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I read on https://www.walmart.com/ip/Isdin-UV-Care-FotoUltra-Active-Unify-Fusion-Fluid-Color-SPF-50-50ml/304385886:

Isdin UV Care FotoUltra Active Unify Fusion Fluid Color SPF 50+ 50ml is a tinted sun fluid with depigmenting action with SPF 106 and UVA in-vitro 49, that clarifies and evens the complexion.

What does "UVA in-vitro 49" mean in a sunscreen's description? I'm guessing it's talking about the reduction of UVA radiation on the skin but I don't get the unit.

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@fygsin has this more or less correct, but to expand on the answer:

This is long so TL:DR - In vitro = in glass - lab based measurements, not performed using real sunlight. Higher number = better protection. Tests measure absorbance of UV by the chemicals, not reflection.

UV is classified into 3 types of rays based on wavelength. Each subtype has, in general different characteristics on how it behaves, with UV-A (longest wavelengths) being considered the less damaging of the 3 and UV-C being the most damaging (shortest wavelengths). If you have ever seen a movie where they show a lab with blue lights glowing - those are the UV-C lamps used as sterilizing/germicidal lights in bio-containment. Basically you can think of it this way - shorter wavelength, more damage.

However, despite the damaging nature of UV-C, very little of it reaches us on the surface, almost all of it is absorbed by the atmosphere. On the other hand, almost all of UV-A reaches the surface, very little is absorbed by the atmosphere, but largely doesn't contribute to the reddening and burn of sunburn (it does to aging and invisible damage though) . UV-B is absorbed partially and primarily causes the reddening and pain you associate with sunburn.

Sunburn is actually really cool (figuratively) - it's your bodies response to radiation burns - actual physical damage to the DNA causes the living layer of skin cells to spontaneously kill themselves through a process known as "Apoptosis". As a result of this your body activates the immune system to clean up the debris from all those dead cells - this results in the swelling, redness and pain.

Sunscreens are chemical compounds that are applied to the dead keratinized layer of skin on the outside. There the chemicals absorb into the dead layer and absorb the energy of the sun's rays.

Sunscreens typically target UV-B and generally do a pretty good job of blocking most UV-B wavelengths. If you have a sunscreen that states "Broad Spectrum" or something similar, it blocks both UV-B (decent blocking) and UV-A (generally partial block). The higher the number the greater the blocking ability.

However, you should note that numbers higher than about 50-70 are more or less meaningless as there's little evidence that higher amounts provide more meaningful protection. You should also note that a number like "106" is meaningless - the measurements are not precise enough to measure in units less than about 10. Yes, you can get measurements that will give you an average value of 106 (or any other number), but the precision is lacking to make it meaningful. I would be wary of any product that uses such numbers in advertising - it shows a fundamental lack of knowledge (at least by the marketing division) of how these things work and possibly a lack of actual science to back up their sun protection claims!

To further complicate things the tests for UV-A are not well defined and there isn't a general consensus on how to measure the protection/absorbance associated with UV-A, so different companies come up with different tests to measure the protection. Whether or not they are using the ISO standard linked in @fygsin's answer is another question.

Often these tests are in vitro (translates to "in glass") and means that they are performing experiments in the lab using a laboratory measure based on some absorbance factor. This might be a transmission/absorbance measurement (i.e. seeing how much comes through a defined thickness (pathlength) at a certain concentration, or it might be performed on cells grown in the laboratory - though this is sometimes is referred to as ex vivo (from life), and may be measured by activation of certain cellular pathways or any number of potential read-outs. Experiments performed on animals or human subjects are known as in vivo (in life).

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In vitro refers to tests being done "in the glass", i.e. in petri dishes or test tubes under laboratory conditions. After some googling I come to the conclusion that it very likely refers to the way the specific protection of this sunscreen, i.e. the spectral absorbance curve, is evaluated in laboratory conditions.

There is apparently an ISO standard which, well, standarises this process: https://www.iso.org/standard/46522.html

This would thus mean that in laboratory conditions the sun screen you linked had a UVA Sun Protection Factor of 49.

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