Sunscreen decoded: SPF, PA and broad spectrum
What SPF numbers, PA plus signs and broad spectrum really mean, mineral vs chemical filters, how much to use and how often to reapply.
If you only use one skincare product, dermatologists generally agree it should be sunscreen. Ultraviolet (UV) light causes sunburn, contributes to skin cancer and drives most visible skin aging, including dark spots and loss of firmness. It also makes dark marks from acne last longer.
UVA vs UVB
| UVB | UVA | |
|---|---|---|
| Main effect | Sunburn | Tanning, deeper damage, aging |
| Strength through the day | Peaks around midday, varies by season | Fairly steady all day and all year |
| Passes through window glass | Mostly blocked | Largely passes through |
| Measured by | SPF | PA rating, PPD, broad spectrum test, UVA circle (EU) |
A memory trick: B for burning, A for aging. Both contribute to skin cancer.
What SPF means
SPF (sun protection factor) measures protection against UVB. It is tested in a lab with a thick, even layer of sunscreen. Roughly:
- SPF 15 filters about 93% of UVB
- SPF 30 filters about 97%
- SPF 50 filters about 98%
The jump from 30 to 50 looks small, but it means roughly a third less UVB reaches the skin. More important than the number is using enough and reapplying. The American Academy of Dermatology recommends a broad-spectrum, water-resistant sunscreen of SPF 30 or higher.
Broad spectrum
In the US, "broad spectrum" on a label means the product passed the FDA test showing UVA protection that rises in proportion to its UVB protection. A sunscreen without that phrase may only protect well against UVB.
The PA rating
PA ratings come from Japan and are common on Asian and Korean sunscreens. They are based on PPD (persistent pigment darkening), a measure of UVA protection:
| Rating | UVA protection | PPD range |
|---|---|---|
| PA+ | Some | 2 to under 4 |
| PA++ | Moderate | 4 to under 8 |
| PA+++ | High | 8 to under 16 |
| PA++++ | Extremely high | 16 or more |
European sunscreens may show "UVA" in a circle, which means UVA protection is at least one third of the labeled SPF.
Mineral vs chemical filters
- Mineral (physical): zinc oxide and titanium dioxide. Gentle for many sensitive skins and usually fine around the eyes. Can leave a white cast, especially on deeper skin tones, although tinted versions help.
- Chemical (organic): filters that absorb UV, such as avobenzone, octocrylene and newer filters approved in some countries. Usually lighter and invisible on all skin tones.
Both work when used properly. Choose the one you will actually wear every day.
How much and how often
- Face and neck: about a quarter teaspoon is a common guide, or two finger lengths of lotion for the face. Most people apply half that.
- Whole body: about one ounce, enough to fill a shot glass.
- Apply about 15 minutes before going outside.
- Reapply every two hours outdoors, and right after swimming, heavy sweating or towel drying. "Water resistant" labels in the US last 40 or 80 minutes.
Over makeup, a sunscreen mist or SPF powder can top up protection, but they are hard to apply thickly enough, so treat them as a bonus, not your main layer.
Daily habits beyond the bottle
Seek shade around midday, wear a wide-brimmed hat and UV-blocking sunglasses, and be extra careful when using retinoids or AHAs, which can make skin more sun-sensitive. Remember lips (SPF lip balm), ears, the back of the neck and the tops of feet.
General information only. Check any new, changing or bleeding mole or spot with a dermatologist.
