UV light, sunblocks and SPF

UV (ultraviolet) light is a kind of non-ionising radiation emanating from the Sun. It has a genuine benefit: UV-B drives cutaneous synthesis of vitamin D, which matters a great deal to health. Set against that are hazards that are well characterised and worth stating plainly.

• Exposure to ultraviolet radiation makes a person more vulnerable to sunburn.
• Cumulative UV exposure is the principal modifiable risk factor for skin cancer.
• UV also raises the risk of eye disease, including cataract and pterygium, which is why UV-rated eyewear belongs alongside sunscreen.

It is indispensable that sunscreens are used by people for whom exposure to the Sun can't be avoided.

"Sunblock" versus sunscreen, and why the word matters

Products offering protection against UV rays were long sold under two names: sunblocks and sunscreens. That distinction no longer stands up, and in one major market it is no longer permitted.

• The US FDA's 2011 sunscreen labelling rule prohibited the term "sunblock", along with "waterproof" and "sweatproof", on the grounds that all three overstate what a product does. No filter blocks ultraviolet light completely. Even a well-applied SPF 50 lets roughly 2% of UV-B through.
• The mechanism was misdescribed too. Zinc oxide and titanium dioxide were said to physically stop UV at the skin surface. In cosmetic grades they are milled fine enough that absorption dominates; scattering and reflection contribute a minority of the attenuation. Inorganic filters and organic filters are doing more similar work than the old labels suggested. See Sunbest-ZnO and Sunbest-Ti+.
• What does still differ is cosmetic behaviour and tolerance. Inorganic filters are visible on skin and can leave a cast, but have a very low sensitisation rate, which is why they remain the usual recommendation for infants and reactive skin.

SPF (sun protection factor)

SPF is a ratio of doses, not a measure of time. It is the UV dose required to produce a just-perceptible redness (the minimal erythemal dose) on protected skin, divided by the dose required to produce the same redness on unprotected skin. The test is run at a standardised application density of 2 mg/cm².

• The common misreading is that SPF 15 buys fifteen times longer in the sun. Under laboratory conditions the arithmetic is roughly that, but two things break it in practice. Most people apply between a quarter and half of 2 mg/cm², which cuts delivered protection sharply; and sweat, water, towelling and rubbing all remove film. Reapply about every two hours, and after swimming, whatever the number says.
• Protection does not scale linearly with the number. SPF 15 absorbs about 93% of UV-B, SPF 30 about 97%, and SPF 50 about 98%. Doubling the SPF does not halve the exposure, and the step from 30 to 50 is much smaller than it looks.
• Delivered protection also depends on skin type, how much product went on, and conditions: altitude, latitude, surface reflectance from water, sand or snow, and the sun's angle. Mid-afternoon UV at sea level is far stronger than early-morning UV.
• SPF only describes UV-B. A high SPF says nothing about UV-A. For that you need a broad-spectrum claim; in the EU, UV-A protection must be at least one third of the labelled SPF, shown by the UVA-in-a-circle mark.

For a formulator, those last two points set the brief: choose filters that cover both bands and hold up under irradiation. Photostable triazines such as Sunbest-BMT and Sunbest-HEB exist for exactly that reason.