Instead of talking about sunscreen like I usually do, we’re going to dive into some of the things you can use to protect your skin apart from sunscreen (I can feel my sunscreen propaganda shillbucks slipping away right now).
As much as we all love sunscreens, they are a bit of a pain in the butt. You have to apply them right, you have to apply enough, they can sometimes give you clogged pores and rashes, they rub off, they can be uncomfortable…
So it seems like there has to be a better option, right?
Well, sometimes there is!
Today’s topic is hats, umbrellas and shade in general: how much protection they give, what to look for, and whether you can use them instead of sunscreen.
The video is here on YouTube, keep scrolling for the text version…
Swiss cheese and sun protection
I think most of us have seen this Swiss cheese diagram, with respect to protecting ourselves from COVID-19:
This is a model used in risk analysis. The idea is that although each layer of protection has shortcomings, enough layers of imperfect protection will give you pretty decent protection overall.
And this is how we should think about sun protection too – sunscreen is an important layer, but it’s not the only layer.
In Australia, we have the slogan Slip Slop Slap Seek Slide: Slip on a shirt, slop on sunscreen, slap on a hat, seek shade, and slide on sunglasses.
But how well do each of these work? Why do we still get burnt in the shade? Can you just wear a hat instead of sunscreen?
Let’s start by talking about where UV comes from. It’s the sun, obviously! But there are two types of UV exposure, which you might remember from my post about whether or not you need to wear sunscreen indoors.
Direct UV exposure is when the sun is directly shining on you. When you’re in full sun you’re getting direct exposure, but when you’re in the shade you have no direct exposure.
The other type is indirect or diffuse UV exposure. Basically, the sun’s UV bounces off something else onto you. The “something else” is mostly air molecules up in the sky, but UV can also be reflected by other objects onto you.
Diffusion also happens with visible light, and it’s why you can still see things that are in the shade. Shorter wavelengths get diffused more, so there’s more diffuse UV around than diffuse visible light, and more diffuse UVB than UVA.
If you’re standing in the middle of a field, you’re getting 60% diffuse UV and 40% direct UV as a very rough estimate.
The actual percentages change with lots of factors such as how high the sun is in the sky, the wavelength of UV you’re talking about, and where you are in the world.
The key takeaway is that UV shining directly on you is often less than half of the UV you could be getting.
How much UV?
Working out how much direct UV you’re getting is straightforward: if there’s sun shining directly on you, you’re getting that 40% direct UV exposure.
Working out diffuse UV is trickier – we need to think about what’s bouncing UV onto us.
Diffuse UV and Sky View
If you’re outside, the main thing diffusing UV onto you is probably the sky. We can use something called Sky View to estimate how much UV is bounced off the sky.
If you’re standing in the middle of a field, you’re exposed to the inside of a continuous dome of sky. That’s 100% sky view, which translates to 100% exposure to diffuse UV.
The amount of diffuse UV you get is proportional to the amount of sky you can see. So if there’s stuff blocking your view of the sky, then you get less sky view and lower diffuse UV.
If 50% of the sky is blocked out, then you end up with around half the diffuse UV:
Let’s combine this with our estimates of direct UV.
If you’re in direct sun and half the sky is blocked by, say, a barn, then you’re getting around 40% direct UV plus half of 60% diffuse UV (30%). 40 + 30 = 70%, so you’re getting 70% of the UV you’d get standing in the middle of a field.
Let’s say the sun moves behind the barn, so you’re now fully in the shade but half the sky is still blocked. Then you’re getting none of the 40% direct UV, but you’re still getting half of the 60% diffuse UV. This leaves you with somewhere around 30% of the UV compared to being in the middle of a field with 100% sky view.
When the ground betrays you
In some situations, the ground can also reflect a significant amount of UV. Here’s a table of the UV reflected by some different surfaces (extracted from here):
Most surfaces absorb most of the incoming UV and won’t reflect much. Grass, dirt, asphalt (bitumen), wooden boards, most types of concrete reflect less than 10% of UV, so the ground ends up contributing less than 5% of your total UV exposure.
However, a few types of surfaces can reflect more. In particular, I’ve been told some countries apparently have tiny chunks of ice that fall from the sky sometimes (they call it “snow”), and this can reflect up to 90% of the incoming UV. This can mean you get up to the same amount of UV from the ground as from the sun and the sky combined.
Choppy surf and metal surfaces can reflect 25%, while some types of sand can reflect up to 40% (although most sand has low reflection).
So depending on the UV index that day, the contribution of diffuse UV from the ground when you’re standing in the shade could still be quite high.
Burning in the shade
So the contributions from the sky and the ground is why it’s still possible to get burned when you’re entirely in the shade.
If you’re facing the sky, or a ground surface that’s bouncing direct UV towards you, you can still be getting a lot of UV – especially those short wavelengths of UVB that get scattered really well and cause burning.
And it’s a bit deceiving, because when you’re in the shade you’ll feel cooler, even if you’re getting a lot of UV.
UV doesn’t produce a lot of perceptible heat. It’s mostly visible light and infrared from the sun that make you hot, and they account for over 90% of the energy from the sun.
Visible and infrared rays also don’t bounce around as much as UV.
So if you face away from the sky while you’re in the shade, you’ll cut down the UV exposure on your face by a lot – even though you’ll probably feel pretty similar temperature-wise.
So with all this background, we can work out how much UV protection hats provide.
Hats are usually made with thick material that blocks out almost all the direct UV. For example, an Akubra hat will only let in 0.05% of direct UV (1/2000th of the incoming UV).
Of course, if you have a hat made out of thinner material like fabric, then it’ll be different. The factors discussed in my post on UV protective clothing apply here as well.
Related post: How to Choose UV Protective Clothing
For a hat that blocks out more UV, your best bet is:
- synthetic fabric
- tightly woven with as few and small holes as possible
- thicker fabric / multiple layers
- intense colours (dyes can absorb UV)
A UPF rating will mean you don’t need to guess.
Size and shape
Unsurprisingly, the bigger the hat, the more sky it’s going to block out and the more diffuse UV you’ll be protected against.
The angle of the hat brim will also make a difference – the same size hat with the brim turned down blocks out more sky and more diffuse UV.
Different parts of the face will also get different protection.
Again, we can think about sky view. For a baseball cap:
- A tiny person standing in the middle of your face won’t see as much sky and will get less UV.
- A tiny person standing on your cheek sees almost all the sky and gets all the diffuse UV (plus they’ll get direct UV some of the time).
But even with a broad brimmed hat there can be a big difference.
The further up towards the hat, the higher the protection since the tiny person sees less sky. This means the forehead and nose get more protection, while cheeks and chin get less.
Hats also block more UV for parts of your face that are angled upwards, compared to parts angled sideways – a little dude lying on your nose is going to see less sky than a little dude leaning against your cheek.
Hats obviously can’t protect you from any UV bouncing from below, which could be a significant problem if you’re on a very reflective surface like snow (with common hat designs, anyway).
Overall UV reduction in practice
As an illustration, one study estimated the annual UV reduction when an outdoor farm worker in Queensland wore a broad brimmed Akubra Drover with only 0.05% UV transmittance all year round.
Forehead UV exposure was reduced to about one-seventh, while nose UV dropped to one-third. However, cheek exposure dropped by less than half.
The overall facial UV exposure reduced by an average of 2-6, but up to 100 times in some facial regions.
Size and distance
We can think about handheld umbrellas much like we thought about hats.
Umbrellas are larger and further away from your head. The further away it is, the bigger it has to be to block out the same amount of sky.
If the umbrella doubles in distance away from, say, your nose, then the width (diameter) has to double to give the same protection.
One big difference between umbrellas and hats is the thickness of the material. Umbrellas tend to be made of thinner material, so more direct UV can get through than with the average hat.
Unsurprisingly, given what we know about UPF clothing, darker-coloured umbrellas and UPF-rated umbrellas block more UV.
Another difference between hats and umbrellas is that you’re usually off to the side with an umbrella.
Depending on the size of the umbrella, how far above your head it is and how you hold it, the umbrella can end up giving you worse protection on one side of your face than a hat would (but on the flip side, you won’t get hat hair).
Overall UV reduction in practice
Annoyingly, most studies on umbrellas are with beach umbrellas that are stuck into the ground and much bigger than a handheld umbrella.
One study estimated that beach umbrellas can give a protection factor of 3-17 depending on the size of the umbrella, where you are relative to the umbrella, where the sun is and so on.
Another study estimated that If you’re sitting in the shade of an umbrella in summer in the middle of the day with no other protection, you could still burn in 2 hours.
Hats and umbrellas are great for giving UV protection that you don’t need to reapply, and won’t wear off quickly.
They also protect areas where you might not be able to get to with sunscreen or clothing, like your scalp and the edge of your eyelids (although there are sunglasses, but that’s going to be a separate post).
They’re particularly good for protecting against direct UV, but that 60% diffuse UV from the sky is what gets you. They’re usually good at blocking the circle of sky right above your head, but UV is also coming in all around from the rest of the sky “dome”.
To protect against diffuse UV, you’re going to need “sideways” protection that gets between your skin and the sky.
Walls and trees can be helpful depending on location. For more portable protection, sunscreen and clothing will be more suitable.
If you’re in a high sun exposure situation, then remember the Swiss cheese layers and use multiple types of sun protection rather than just relying on one type. High sun exposure includes the obvious situations like going to the beach and exercising in the sun, but also when you’re at the snow since the ground reflects a lot of UV.
In lower UV exposure situations, like if you’re only going outside for a few minutes or in the middle of winter (assuming you’re not in this mythical snow), you can get away with less layers of Swiss cheese.
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