The Science of 'Persistent Pigment Darkening' (PPD)
The UVA effect behind Asia's PA rating system.
About 95% of UV radiation reaching the ground is UVA; only ~5% is UVB.
Dermatologists summarize it simply: UVB causes the burn, UVA drives the aging.
Frequently asked questions
What does the UV index mean and when should I protect my skin?
The UV Index, from the EPA and WHO, rates the strength of midday UV on a scale from 1–2 (low) to 11+ (extreme). Dermatologists generally advise sun protection whenever it reaches 3 or higher. It appears in most weather apps and rises with sun elevation, altitude and proximity to the equator.
Is visible light bad for skin?
Visible light makes up about 45% of sunlight. Research shows high-energy visible (blue) light can worsen pigmentation and melasma, especially in medium-to-deep skin tones. Tinted sunscreens containing iron oxides help shield against it, which clear sunscreens generally do not.
How does sunscreen actually work?
Sunscreens use UV filters in two groups. Organic ('chemical') filters such as avobenzone absorb UV and release it as a small amount of heat. Mineral ('physical') filters — zinc oxide and titanium dioxide — mainly absorb UV and also scatter some. Both cut how much UV reaches living skin cells.
r/SkincareAddiction: 'Do I really need SPF if I'm inside all day near a window?'
Sources & citations
- EPA, 'A Guide to the UV Index' (U.S. Environmental Protection Agency, 1994/updated)
- epa.gov ↗