"The protection factor (SPF or UPF if you prefer) will be roughly proportional the fractional area blocked by the fibres themselves.".
As far as I understood from this, the fabric is like a grid, where you have fibers blocking UV rays and gaps where those UV rays can go through.
If the gaps are 5% of the whole area, it means that 95% of the skin is protected, so basically this means around SPF 20/30 because it blocks 95% of the UVs. Did I understand correctly?
So what I still have to get my head around is:
That 5% of unprotected skin (gaps) gets UV rays and should theoretically be damaged by them, so you should see a sort of grid on your skin (red skin where the gaps are because it's where the UV rays go through and hit your skin), but it doesn't seem to be the case, at least I've never seen it myself in the past (it's always red where you are not protected and pale where you have clothing on). How does it work?
Same thing applies for sunscreen, if it blocks 95% of the UV rays, it means that 5% of them still go through, so you still have skin that is not protected at all and should see red spots (erythema) where those photons keep hitting, but I've never seen them.
And this leads to another question, with SPF 50 it takes 50 times longer to get erythema. Yes, I understand that it's because you get less photons, but if the few photons that go through keep hitting the same spot shouldn't you get erythema at least on that spot?
With incomplete sunscreen application, as with leaks through fabrics, I think there will be SOME protection for all skin surfaces. In the case of clothes, any movements mean that each spot of skin is sometimes illuminated and sometimes not illuminated.
Could it also be that the photons can't go through directly and sort of lose power due to scattering while "entering" the gaps (if they are small enough)?
Also I think that if the sun is high in the sky and you are wearing a t-shirt, the "grid" created by the fabric is sort of at an angle and it means that the gaps are even smaller if not completely "closed". A little like when you look through a fence, if you are facing the fence at a 90-degree angle, you can see through it, but as the angle decreases the less you can see until all the gaps between the posts are sort of closed.
So you should basically lie down and face the sun for UV rays to go through easily, the only parts that this doesn't work for are the shoulders, because they face the sun.
So I guess that if a t-shirt is not the best at blocking UV rays at a 90-degree angle (worst-case scenario), it's probably quite good in a real scenario (apart from the shoulders where you could apply sunscreen before putting the t-shirt on).
This is just my assumption so I'd love to know your opinion Richard, does this make sense?
What you say all makes sense. The angular effect you mention is also an important factor leading to the much lower UVI when the sun is low in the sky. The UVI is measured on a horizontal surface, so when the sun is low in the sky, not only is there a longer absorbing atmospheric path, but also, the radiation is 'diluted' on that surface which in inclined at an angle to the direct rays. For example, when the sun's elevation is only 30 degrees, the direct component of sunlight arriving on a surface perpendicular to its rays is twice as large as that on a horizontal surface. It also follows that for surfaces facing the sun, the effective UVI can be considerably higher than as advertised (as I discussed in my 'twilight' posting). By the way, photons are infinitesimally small. They have no problem penetrating the tiniest of air gaps in fabric.
Understood! I wear long sleeve merino all year round when out running, locally and globally, and just recently started using sun defense stick on face (SPF 50). Have no idea what that means but feels good on my skin, and no one hassles me about applying sunscreen protection (greasy stuff)!
Good on you! The stick sounds good. When I play tennis my sweat causes the sunscreen on my face to get into my eyes. The SPF50 means that it reduces skin damage by a factor of 50 if applied at the correct thickness. Does it say in the package how thick the application should be?
At least I understood this!!
Well done. Success!!
Thanks Richard!! As usual, amazing post!
"The protection factor (SPF or UPF if you prefer) will be roughly proportional the fractional area blocked by the fibres themselves.".
As far as I understood from this, the fabric is like a grid, where you have fibers blocking UV rays and gaps where those UV rays can go through.
If the gaps are 5% of the whole area, it means that 95% of the skin is protected, so basically this means around SPF 20/30 because it blocks 95% of the UVs. Did I understand correctly?
So what I still have to get my head around is:
That 5% of unprotected skin (gaps) gets UV rays and should theoretically be damaged by them, so you should see a sort of grid on your skin (red skin where the gaps are because it's where the UV rays go through and hit your skin), but it doesn't seem to be the case, at least I've never seen it myself in the past (it's always red where you are not protected and pale where you have clothing on). How does it work?
Same thing applies for sunscreen, if it blocks 95% of the UV rays, it means that 5% of them still go through, so you still have skin that is not protected at all and should see red spots (erythema) where those photons keep hitting, but I've never seen them.
And this leads to another question, with SPF 50 it takes 50 times longer to get erythema. Yes, I understand that it's because you get less photons, but if the few photons that go through keep hitting the same spot shouldn't you get erythema at least on that spot?
Fabio
Hi Fabio
With incomplete sunscreen application, as with leaks through fabrics, I think there will be SOME protection for all skin surfaces. In the case of clothes, any movements mean that each spot of skin is sometimes illuminated and sometimes not illuminated.
Could it also be that the photons can't go through directly and sort of lose power due to scattering while "entering" the gaps (if they are small enough)?
Also I think that if the sun is high in the sky and you are wearing a t-shirt, the "grid" created by the fabric is sort of at an angle and it means that the gaps are even smaller if not completely "closed". A little like when you look through a fence, if you are facing the fence at a 90-degree angle, you can see through it, but as the angle decreases the less you can see until all the gaps between the posts are sort of closed.
So you should basically lie down and face the sun for UV rays to go through easily, the only parts that this doesn't work for are the shoulders, because they face the sun.
So I guess that if a t-shirt is not the best at blocking UV rays at a 90-degree angle (worst-case scenario), it's probably quite good in a real scenario (apart from the shoulders where you could apply sunscreen before putting the t-shirt on).
This is just my assumption so I'd love to know your opinion Richard, does this make sense?
What you say all makes sense. The angular effect you mention is also an important factor leading to the much lower UVI when the sun is low in the sky. The UVI is measured on a horizontal surface, so when the sun is low in the sky, not only is there a longer absorbing atmospheric path, but also, the radiation is 'diluted' on that surface which in inclined at an angle to the direct rays. For example, when the sun's elevation is only 30 degrees, the direct component of sunlight arriving on a surface perpendicular to its rays is twice as large as that on a horizontal surface. It also follows that for surfaces facing the sun, the effective UVI can be considerably higher than as advertised (as I discussed in my 'twilight' posting). By the way, photons are infinitesimally small. They have no problem penetrating the tiniest of air gaps in fabric.
Understood! I wear long sleeve merino all year round when out running, locally and globally, and just recently started using sun defense stick on face (SPF 50). Have no idea what that means but feels good on my skin, and no one hassles me about applying sunscreen protection (greasy stuff)!
Good on you! The stick sounds good. When I play tennis my sweat causes the sunscreen on my face to get into my eyes. The SPF50 means that it reduces skin damage by a factor of 50 if applied at the correct thickness. Does it say in the package how thick the application should be?
Probably more cosmetic but it's called an eight hour cream -SPF 50 High Protection. Thanks Richard