Stasya has put me on the spot.
Hi Colin, I have a question that’s been at the back my mind for some time now, and after having made my way back to your blog after some time and read your post on titanium dioxide, I figured I’d ask you. I’m sure you might agree with me that it is quite frustrating how as of date there is no universally-agreed method of categorising levels of protection against UVA radiation. I was wondering if you might be able to do a cross-comparison between the East-Asian PA (+ to +++) system based on PPD, and the Boots star rating system based on absorbance of UVA? For instance, what would PA++ be roughly equivalent to, using the Boots star rating system? Thank you for your help Stasya
Now there is a tough question!
I’ll answer the easy bits first. Yes it is a problem that there is no universally agreed method for assessing UVA protection. UVA is the high energy radiation that causes skin to age more quickly and to increase the risk of skin cancer. So it would be good if there was a way to accurately measure the effectiveness of UV protection.
This is where it gets difficult.
The Boots system was devised by Brian Diffy from Newcastle University, who certainly knows his stuff about sun screens. You can see him on YouTube here giving a very good five minute rundown of the subject. But as I am sure he would agree, there is a problem with his method. It works simply by measuring how much UVA is absorbed in the laboratory. This is better than nothing, but it isn’t actually measuring the biological effect. It is simply a physical test,
The East Asian PA system works by assessing the darkening of the skin following application of the sunscreen. So this is directly measuring the effect of the radiation on the skin. So on the face of it this is a better method.
So that sorts out which system is better. But the trouble is, it still probably isn’t all that good. What we are interested in is the long term effect of the UVA. Is the reaction measured in the East Asian PA system a good proxy for this? We don’t really know.
I don’t think it is going to be much help trying to work out a conversion factor between the two systems. They really are measuring different things.
I am far from convinced that the use of these numbers actually helps that much. I think that they can only ever be a rough guide. I suppose you might want to use a really high one if you are say travelling to a place with a lot of sun from somewhere with very little. But if you are looking for a product to use over a prolonged period of time it might well be better to choose one that is easier to apply. The number of applications you make is as big a factor as the UVA absorbance level. After all, if you apply it twice as often you’ll probably be roughly doubling the effectiveness.
But in any case we don’t even know very clearly just how much UVA we need to be blocking out. With UVB we can tell from rapid change in skin colour how well the sunscreen we are using is working. Given that UVA’s effects are a lot more long term it is much harder to tell what is going on.
So all in all, I’d go for a product that you like rather than one with a particularly high rating.