UV Alerts

Too dumbed-down by far ...

Updated and improved 15 Nov 2023.

Many years ago, Sunsmart New Zealand began implementing a new way to advise the public about the daily UV risk during summer months. They called it the UV Alert. The alert period reported is simply the time interval between the start and end times for which the UVI will exceed 3 that day at the specified site. It’s a bit boring because it’s based on a calculation for clear-skies without taking any account of cloud effects which are just about always important.

An example, taken from the Sunsmart website on 3 November is shown below. As you can see, the warning applies for a big chunk of the day - even though we weren’t yet into summer. The alert period for the next day was therefore very similar. No more than 5 or 10 minutes longer. And in the peak of summer those day-to day differences are even smaller.

The highest UV any day will occur half way between those two times shown. But the Alert gives no information about how intense that UV will be then. In spring or autumn it might remain close to 3 or 4, but in the middle of the summer, the UVI could exceed 12, which is more than 4 times higher than the alert threshold. The text also makes the questionable assertion that it applies also on cloudy days. Our measurements show that’s clearly (if you’ll excuse the pun) not the case, as shown for example by the plot below. But, better safe than sorry, I guess…

My question is this. Why dumb the message down so much that you lose these important pieces of information?

I’m a LOT more careful about my UV exposure when the UVI is 12 than I am when its only 3. If it takes an hour to damage my skin when the UVI is 3, it will take only 15 minutes when the UVI is 12. Why don’t these agencies just show us how the UVI will vary over the day, and give appropriate message about the maximum exposure period allowed at noon before there’s perceptible skin damage (i.e., skin-reddening, or ‘erythema’ to use the technical term)?

I’m not a great fan of the alert threshold (UVI = 3) either. When the UVI is 3, skin damage (as evidenced just perceptible erythema 24 hours later) can occur after about 1 hour’s exposure. At that UV level, there’s no real problem for exposures of less than an hour. To specify an ALERT at UVI = 3 seems a bit arbitrary. There’s no real threshold anyway. Damage still occurs at UVI = 1.5. It just takes twice as long.

The Alert Threshold of 3 was set after extensive discussions and soul searching many years ago, but to me it just signifies the period when you need to be aware of the desirability of covering up, rather than a ‘danger’ period as interpreted by some.

That threshold seems overly conservative. The at-risk population in New Zealand are descended from the British Isles, where the peak UVI is about 7. According to current theory, skin colours gradually lightened as populations moved poleward, to allow adequate vitamin D to be made at those higher latitudes with their lower UV levels. Surely with that adaptation the skin still be able to cope with the ambient range of UV? But perhaps we all live too long nowadays …

But, even that seemingly too-low low threshold is not really correct. As we showed several years ago, skin damage can occur even for UVI levels as low as 1, but only for exposure periods of 3 hours or more (during which time there would be substantial changes anyway). So that UVI = 3 threshold could also be construed as being too high! Oh dear!

The Alert message should at least include information about the peak UVI expected.

My preference would be to include graphs of the daily variation in UV with and without cloud effects. Like the daily curves shown for several sites on the NIWA ozone/UV web site.

You can see that on this particular day, the actual UV (black dots) was far below that forecast for clear skies (blue curve). In the morning the predictions that included cloud effects were quite good, but the data showed that it remained cloudy in the afternoon while the model showed a dissipation in clouds.

Measurements like these are available at only a few sites and not everybody lives at those sites. But that’s OK. Similar plots can be read from the UVNZ smartphone app that my colleague, Jerry Burke, and I developed. They also include the recommended behavioural advice – dependent on your selected skin type -  for exposures to sunlight at any time of the day.

Here’s an example of what the apps show. Ideally, daily UV reports to the public should do something similar. Note at the forecasts can include cloud effects - as shown by lower white curve, or be just for clear-skies - symmetrical upper black curve.

The app, which is endorsed by the Cancer Society, gives appropriate behavioural advice (i.e., the number minutes for skin damage) for the skin-type selected whenever the UVI is 3 or more. As you slide your finger across the screen to different times, the app revises the messaging appropriately. When it’s less than 3 we refrain from saying ‘no protection needed’. You might still need protection then if you were outdoors for more than an hour or so, playing golf for example.

Let’s not dumb things down in the Alerts to the lowest common denominator. Some want more ..

Thinking people who live in New Zealand can download the UVNZ app. Tune it to your skin type and use it regularly to inform yourself of best UV exposure practice. If you’re outside New Zealand, you’ll have to make do with the GlobalUV app.

If you’re not a thinking person, you probably haven’t got this far … 😊