You already know about SPF, UVB, sunburn and its relationship with skin cancer… but what about UVA protection? Newer research has really highlighted the important of UVA protection, so this post will tell you all about what UVA is, what it does and how to protect your skin against it (i.e. lots of nerdy sunscreen talk).
The video version is here, scroll down for the article version…
What Is UVA?
As you no doubt know, the sun emits UV radiation, along with infrared radiation and visible light. I’ve talked about visible light and the damage it causes before.
UV can be divided into 3 types according to their energy, which is inversely proportional to wavelength (in other words, short wavelength = more energy):
UVC (wavelengths 100-280 nm)
This is the most harmful type of UV, with the highest energy and lowest wavelength. But luckily, it’s also the least penetrating! UVC is almost completely blocked by the atmosphere and the ozone layer, so we don’t need to worry too much about it. It’s also known as germicidal UV since it can be used to kill microbes in sterilising procedures.
UVB (wavelength 280-315/320 nm)
UVB is the type of UV that you’ve probably heard about the most. About 90% of UVB from the sun is absorbed by the ozone layer, which means it’s a bigger issue for Australians and other people located near the South Pole, since we’re near the Antarctic ozone hole and we have excellent sunny weather which means we’re more likely to be hanging around outside.
UVB causes sunburns and skin cancer, but it’s mostly blocked by regular window glass. It’s also the type of UV light that’s involved in the production of vitamin D in the skin. UVB is strongest in summer, and in the middle of the day.
Related post: Sun Protection and Vitamin D Deficiency
UVA (wavelength 315/320-400 nm)
UVA penetrates the atmosphere better than UVB and UVC, and makes up ~95% of the UV radiation reaching the Earth. It also penetrates deeper into your skin than UVB.
UVA forms highly reactive free radicals in the skin, which randomly react with whatever’s around – for example, your skin’s proteins, DNA and lipids. This damage is known as oxidative stress, and it leads to visible wrinkles and old-looking skin, as well as deadly melanomas It’s the type of UV light that’s used in tanning beds and black lights.
Like UVB, UVA levels vary depending on the time of day and the season. However, it varies a lot less, so it’s important to think about UVA protection at off-peak times as well.
(The exact line between UVA and UVB depends on the regulations used, but 315 nm is the newer standard.)
UVA is further subdivided into UVA1, which has longer wavelengths closer to the visible range (above 340 nm) and UVA2, with shorter wavelengths closer to the UVB range (below 340 nm).
The reason UVA is less famous than UVB is that the dangers of UVA weren’t well researched until relatively recently. The effects of UVA are also less immediate and build up over time, which makes them arguably more dangerous.
UVA’s penetrative power is also scary – it can penetrate through glass, like the glass in your car’s side windows, which is why there are lots of asymmetric skin cancer and wrinkle patterns emerging. UVA damage is responsible for the asymmetric wrinkling on this veteran truck driver:
|Gordon JR & Brieva JC, Unilateral dermatoheliosis, N Engl J Med 2012, 366, e25. DOI: 10.1056/NEJMicm1104059.
He drove a truck for 28 years with the windows closed, with the sun shining through the window and hitting his left side (our right when facing him). His right side received only indirect sunlight – the difference in skin aging is massive!
Even if you never go to the beach or lie in the sun, you’ll still be affected by UVA – it’s been estimated that 80% of UVA damage comes from everyday exposure, during activities like walking, driving and even sitting near windows. So how can we protect ourselves?
UV Protection in Sunscreens
Before we talk about UVA protection, let’s talk about SPF (Sun Protection Factor), the most well-known measure of a sunscreen’s protection.
Sunscreens are almost always labelled with an SPF value. It tells you the sunscreen’s ability to protect you from erythemal UV – the type of UV that causes redness and burning. The higher the SPF, the higher the protection against erythemal UV.
Erythemal UV is mostly UVB, but shorter wavelengths of UVA are also involved. It’s estimated that about 80-91% of erythemal UV is UVB, with the remaining 9-20% UVA. If a sunscreen only protects against UVB and offers no protection against UVA, the highest protection that can be achieved is only SPF 11!
SPF was introduced as a measurement of sunscreen protection during the 1970s, before all the UVA-is-dangerous research, when it was assumed that erythemal UV was the only type of UV to be concerned about.
The problem is that SPF doesn’t tell you much information about UVA protection, particularly the longer wavelengths of UVA that aren’t involved in burning. That’s where other regulated terms for UVA protection come in.
Related post: What Does SPF Mean? The Science of Sunscreen
In most countries the term “broad spectrum” is regulated to show how far the UV protection of a sunscreen extends into the UVA region.
There are a couple of different ways of measuring broad spectrum: the critical wavelength method and the UVAPF-to-SPF ratio.
Critical wavelength >370 nm
For the critical wavelength test, the UV absorbance of a sunscreen from 290 to 400 nm is measured, and the area under the curve is calculated. To be “broad spectrum”, at least 10% of the area under the curve must be at a wavelength of 370 nm or longer. The critical wavelength is the longest wavelength at which the 10% mark is met.
UVAPF-to-SPF Ratio > 1:3
The second criteria is the UVAPF-to-SPF ratio. To be broad spectrum under this criteria, the UVA protection must be at least 1/3 of the labelled SPF value. This can be measured using special plastic (PMMA) plates. This means that when this ratio is used, a higher SPF label will give you better UVA protection.
While critical wavelength tells you how far the protection extends, it doesn’t tell you how evenly spread out the protection is. Critical wavelength is an easier criteria to pass. Only the critical wavelength criteria is required in the US, but in the EU and Australia you need to have both for “broad spectrum”. That means that some sunscreens that are classified as broad spectrum in the US won’t qualify in the EU and Australia. One study tested 20 sunscreens, and found that 19 would be broad spectrum in the US, but only 11 would be broad spectrum in the EU.
In some Asian and European countries, there are numerical ratings for UVA, although there’s no one standard yet. The most common are:
PPD stands for “persistent pigment darkening” and measures how protective a sunscreen is against a long-term tan caused by UVA – it’s essentially the equivalent of SPF for UVA radiation. It’s commonly used for European sunscreens.
PPD 10 blocks about 90% of UVA rays, and is generally considered to be suitable for everyday use. For some reason PPD values are usually hidden away on the sides of packaging.
PA is a similar concept to PPD, but rates sunscreens on their UVA protection with plus signs (+). It’s more popular in Asian countries. PA+ is roughly equivalent to PPD 2-4, PA++ is PPD 4-8, PA+++ is PPD 8-16 and PA++++ is PPD 16 and above. (If you’re mathsy, the conversion is 2number of plus signs.) Many Asian sunscreens have PA ratings prominently listed.
UVA-Protective Sunscreen Ingredients
Most sunscreen ingredients don’t give great UVA protection, especially longer wavelength UVA1. (Note: “physical” inorganic sunscreens zinc oxide and titanium dioxide mostly absorb UV, so they don’t protect against all wavelengths, contrary to what you might read about them protecting against visible light.)
For a long time, the best ingredients for protecting against UVA1 were avobenzone and zinc oxide. These both had issues:
Avobenzone gives very high protection, but it’s photounstable, which means it breaks down quickly in UV light (although it can be stabilised)
Zinc oxide mostly gives even protection across UV wavelengths, but the protection it gives is quite low, which means you need a high concentration… but that means unpleasant sticky textures and white cast:
But luckily there are newer UVA-protective sunscreen ingredients on the market! They also tend to be photostable and don’t absorb into the skin much. Unfortunately they’re mostly not available in the US unless you import your sunscreen.
The filters are:
- Tinosorb S (bemotrizinol)
- Tinosorb M (bizoctrizole)
- Mexoryl XL (drometrizole trisiloxane)
- Uvinul A Plus (diethylamino hydroxybenzoyl hexyl benzoate)
- Mexoryl SX (ecamsule)
- Neo Heliopan AP (disodium phenyl dibenzimidazole tetrasulfonate)
Oxybenzone, octocrylene and titanium dioxide are less effective but still block some UVA wavelengths (mostly UVA2).
(For the super nerdy – this article shows the absorption spectra of a bunch of different sunscreen filters. The higher the line, the higher the protection at that wavelength – higher concentrations of that sunscreen filter will raise the curve and increase protection.)
More information on choosing a sunscreen, and setting up a skincare routine: The Lab Muffin Guide to Basic Skincare
As you can probably guess by now, sunscreens aren’t the best sun protection – many don’t have complete UVA protection, they need reapplying, and it’s really easy to miss a spot or apply too little. Luckily, there are other options:
- Wear sun protective clothing, hats and sunglasses. Hats are especially good for protecting your scalp – I know I never put sunscreen on my scalp!
- Seek shade and avoid sun exposure at peak times. Reflected UV will still cause damage, but less than direct UV.
- Add UVA tints to windows that you go near frequently, such as at home or in your car. Many windshields are already designed to block UVA.
Sayre RM et al., Commentary on ‘UVB-SPF’: the SPF labels of sunscreen products convey more than just UVB protection, Photodermatol Photoimmunol Photomed 2008, 24, 218-20. DOI: 10.1111/j.1600-0781.2008.00360.x.
Wang SQ et al., Comparison of ultraviolet A light protection standards in the United States and European Union through in vitro measurements of commercially available sunscreens, J Am Acad Dermatol 2017, 77, 42-47. DOI: 10.1016/j.jaad.2017.01.017.
Osterwalder U & Herzog B, The long way towards the ideal sunscreen – where we stand and what still needs to be done, Photochem Photobiol Sci 2010, 9, 470-81. DOI: 10.1039/b9pp00178f.
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