Why does sunscreen not work all day?

Question

All over the internet it says that sunscreen does not work all day, but does not say anything about why.

Why does sunscreen not work all day?

Does the matrials get destroyed by the UV rays? Or is it because they evaporate?

A product called p20 claims 10 hours but does not provide any evidence, which does not seem trustworthy.

The general principle is to reapply every two to four hours," dermatologist James Spencer, MD, of St. Petersburg, Fl, told WebMD. "Sunscreen does go away with time.

http://gizmodo.com/how-sunscreen-works-and-why-youre-wrong-about-it-508910004

Answer 1

All sunscreens are physical barriers, designed to absorb or reflect UV radiation. They rub off over time, and they will rub off faster when exposed to water/moisture, most commonly from sweating or swimming. Friction (e.g. from towel-drying) also removes sunscreen.

Some chemical sunscreens do break down when exposed to sunlight and require the addition of other chemicals as stabilizers. This is less of a concern with mineral (zinc or titanium oxide) sunscreens.

The most common recommendation is to reapply sunscreen every 2-3 hours, but this doesn't take into account many variables such as individual sun-sensitivity, SPF of the sunscreen, and type/level of activity. So the 2-3 hour guideline is a blanket recommendation that won't necessarily prevent one from burning. In fact, a 2001 study suggests that earlier re-application is more effective:

Advice given to sunscreen users should be to apply sunscreen liberally to exposed sites 15 to 30 minutes before going out into the sun, followed by reapplication of sunscreen to exposed sites 15 to 30 minutes after sun exposure begins. Further reapplication is necessary after vigorous activity that could remove sunscreen, such as swimming, towelling, or excessive sweating and rubbing.

As for claims of all-day (or x-hour) protection, time estimates are based on a rough calculation using SPF. According to the Environmental Working Group,

SPF is a measurement of sunburn protection, primarily caused by UVB rays. If your skin would normally burn after 10 minutes in the summer midday sun, for example, wearing a thick layer of an SPF 15 sunscreen would theoretically allow you to stay in the sun for 150 minutes (10 x 15) without burning. This is a rough estimate, however, and your own skin, the type of activities you do in the sun (e.g. involving water or sweat) and the intensity of sunlight may affect how much safety it gives you. SPF ratings can be confusing or misleading. The numbers do not reflect the degree of protection from UVA rays, which cause skin aging, immune suppression and cancer. The FDA has warned that high-SPF products can create a false sense of security, contain higher concentrations of allergenic or irritating ingredients and offer little additional sun protection (Branna 2011).

Moreover, manufacturer claims are likely based on a best-case scenario that would involve applying a thick layer ~20 minutes before going outdoors to allow for the lotion to bind to the skin, minimal sweating and friction, and perhaps even use of additional protection measures such as a hat or umbrella.

Answer 2

I'd like to add to the existing answer that while sunscreen effectiveness does decrease as it is absorbed into the body, wears off, or is washed off, high SPF sunscreens will still block most sun after the two hours. The FDA recommendations for reapplication every two hours are basically to provide maximum protection, because the level of protection does decrease over time. I don't know the history of how the 2 hour period was introduced, but it is the usage that the FDA considers proven to prevent skin cancer.

From my personal experience: The two hour rule is pretty conservative, if you use large amounts of high SPF sunscreen. I used to spend most of June on a boat in the sun in the desert. I'm very pale, although I do tan a little. Without sunscreen, I would burn in 15-30 minutes. With an SPF 30, I would get lightly burned after 8-10 hours in the sun, unless I reapplied at least once, maybe twice during the day. With a zinc-based high SPF (Blue Lizard brand), I could apply the sunscreen before dawn, and wouldn't be any darker by late afternoon when I finished. Other coworkers used different brands, but for all of us, using the highest SPF available, we only had to apply it once to keep from burning. This sort of usage hasn't been proven to prevent skin cancer, but I'm not sure it's been specifically tested either. It probably does, since it prevents burns, but it may not do so as well as frequent reapplications.

To summarize: Sunscreen protection decreases the longer you wear it, and it decreases faster if you do activities that will rub or wash it off (sweating, swimming, drying yourself with a towel). Depending on the type of sunscreen, absorption into the skin will also decrease effectiveness. But the protection won't magically disappear after two hours, and may still be sufficient to prevent burning after all day long, depending on the sunscreen you used, your activities, and how thickly you slathered it on.