UVI & time(s) for skin damage

How to relate them ...

After a recent post about the UV Index (UVI), my old Austrian colleague, Mario Blumthaler, reminded me that - as far as the public’s concerned - it’s really the UV dose that matters, rather than the instantaneous dose rate that’s represented by the UVI. It’s the same as the difference between energy and power (the rate at which energy arrives). The total energy is the power multiplied by the time.

Let’s take a household example that you’ll be familiar with. When you pay your electric ‘power’ bill, you’re really buying the energy you’ve used, which is the product of the average power you consumed (usually in kilowatts) multiplied by the number of hours you used it for (i.e., in kWhr). Typically in New Zealand, electricity costs about 35 cents per kWhr. So, if you use a 2 kW heater for an hour, it costs 70 cents. And that’s what matters 😊.

Similarly, Mario wondered if we should re-emphasise a measure like the ‘UVI-hour’ (UVIhr), which had been suggested a few years back for assessing UV risk for the public. For example, a dose of 10 UVIhr can be reached by having a UVI of 10 for 1 hour, or a UVI of 5 for 2 hours, or a UVI of 1 for 10 hours (which would admittedly be extremely unlikely, but hopefully you get the idea), etc.

It turns out to be a useful idea. If you know the amount of UV energy coming in, and the amount needed to cause skin damage, you can work out how long it would take for damage to occur. The skin-damaging dose depends on skin type. As it happens, for the most sensitive skin types, a dose of about 3 UVIhr is enough to cause the first signs of damage (skin-reddening, which medics call ‘erythema’). So, for a UVI of 10, just over three of those damaging doses can be delivered in an hour (i.e., for a dose of UVIhr = 10). Or to put it another way, one damaging dose can be reached in about 20 minutes.

If you think that’s all a bit complicated, don’t worry too much. A solution is in store.

Basically, the idea of UV dose was the thinking behind the old ‘burn time’ that was used to describe the UV risk in New Zealand way back in the 1990s. Although the idea has appeal, there are problems. Firstly, the dose-threshold calculated was far smaller than what people would think of as a ‘burn’. That created an incorrect perception - that persists to this day - that the UV is much higher in New Zealand than other places. Secondly, the dose depends strongly on skin type. Much larger doses are OK for darker skin types. If you don’t take it into account, that raises ethical issues. But taking it into account makes it even more complicated.

Now the good news …

Really, what we all want to know is how long each of us (personally) can stay in the sun before skin damage becomes a problem. Luckily, the UV smartphone apps (UVNZ and GlobalUV) that we’ve developed do all the hard yards for you. All you need to do is provide them with your skin type when you first use them. They do the rest. As a by-product, they even give you the (very approximate) exposure time needed to maintain sufficient levels of vitamin D for your selected percentage of skin exposed (here taken as 35%). For high UVI values, these exposure times are far shorter than those that cause skin damage.

I’ll illustrate with a couple of examples from the UVNZ smartphone app. The screen shots below were taken on 13 January, shortly after 1 pm local time (13:13 and 13:15 NZST), which was a few minutes before the highest sun elevation that day.

In the first one (below), I’d set the app to Skin Type 1 (the most sensitive). In the lower box, it tells us that with the current UVI of 11, it takes 14 minutes for skin damage (and 2 minutes for enough vitamin D, as shown by the smaller number below the curve). You can niftily specify other times by sliding your finger across the screen. Here I’ve set that to 9:45 am. The behavioural advice for that time is shown in the upper box, where the predicted UVI is 3.6, corresponding to a skin-damage time is 41 minutes (and 6 minutes to make enough vitamin D).

In the second screenshot (below), Skin Type 3 (olive skinned) has been selected. You are reminded of that selection by the different-coloured bar at the top. The reported UVI is unaffected, but the damaging exposure times for this skin type are longer by a just over factor of two. In this example, I’ve selected a later time, 16:55 (4:55 pm) for the second reading.

If I’d selected skin-type 5 (dark brown skin), the time for damage at UVI = 11 would increase to about 50 minutes, and for skin type 6 (black skin) it would take more than 90 minutes.

The times for skin damage for all skin types are tabulated below for all realistic UVI values anywhere in the world (taken from here). Notice that the times shown there for damage for skin-types 1 and 3 are a bit shorter than shown in the app : 14 and 22 minutes (rather than 15 and 30 minutes). This is partly because of disagreements in the assumed damaging doses, which for each skin type are not very well defined. But part of the difference can also be explained by the app taking proper account of short-term changes in UVI. For example, over the period starting 13:15, the UVI continues increases over that initial value of 11, so the exposure time for damage decreases in the same proportion.

Time for skin damage. Where MED = Minimum Erythemal Dose = minimum UV dose needed to produce a just-perceptible reddening of the skin (erythema), and SED = Standard Erythemal dose (100 Jm-2)

If you want to know your skin-damage time at places outside New Zealand, you’ll need use the GlobalUV smartphone app instead of UVNZ. Download it and have a play to see how it works. They’re both free.

By now - a couple of weeks later than the examples shown above - the peak UVI for Alexandra is less. But if you live in Northern New Zealand or Australia (or anywhere from the tropics down to latitudes near 40 S), the app may show that your current risk is higher. Check it out.

If you want more details on the conversion between UVI and UV doses, take a look at this substack post, or this paper.

Otherwise just use one of the apps to guide you, and take care …