A guide to sunscreen use

How high, how much and how often?

The question below is another posed by Fabio, from Italy.

When I put sunscreen on, how does it work? How does it stay on when I put my clothes on or when I, let's say, play sports and have contact with other people or maybe go on slides in water parks and so on? Is it absorbed in any way? Does it stay on if I touch things with my hands, arms etc? I'm always scared of rubbing it off by touching things, especially right after putting it on and using my hands (it's difficult to not touch anything after putting it on your hands, not sure if you need to wait a little before doing so). Also, when the sunscreen sort of dries off after a few minutes, sometimes my hands are quite dry and it feels like it's not there anymore, but I guess it's still there, or isn't it? Is there a way to actually test this at home and see if the sunscreen is still there? Has someone done any studies on this? Hopefully what I'm asking is clear, and I hope you can maybe give me some advice and explanation.

Nice question, but a hard one - not least because there are so many different sunscreens on the market. No doubt there will be differences between them. If you believe their advertising, there certainly are. These include (most importantly) the SPF rating, but also stickiness, water-resistance, stability, cost, ….

Sunscreens fall into two main categories. They protect your skin by either (1) reflecting, or (2) absorbing incoming UV radiation from the sun.

Reflective sunscreens. These are sometimes called inorganic sunscreens, including zinc. They have two advantages. Firstly, it’s easier to see if they’re still present (though cosmetically that may be considered a disadvantage). Secondly, because they are applied to the outside of the skin rather than being rubbed into it, there’s less potential for absorbing any harmful chemicals they may contain. I haven’t used these much myself, but you often see the white reflective surface of these sunscreens on the noses and cheekbones of professional cricketers, whose faces would otherwise be exposed to huge amounts of UV.

Absorbing (or organic) sunscreens. These are currently the most prevalent, at least in New Zealand. When using them you are usually advised to apply 20 minutes before sun exposure, and to reapply regularly. This reapplication instruction is in recognition that it can simply wear off over time. But the UV in sunlight can also photo-degrade the active chemicals into components that are less effective at blocking skin-damaging UV radiation.

To get the advertised sun protection factor (SPF), you need to apply it quite thickly too. The recommended application amount is 2 milligrams per square centimetre of skin area. The density of sunscreens is about the same as for water, around 1 g/ml, so that application rate corresponds to 0.002 ml per square centimetre. You can use this on-line tool (which uses the Mosteller formula) to calculate your skin area from your height and weight. For me, it’s about 2 square metres, or 20,000 square centimetres. So, for the advertised SPF, I need about 40 ml of sunscreen per full-body application.

For the cooks, you’d need about 8 teaspoons for full-body protection if wearing only bathing briefs. If its just your face and neck that’s exposed, you’ll need about 1 teaspoonful. The photo below is of the sunscreen I’m currently using, alongside an eight-inch crescent for a scale.

Its 20 cm high (8 inch) container holds 400 ml - enough for only ten full-body applications - costs NZ$26. At that rate, the cost for sunscreen alone for a day at the beach could be 5 to 10 dollars per person.

For that reason, and also because it’s safer for your skin, you’d be better advised to exclude the period within 2 hours either side of solar noon (about 1:30 pm in the NZ summer) on your visits the beach. And cover up.

I usually wear a sunhat, a short-sleeved shirt with a collar, and light trousers if I’m going to be outdoors for long periods in summer. I also actively seek shade. There’s no need for a sunscreen for skin surfaces covered by clothing. For exposed surfaces - my face and neck (including ears and lips), lower arm and the outside of my hands - I use a sunscreen, but the amount needed there is closer to 2 or 3 teaspoons (10 to 15 ml). I don’t apply sunscreens on palms of my hands, as they’re much less exposed to sunlight, and it would soon wear off anyway. The container pictured is enough for 30 to 40 such applications, which is much more reasonable. I therefore tend to err on the generous side - also because I’m not very good at remembering to re-apply it. In my defense though, most of my outdoor activities these days (e.g., tennis or 9-holes of golf) are less than 2 hours. But for a full 18-hole game of golf I try to remember to re-apply at least once during the round.

This particular sunscreen recommends re-application every 2 hours - or more often if swimming, sweating, or towelling. The sunscreen can also be absorbed into the skin over time and for that reason, certain chemicals (e.g., PABA) have been banned. I heard at a skin cancer meeting last August that the US Surgeon General at the time was also looking into other potentially problematic ingredients in current sunscreens - but no doubt that research has been stopped under the current crazy regime.

It’s very important not to skimp on the thickness, as illustrated in the table below where I’ve calculated the effective SPFs for a range of commonly advertised SPFs, for various application thicknesses.

Effective SPFs calculated assuming Beer’s law applies. Results for sunscreen pictured above are in bold text.

When applied correctly (at 2 mg/cm2), the SPF will be as advertised, but it decreases much more rapidly than you might have expected if you skimp. With three-quarters of the recommended thickness, all but two have an effective SPF less than 19. With half the recommended application thickness, none of the effective SPF exceed about 12. And with only one quarter of the specified thickness, all have an effective SPF of only 2 or 3.

One study has shown that people typically apply only between 0.4 and 1.0 mg/cm2. That lower number is less than one quarter of the recommended thickness. With that, the effective SPF of the sunscreen pictured above would be only 2.2 rather than 50! For that reason, the study authors suggest applying sunscreen before sun exposure, and once again within an hour of exposure. Higher SPFs are also recommended.

How long do they last?

As I showed in a previous post, sunscreens seem to survive for years if they’re kept in their containers and not exposed to sunlight.

But what about when they’re applied to the skin and used outdoors? Is it still there after it starts feeling dry, Fabio asks? In a follow-up message, he kindly pointed me to several relevant papers in the medical literature (as listed at the end, along with their main conclusions). The results aren’t always consistent but in general, they show that the effectiveness of sunscreens does seem to diminish over time with sweating and abrasion (and after immersion in water for non water-resistant ones). After an hour or two you should assume the effective SPF to be significantly less than when first applied. It’s therefore prudent to re-apply occasionally, especially since the initial application may have missed some skin areas.

It would be great to further quantify the photo-degradation of sunscreens used in this part of the world over time using a couple of personal UV dosimeter badges like those discussed here. A reference dosimeter could measure the ambient UV throughout the day without any sunscreen application, and an identical one could be used to measure the UV received after passing through an application of the sunscreen to its Teflon entrance optic. The protection factor for the sunscreen is then just the ratio between the two readings. Any degradation in the protection factor between morning and afternoon (at the same sun elevation angle) would be due to photo-degradation or evaporation of the chemicals.

Other similarly sunscreen-coated dosimeters could be subjected to stresses such as soaking in water (fresh or salty) and/or towelling to ascertain the increased rapidity of performance degradation with those additional stresses. With such objective measurements the stressors could be much more tightly controlled, though it wouldn’t be quite the same as the experiments linked below because, unlike human skin, no chemicals would be absorbed at the surface of the input optic of the dosimeter badges. I’d love to see the results of such an experiment, but here in New Zealand it would have to wait until next summer because daily doses of sunburning radiation are then about twenty times more than in the winter we’re currently approaching. If somebody has already done the experiment, please let me know.

Thanks for the question, Fabio. I hope this reply was as useful for you as it was for me 😊. Thanks for the literature links you found too.

Summaries are listed below. Follow the links if your need more ...

1. Bodaekaer et al., 2008.
Found that one application of either an inorganic or an organic sunscreen reduced the erythema caused by UVB during a day with physical activity and bathing. After 8 h the sunscreens still provided approximately 43% of the initial protective effect.

2. Beyer et al, 2009
Found a similar decrease in SPF of the organic and inorganic sunscreen during 8 h without activities, and is thus not likely to be caused by absorption or evaporation from the skin. The SPF decrease after 8 h is about 55% when the participants perform activities and 25% without activities.

3.Petersen & Wulf, 2013
Showed that sunscreens under natural conditions are applied insufficiently with amounts about 0.39 to 1.0 mg/cm2, which decreases the protection factor considerably. Suggest a simple teaching strategy: (1) Apply before sun exposure and (2) Reapply once within 1 h.

4. Odio et al., 1994
Showed that 1 or 4 sunscreen applications yielded comparable erythema protection after a 6-hour sun exposure, totaling 13 minimal erythema doses (MED). However, after an 8-h, 21 MED exposure, incidence and severity of erythema was greater at body sites treated with 1 compared with 5 sunscreen application

5. Ruvolo et al., 2020
Found that sunscreen efficacy is maintained over time in the absence of sweating or rub-off. After two hours of sweating, an 80 minutes water-resistant sunscreen does not need to be re-applied "at least every 2 hours." HERESY do I hear you say?

6. Kobwanthanakun et al., 2021
Found that sunscreen coverage declined most rapidly during the first 2 hours with a mean reduction of 18.31%. By the end of the 8‐hour study workday, the mean decrease in sunscreen coverage was 31.63% (range: 17.39%‐45.29%). Reapplication of sunscreen is essential among outdoor workers. After 4 hours, the amount of sunscreen remaining on the face may not be sufficient for protecting the skin from harmful ultraviolet radiation.