Sunscreen 101: A Dermatologist's 3 Tips for Real UV Protection

This is an English adaptation of a clinical article Dr. SangYoul Yun — board-certified dermatologist and Medical Director of Delight Dermatology in Gangnam, Seoul — originally published in Korean. Read the Korean original on Naver. It has been restructured and translated for international readers.

Late summer and early autumn are when your skin sees the most ultraviolet (UV) light of the year — and UV is the single largest cause of skin aging, on top of triggering a range of skin problems. After ten years in dermatology, the questions I hear most about sunscreen are always the same three: which type should I use, how much should I apply, and what do the numbers mean? Here is how I answer them.

First, a quick word on the UV that reaches you

There are three types of ultraviolet light — UVA, UVB and UVC.

• UVC is absorbed by the ozone layer and does not reach the ground.
• UVB is largely absorbed by the ozone layer, with only part reaching the surface. It is mostly blocked by glass, but it carries strong energy — long exposure can cause sunburn, skin cancer, photoaging, pigmentation and cataracts.
• UVA has a longer wavelength, so most of it reaches the ground and it passes straight through window glass. Over time it drives wrinkles and photoaging.

Because both UVA and UVB matter, you want a sunscreen that handles both — which is exactly what the numbers on the tube are telling you (more on that below).

Tip 1 — Choose the right type for your skin (mineral vs chemical)

Sunscreens split into two broad families: those that leave a white cast and those that don't — in other words, mineral (physical) and chemical filters.

So who should use which?

• Most people reach for a chemical sunscreen, simply because it spreads more comfortably.
• Sensitive skin may do better with a mineral sunscreen, because chemical filters can occasionally provoke an allergic reaction.
• Skin prone to folliculitis around the face is often better with a chemical formula — a mineral sunscreen can block pores and aggravate the inflammation.

Tip 2 — Use enough, and reapply

The most common mistake is using too little. The face averages roughly 400 cm², and the recommended amount is about 2 mg/cm² — which works out to around 800 mg per application. In practice that means squeezing sunscreen along your fingers and applying until your face looks noticeably white for a moment. A useful guide is the "two-finger" method: a strip laid along your index and middle fingers.

Applying that much in one go is honestly difficult, so when you're outdoors, plan to reapply once every 2–3 hours.

Tip 3 — Read the SPF and PA numbers correctly

SPF tells you how well a product blocks UVB. The jump from SPF 30 to SPF 50 sounds like double the protection, but in practice SPF 30 blocks about 97% of UVB and SPF 50 about 98% — a small real-world gap. A higher number is still a little better, all else being equal.

PA indicates how much UVA a product blocks. For everyday use, PA3+ (PA+++) is generally enough. A sunscreen that carries both a solid SPF and a PA rating gives you close to maximal everyday protection.

Since UV is the biggest driver of skin aging — and a major contributor to melasma and other pigmentation — diligent daily sun protection is one of the highest-value habits for keeping skin younger for longer. Treating existing pigment is far harder than preventing it, so consistent sunscreen belongs at the core of any skin-care routine.

Medical disclaimer. This article is general information and does not replace individual consultation. The best sunscreen type depends on your skin, and any persistent pigmentation, sunburn reaction or skin lesion should be assessed by a dermatologist in person.