Do You Need To Wear Sunscreen Indoors?

What better question to answer now that we are locked indoors for the foreseeable future. With work/home life changing forever and with it the amount of time we spend travelling and generally being outside the question I’ve been asked many times over is ‘do I still have to wear sunscreen?’ I get it, even I don’t wear make-up anymore, so if something feels unnecessary why bother? But is it actually unesscessary?

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The answer? Well, kinda like everything – it depends. And what it depends on is down to what your ‘indoors’ is. Are you working on your laptop all day by the window? Are you living in your parents basement (no judgement, whatsoever). Or are you flitting between two situations on either end of the windowsill – basement spectrum? Do you live in a hot climate or do you live in Russia? The first step in understanding what difference these variables make is to look at how exactly you can be exposed to sunlight.

Direct and diffuse exposure

There are two ways you can be exposed to sunlight – direct and diffused exposure. Direct exposure is when the sun is shining on you. In full sun you’re getting direct exposure, in the shade you have no direct exposure. Diffuse exposure is when the sun’s UV is bounced onto you – mostly by air molecules high up in the sky, but also by objects around you. Shorter wavelengths get diffused more, so there’s more diffuse UV around than diffuse visible light.

Obviously there’s more UV (both direct and diffuse) outside; when you’re indoors, there are things between you and the sun and sky. In the shade, solid walls and roofs block out direct UV, but you can still get diffuse UV.

What this tells us it that it is very easy to tell whether or not you’re affected by direct sunlight depending on whether or not you are in the shade but it’s much harder to tell whether you are affected by diffuse UV. So, how do we figure this out?

Sky view

To estimate how much diffuse UV you’re getting in any particular location we can use a concept called sky view. Sky view suggests that the amount of diffuse UV you’re getting is proportional to your sky view.

For example, if you are standing outside with nothing obstructing your view of the sun i.e in the middle of a field, the sky view is 100%. Once you start adding objects; tree’s, brick wall, windows (beginning to describe a house), cars etc your sky view percentage decreases.

But it’s still pretty complicated to work out sky view. Let’s say you’re sitting on your windowsill and outside there is only the sky. No tree’s obstructing the window. Your sky view is likely to be 50%. What if you have the same set up but are slightly further away from your window? Well, then your sky view decreases exponentially. And what if your window is bigger in one room than another? Then your sky view percentage changes again. Not shockingly there is a complex mathematical way to work this out, which for my sake I am not going to go into. Sky view, although helpful in guesstimating UV exposure, can be a pretty inefficient way to decipher whether you need sunscreen or not.

Thankfully, we also have to consider the fact that UV levels depend on where you live, the time of day and the time of year. Because of this there is an easier way to figure out whether or not you should use sunscreen everyday and for it, you need to know what the UV Index is.

The UV Index

The UV Index is a standard international measure of ultraviolet radiation emitted by the Sun. It tells you how much erythemal or sunburn-causing UV you’re getting at a particular time. The UV Index is the best way to work out the UV in your area, and you can generally look this up online.

You can use the UV Index to approximate UVA levels as well. This is particular useful for when you are inside because UVA is the one that can penetrate glass. Most UVB is blocked by glass but only about a quarter of UVA is.

So should I wear sunscreen?

The truth is that not everyone needs to wear sunscreen everyday and in all honesty there are a few benefits to UV exposure (shock horror, I know). Our body actually produces vitamin D after UVB exposure and nitric oxide after UVA exposure. Although, I still recommend using sunscreen dilgently, especially when it’s a sunny day.

However, there are variables (like in all good scientific research) to consider. Lets take for example the fact that most research on this subject is done on lighter skin tones. Hence, it’s hard to tell how much of a difference melanin (skin pigment, that acts as a natural sunscreen, albeit not a very strong one) has on incidental exposure, but my guess it that it has some benefit.

Your skincare products are also another variable. You might be using skincare that increases sun sensitivity. AHA exfoliants, for example, will increase how susceptible your skin is to UV damage.

Or you might just have a photosensitive disease or a family history of skin cancer that means you should be more careful about UV exposure.

So, the actual answer is dependant on the variables discussed in this article. If you have a family history of skin cancer and sit by the window to work on your laptop in the sunshine everyday – wear sunscreen. If it’s been raining all day, clouds in the sky and you are staying away from the window anyway, you’re probably fine for the day.

Sky view can give you an estimation as to how affected you are by UV at any given moment, whereas the UV Index can give you more of a quantifiable idea of your UV exposure.

Combining these two metrics with your personal data will give you a good idea as to whether it’s a day for sunscreen or not. And if this is all too confusing for you a solid rule to live by is if the UV index reaches a maximum of 3 or above you should wear sunscreen.

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