The two kinds of UV light: UVA and UVB
The UV light that reaches the ground splits into two types by wavelength. They leave different marks on skin, which is why each gets its own number.
- UVB (290 to 320 nm) works on the outer layer of skin. It is the reason you burn, and it is linked to skin cancer. It peaks around midday and in summer, then fades in the morning, evening and winter.
- UVA (320 to 400 nm) reaches deeper into the skin. It drives photoaging, the wrinkles, dark spots and loss of firmness that sun leaves behind, and it is also linked to skin cancer. It stays fairly steady all year and passes through window glass.
SPF tells you how well a product blocks UVB, and PA tells you the same for UVA. Pick by one number alone and you are only half covered, so always read both.
What SPF means: the number for burning rays (UVB)
What SPF measures
SPF (Sun Protection Factor) is the UV dose that starts to redden skin with sunscreen on, divided by the dose that does it with no sunscreen. That reddening dose is called the minimal erythema dose (MED). In Korea it is measured on real skin under the MFDS standards for UV protection (in vivo, ISO 24444).
Put simply, SPF 30 means skin takes 30 times as long to redden with the product on.
How it is tested may change soon. In August 2026 the MFDS put out a proposed amendment that would also accept lab-based (in vitro) test data as grounds for SPF and PA values. As of this October 2026 update, the regulation in force (amended December 2025) does not include it yet.
SPF 30 and 50 are closer than they look
In theory SPF 30 blocks about 96.7% of UVB and SPF 50 about 98%, a gap of only about 1.3 percentage points. People still reach for 50 because almost everyone applies less than the tested amount, and a higher number helps make up for the shortfall.
Korea's labeling rules
Under MFDS rules, SPF 50+ is the highest grade a Korean label can show. Anything that tests above 50 is printed as "SPF 50+" no matter how high it went. That means you cannot compare Korean products past 50+ by the number alone. The U.S. FDA has proposed a 60+ cap but has not finalized it, and says it will set the maximum SPF in a future order.
What PA means: the grade for tanning and aging rays (UVA)
What PA measures
PA (Protection grade of UVA) started in Japan and is used in Korea and parts of Asia. UVA does not turn skin red right away. It slowly darkens it instead, and PA is based on how much UVA it takes to cause that lasting tan, called persistent pigment darkening (PPD).
More plus signs, more UVA protection
- PA+: low
- PA++: moderate
- PA+++: high
- PA++++: very high
Each extra plus sign doubles the threshold. The scale used to stop at PA+++ until the MFDS added PA++++ in 2016 and made it four grades.
Europe uses a UVA seal instead of PA, the letters UVA inside a circle. That mark means the product blocks UVA at a level of at least one third of its SPF.
Choosing the right grade for each situation
The MFDS suggests SPF 15 to 30 with PA+ or PA++ if you are mostly indoors or only out briefly. For long hours of strong sun like hiking or a day at the beach, it suggests SPF 50+ with PA+++ or higher. The American Academy of Dermatology (AAD) recommends SPF 30 or higher for everyday use. Put the two together and this works well.
- Commuting and mostly indoors: SPF 30 or higher, PA++ or higher
- An hour or two outside: SPF 50+, PA+++ or higher
- Long hours of strong sun at the beach, hiking or skiing: SPF 50+, PA++++ and a water-resistant label
- Cloudy days: the usual SPF 30 or higher, PA++ or higher. UVA stays fairly strong even under clouds.
How much to apply and when to reapply
The labeled SPF assumes a generous layer
SPF is measured with 2 mg of sunscreen per square centimeter of skin. For the whole face that comes to about 1.2 to 1.5 g, usually described as a coin-sized blob or two finger-lengths of product. It is more than most people expect.
Plenty of research finds that people apply only 25 to 50% of the recommended amount. That is why the labeled protection often does not come through in full. Using enough product matters more than chasing a higher number.
When to reapply
- Day to day, reapply every two hours as a baseline. Do it right away after sweating, getting wet or towel drying.
- During outdoor activity, stick to the two-hour rule. Both the U.S. FDA and the AAD say to reapply at least every two hours.
- After swimming, reapply once the time on the label runs out, even with a water-resistant product. The FDA does not allow "waterproof" on labels. Products say "water resistant" for 40 or 80 minutes instead.
For the first coat, put it on 15 minutes before you head out. That is also what the FDA advises.
Chemical filters and mineral filters
Sunscreens block UV in two ways.
- Chemical filters: ingredients such as avobenzone and octocrylene absorb UV and turn it into heat. They feel light and clear, though sensitive skin should check for irritation.
- Mineral filters: zinc oxide and titanium dioxide sit on the skin and reflect and scatter UV. They are known to be gentler but can leave a white cast or feel heavy.
Mineral filters are the usual pick during pregnancy for a reason. In its 2021 proposed order, the U.S. FDA proposed zinc oxide and titanium dioxide as generally recognized as safe and effective (GRASE). It asked for more data on some chemical filters such as oxybenzone, which is not the same as finding them harmful.
In practice, few products use only one type. Of the 259 sun care products we have collected, only 36 use mineral filters alone, and the largest group mixes both. If you picked a sunscreen because it says mineral, check the ingredient list for any filter besides zinc oxide and titanium dioxide.
