How does this stuff called “cinnamate” that they put in sunscreen block UV rays? Also, sunscreens used to go up only to 15 SPF, but now they go up to 70 SPF! How can they make sunscreens that powerful?
The chemical absorbs the UV light. To prevent the same chemical from re-emitting the UV light, they chose a chemical that uses the absorbed UV light to break the chemical apart, thus using the absorded energy so it can’t be re-emitted as UV light.
And yes, as the chemical breaks down from the UV light, it’s protection weakens. That’s why it’s suggeste you re-apply every few hours.
SPF is measured by how much the sunscreen increases the minimum erythymal dose (MED): that is, the point at which the skin just turns red. This dose is different for everyone, so several subjects are used to arrive at an average SPF. The MED is determined for a limited spectrum, so the actual protection from natural sunlight may be different.
Sunscreen chemicals block a spectrum of UV radiation, but are especially effective at a peak wavelength. 2-ethylhexylmethoxycinnamate is only one of many sunscreen chemicals; by adding several to the sunscreen formulation, the spectra overlap and provide more effective protection. Merely adding more sunscreen will also increase SPF, but not as much, and not as effectively in natural sunlight. The additive effect of sunscreens is greatest when a UV-A blocker is included. The best UV-A blocker in the currently available list is avobenzone, and I highly recommend AVOIDING high-SPF sunscreens that don’t contain it.