it's a tested one. A genuinely water-resistant sunscreen needs specific water-repelling polymers in its formula, plus a standardized wear test, typically 40 or 80 minutes of water immersion, confirming the SPF holds up afterward. A sunscreen that simply feels smooth or slightly water-repellent due to silicones doesn't automatically meet this tested standard.
The Bottom Line
Choosing sun protection well means looking past the SPF number alone, checking for a PA rating for UVA protection, and treating water resistance as a verified claim rather than an assumption based on texture.
Frequently Asked Questions
Is PA++++ always better than PA+++? Yes, PA++++ represents the highest tested UVA protection level currently available, PA+++ is still strong protection, just a tier below.
Can a sunscreen have high SPF but low PA? Yes, this happens when a formula is optimized for UVB filters without an equally strong UVA filter, which is why checking both numbers matters.
Do I need to reapply sunscreen even if it says water resistant? Yes, water resistance ratings apply for a limited, tested duration, typically 40 to 80 minutes, reapplication is still necessary for extended sun exposure.
This article is for general skin education and reflects published research at the time of writing.