Since so many of us are staying indoors more at the moment, I’ve had a lot of people ask me whether they still need to wear sunscreen inside. And I personally haven’t been wearing sunscreen indoors, even as a sunscreen nerd who owns all of the sunscreens.

So do you need to wear sunscreen indoors? The truth is… it depends. “Indoors” spans a huge range of situations: from a windowless basement, to sitting under a whisper of netting in the summer sun. I’m going to break down the factors that affect whether or not you need to wear sunscreen indoors from a scientific perspective, so you can

Here’s the video on YouTube (I highly recommend watching the video since the topic’s quite complex, but also my editor did a great job on cutaways… and the sunscreen in my eyes stung for 10 minutes so it would be great to maximise the mileage of my pain). Scroll down for the blog post!

Why wear sunscreen?

First, let’s quickly run through why we wear sunscreen.

Sunscreen’s main job is to cut down how much UV reaches your skin.

Sunscreens have an SPF rating which mostly measures how much UVB the sunscreen protects you from. These shorter wavelengths generally cause you to burn (erythema), and they’re the wavelengths most strongly associated with skin cancer (basal and squamous cell carcinomas, and melanoma). They don’t go as deep into the skin, but they do still cause your skin to age faster.

Related post: What Does SPF Mean? The Science of Sunscreen

There’s also UVA, which are longer wavelengths of UV. These burn less, but they cause more prolonged tanning. They contribute to the development of melanoma, skin aging and hyperpigmentation.

The broad spectrum rating on a sunscreen, or the UVA rating (PPD, PA or Boots star rating) shows you how effective a sunscreen is at protecting against UVA.

Related post: Why you should protect your skin from UVA (and how) (with video)

One of the problems with this is that most of the studies on the risks of UV have been done on people with white skin – we’ll come back to this point later.

(Some people have also asked about the light from screens, especially blue light. Sunscreen will not protect you against blue light, not even mineral sunscreen filters (unless there are special additives, which isn’t the case for the vast majority of sunscreens). Your screens also don’t produce anywhere near enough blue light to make much of a difference – check out my posts on visible light protection and blue light damage from screens for more information.)

To work out if you need sunscreen, there are two factors that need to be considered: how much UV you’re exposed to, and how susceptible your skin is to UV damage.

How much UV is indoors?

There are two ways you can be exposed to UV:

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. (This 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.

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.

How much diffuse UV is indoors?

It’s easy to work out how much direct UV you’re getting based on whether you’re in the shade or not. But how can you estimate the amount of diffuse UV?

This is actually quite tricky. Lots of different factors that contribute to the ratio between direct and diffuse UV – wavelength of UV, height of the sun, altitude – there are some very detailed and dense papers on each of these factors.

I’m using 60% diffuse UV and 40% direct UV in the open as a rough estimate (based on a range of empirical studies).

To estimate how much diffuse UV you’re getting in any particular location, we can use a concept called sky view. The amount of diffuse UV you’re getting is proportional to your sky view – how much sky you’re exposed to (this is called the sky view factor model).

If you’re standing in the middle of a field, you’re exposed to the inside of a dome of sky. This is 100% sky view, which is 100% exposure to diffuse UV.

If there’s stuff blocking your view of the sky – trees, buildings, walls – then you get less sky view and lower diffuse UV.

Inside a typical room, your exposure to the sky will be through windows and doors. That’s why distance from doors and windows matters.

For example, in my room, I have a 1.6 x 1.2 m window. To work out my sky view through that window, there’s… complicated maths involved, so I used some rough approximations to estimate my sky view.¹

If you’re basically sitting in the window and there’s nothing outside your window but the sky, you’re getting 50% sky view, so you’re receiving roughly half of the diffuse component of UV.²

If you’re further back from the window, then the portion of your dome that’s sky is smaller – this decreases exponentially as you move away from the window.

For example, I sit around 3 metres from the window in my room. This means I get no direct UV, and up to about 3% of the diffuse UV from the sky through the window.

But I also have stuff (trees, houses, blinds) blocking out parts of the sky:

So only about 11% of the sky in the window is actually visible (as a generous overestimate; the UV reflected by the objects outside the window is usually less than 10%, so the overestimate covers it).

That gives me 0.33% sky view – which means I’m getting one 300th of the diffuse UV compared to being in the open, and one 500th of the total UV compared to being in the open.³

Even if I was only one metre away from the window that’s less than 2% sky view – with no direct UV, I’m getting about 1% of the amount of UV compared to being in the open.

¹ Unfortunately my friends with advanced maths degrees were too busy to help me out with this (one was working on graduating and the other was doing firefighter training, so it was hard to argue). Since the window is around head height, I figured it was acceptable to approximate the dome portion using the chord lengths as the sides of a rectangle on the dome. The further away from the window, the more accurate the approximation is, but this whole exercise is rough anyway so simply not being orders of magnitude out is OK… I think?

² This is an average, but since we really only care about facial skin, your orientation to the window makes a massive difference. Different regions of your face (ear, nose) have different exposure to the sky through your window, and it’s pretty much impossible to take into account. Again, another rough estimation: if you’re facing away from the window you’re getting essentially none of the diffuse UV in the middle of your face, and if you’re facing directly towards the window you’re getting double the estimated sky view’s worth of diffuse UV.

³ This can be loosely interpreted as having SPF/UVAPF 500, but again this is an average – see footnote 2.

But there’s usually also another thing between us and the sun when we’re indoors…

UV through glass

If your windows are closed, that means there’s also glass between you and the sun. Normal window glass blocks almost all UVB from getting through, but blocks only about a quarter to a third of UVA.

So in my particular situation, I’m going to get none of the UVB, and only 1/670th of the UVA compared to sitting outside in the open (assuming 1/4 of UVA is blocked).

But the amount of UV you get sitting out in the open also changes…

UV varies a lot

UV levels depend on where you live, the time of day and the time of year.

You might have heard of the UV Index, which 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 – for example, Sydney in the middle of summer has a peak of around 12, in the middle of winter it’s around 2:

You can use the UV Index to approximate UVA levels as well.

There’s a myth that UVA levels don’t change throughout the year, or throughout the day – they do, just not quite as much as UVB does. Just like UVB (blue), UVA still peaks around the middle of the day and during summer (red):

So once you roughly know how much UV you’re getting based on the UV index, sun vs shade, sky view and the presence of glass (phew!), you can decide whether or not it’s worth wearing sunscreen for you.

How susceptible are you to UV damage?

Sunscreen every day no matter what?

Not everyone needs to wear sunscreen every day. I know it’s a common saying in skincare circles that the sun is a deadly laser and you must slather up with sunscreen every day with no exceptions, but this doesn’t accurately reflect the latest scientific consensus in many countries.

There are benefits to UV exposure, which I talked about in my SPF myths video: our body produces vitamin D after UVB exposure, and nitric oxide in response to UVA exposure.

Related post: Top 7 Sunscreen Myths (Video)

So in many countries – including the UK and Australia, the skin cancer capital of the world – health authorities actually don’t recommend that you wear sunscreen every day even if you go outside.

The Australian guidelines (from the Australian and New Zealand Bone and Mineral Society, the Australasian College of Dermatologists, Cancer Council Australia, Endocrine Society of Australia and Osteoporosis Australia) base this on the UV Index:

• If the UV index reaches a maximum of 3 or above you should wear sunscreen.
• If not, it’s recommended that you don’t wear sunscreen, and intentionally get some sun exposure on uncovered skin (yes, seriously).

Related post: Sun Protection and Vitamin D Deficiency

You don’t get much UVB through glass so the benefits of vitamin D won’t be there, but the point is this – it isn’t a blanket rule that everyone needs to wear sunscreen if they get any sun exposure.

Skin tone

There’s also a racial bias in sunscreen studies. Like in pretty much every other area of health, the vast majority of studies have been performed on white people.

Skin tone makes a massive difference when it comes to the risks of sun exposure. Different skin tones are aged differently by the sun, and the link between sun exposure and melanoma in dark skin is not well supported by the research.

This mostly comes down to melanin. Melanin is skin pigment, and it acts as a natural sunscreen – not really enough to for people with darker skin to confidently run around without sunscreen on the beach for hours, but certainly enough to make a big difference with incidental exposure. So the benefits of wearing sunscreen for cancer prevention and aging are going to be lower for people with darker skin.

On the flip side, UVA does tend to cause more uneven pigmentation issues in darker skin. This is more of an aesthetic thing than a health issue, so whether you need to protect your skin from the UVA you’re getting depends mostly on how much you care about pigmentation issues.

Related post: Video: My Routine for Fading Acne Marks (Post-Inflammatory Hyperpigmentation)

Skincare products

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

Related post: Do I need extra sunscreen when using chemical exfoliants?

Specific health conditions

You might also have a special condition that means you need to care more about UV exposure – a photosensitive disease or a family history of skin cancer will influence your choice.

Drawbacks of sunscreen

There are definitely some downsides to sunscreen that need to be considered when weighing up whether or not to wear sunscreen.

The obvious one is cost – you have to buy and use sunscreen. You also have to spend time and effort applying it. In a lot of people, sunscreens can cause clogged pores. There’s also problems like irritation, allergy, eye watering.

There are also some potential long-term health concerns about wearing sunscreen. These concerns are usually really small if you’re going outside – the benefits of wearing sunscreen will generally hugely outweigh the potential risks. But if you’re exposed to only a tiny amount of UV then these risks, combined with the other drawbacks of sunscreen, might tip the scales to not wearing sunscreen.

Related post: Sunscreens in your blood??! That FDA study

What’s the real impact of indoor UV exposure?

While all these factors are important in accurately considering the real impact of sunscreen, it’s all still unfortunately pretty vague and abstract to think about. But luckily there’s actually a really well-known example that illustrates the impact of some different types of indoor UV exposure: it’s this well-known photo of a truck driver who drove for 28 years with the windows closed.

Sun struck his left side (our right side) directly, but he only got diffuse UV exposure on the other side, through the side windows that filter UV similarly to window glass. There’s a huge difference between the two sides.

I think it goes to show that direct UV is probably worth worrying about, but whether you need to protect against diffuse UVA through glass really depends on your specific situation.

References

Wang SQ & Lim HW (eds), Principles and Practice of Photoprotection, 2016. DOI: 10.1007/978-3-319-29382-0 This book covers pretty much everything about sun protection

Diffey B, Sun Protection: A Risk Management Approach, 2017. Really fantastic comprehensive book about sun from one of the pioneers in the field (although it would’ve been nice to have more references)

Schippnick PF & Green AE, Analytical characterization of spectral actinic flux and spectral irradiance in the middle ultraviolet, Photochem Photobiol 1982, 35, 89-101. DOI: 10.1111/j.1751-1097.1982.tb03815.x WARNING: DENSE AND MATHSY

Utrillas MP et al., Ultraviolet radiation protection by a beach umbrella, Photochem Photobiol 2010, 86, 449-456. DOI: 10.1111/j.1751-1097.2009.00677.x Explains the sky view factor model

Almutawa F et al., Current status of photoprotection by window glass, automobile glass, window films, and sunglasses (open access), Photodermatol Photoimmunol Photomed 2013, 29, 65-72. DOI: 10.1111/phpp.12022

Vashi NA et al., Aging differences in ethnic skin (open access), J Clin Aesthet Dermatol 2016, 9, 31-38.

Adamson AS, Sunscreen wouldn’t have saved Bob Marley from melanoma, and it won’t help other dark-skinned people, The Conversation, 14 May 2019.

Gordon JR & Brieva JC, Images in clinical medicine: unilateral dermatoheliosis, N Engl J Med 2012, 366, e25. DOI: 10.1056/NEJMicm1104059

Mac-Mary S et al. Assessment of cumulative exposure to UVA through the study of asymmetrical facial skin aging (open access), Clin Interv Aging 2010, 5, 277-284. DOI: 10.2147/CIA.S13044 More case studies on asymmetric indoor sun exposure