Sunscreen

[Steve]

As a fair skinned redhead (at least until it turned grey and fell out) I am a regular user of sunscreen and/or sunblock during the warmer months. The instructions invariably say to apply the sunscreen 15 to 30 minutes before exposure. I recently started to wonder what happens during that 15 to 30 minute period that changes the effectiveness of the sunscreen. Why isn't it effective immediately after application? Is some sort of reaction with the skin a critical component of the effectiveness? Your thoughts would be appreciated.

 

[Dymanic]

The instructions invariably say to apply the sunscreen 15 to 30 minutes before exposure.

Son of a gun, I never noticed that. If it had ever occurred to me to give it a second's thought, I probably would have concluded that anybody who can't figure out how to put sunblock on without reading the instructions might be safer just staying inside. But I just checked three different tubes of sunblock, and they all say that.

It's possible that while studies show pre-application to be more effective, nobody knows for sure why that is. It's also possible that somebody does know, but since they tend to communicate in language like this:

"The number of UV-induced (20 mJ cm-2) reactive oxygen species (ROS) generated in nucleated epidermis is dependent upon the length of time the UV filter octocrylene, octylmethoxycinnamate, or benzophenone-3 remains on the skin surface. Two-photon fluorescence images acquired immediately after application of each formulation (2 mg cm-2) to the skin surface show that the number of ROS produced is dramatically reduced relative to the skin - UV filter control. After each UV filter remains on the skin surface for t = 20 min, the number of ROS generated increases, although it remains below the number generated in the control. By t = 60 min, the filters generate ROS above the control. The data show that when all three of the UV filters penetrate into the nucleated layers, the level of ROS increases above that produced naturally by epidermal chromophores under UV illumination"

...it's almost the same thing as if they know, but aren't telling; you can either guess what they're saying, or just guess about why pre-applying the stuff works better. My best stab at the former is that they're saying that the products they tested worked well for a while, but unless reapplied, a point was reached at which the damage from UV was actually worse than with no blocking agent at all. My best stab at the latter is pretty simple: the stuff tends to stay on better if it has a chance to soak in a little. I think the recommendation may also be based on concerns about the possibility of a reaction occurring if the product is applied to hot skin.

 

[JJM]

... The instructions invariably say to apply the sunscreen 15 to 30 minutes before exposure. I recently started to wonder what happens during that 15 to 30 minute period that changes the effectiveness of the sunscreen. Why isn't it effective immediately after application? Is some sort of reaction with the skin a critical component of the effectiveness? Your thoughts would be appreciated.

Wow, that is a fascinating observation. I made sunscreens decades ago, and there is no reaction needed to make them effective. (As Dymanic suggests, the use seems so simple, I never read the label on a finished product.) I would love to hear from someone who knows; but I will venture the following:

The effectiveness of the sunscreen is determined by experiment- a "research protocol" that involves applying the product and then exposure to UV light. Said protocol, the standard procedure for studying effectiveness, may call for waiting for the product to dry (15-30 minutes) before beginning the UV exposure. Thus, there is no theoretical basis for waiting; however, there is no experimental evidence of protection if you don't.

I am out of date on this subject; but take note- sunscreens are tested for their ability to prevent sunburn, not cancer. We don't know how effectively they prevent cancer. Limit your exposure to the Sun, regardless of your sunscreen.

 

[JoeTheJuggler]

Here's my guess: if you put it on 15 to 30 minutes before exposure, it means you're applying it while still indoors. If you don't, it means you could be out in the sun first, and then putting it on.

Maybe it's just a way to make sure you don't go out in the sun unprotected even for a few minutes.

 

[RenaissanceBiker]

Here's my guess and it is pure speculation. You put it on while still indoors. The lotion part gets absorbed into your skin, moisturizing it and leaving a dry residue of sunscreen material. It may even form a coating like latex paint. Once dry, it is resistant to washing off. You go outside and sweat but the residue does not wash off. If you go outside too soon, sweat may wash off the protective coating before it has properly cured. This speculation is based solely on how I would make a sunscreen work if someone asked me to design some. I may be wrong.

I put sunscreen on my arms and neck before going for a ride. It is a big mistake to put sunscreen on your face before motorcycling. It will get in your eyes and blind you no matter how long you let it cure.

 

[EvilBiker]

I put sunscreen on my arms and neck before going for a ride. It is a big mistake to put sunscreen on your face before motorcycling. It will get in your eyes and blind you no matter how long you let it cure.

Amen to that. Nothing worse than suddenly getting watering eyes while hard in a nice sharp curve on a mountain pass - been there, done that, got the adrenaline rush. Wasn't the wind that did it (fullface), it was the sweat!

Of course, putting sunscreen on your arms and neck implies you do not wear protective gear at all times? Bad, bad boy.

 

[RenaissanceBiker]

Of course, putting sunscreen on your arms and neck implies you do not wear protective gear at all times? Bad, bad boy.

I always wear a 3/4 helmet with a visor. After that, weather dictates. the minimum is boots, full length jeans, gloves, and a tank top. I've been down a few times and this is the level of protection I have chosen.

ETA: SC does not require motocyclists over the age of 18 to wear a helmet.

 

[casebro]

Haven't I heard that after having sun screen on the market for 30 years, our skin cancer rates remain stable? We might as well use hypnosis to prevent skin cancer.

But I don't think sunscreens have even been proven in the lab to prevent cancer. They prevent sun burn, that we assume causes cancer in a dose related fashion.

 

[chulbert]

If you apply sunscreen 15 to 30 minutes before exposure then you're presumably still inside and not sweating. Once you're in the sun you start to sweat and that makes it difficult for the lotion to sink it.

That's my guess.

 

[Dymanic]

Haven't I heard that after having sun screen on the market for 30 years, our skin cancer rates remain stable?

Maybe the use of such products inspires enough confidence that people are less likely to limit their exposure -- and since most probably don't read the directions, let alone follow them carefully, they end up worse off than they would have been had no sunscreen been available.

Besides, with the loss of stratospheric ozone in recent years and the corresponding increase in ultraviolet radiation, those rates staying stable would indicate that something is working to counter the effects.

But I don't think sunscreens have even been proven in the lab to prevent cancer.

I'm not sure "the lab" is the perfect place to "prove" that, but they have been proven to filter UV. I think it's fairly well established that UV exposure increases generation of reactive oxygen species, and this is strongly implicated in skin cancers. That blocking the UV is preventative against skin cancer doesn't seem like a huge logical step along that path.

They prevent sun burn, that we assume causes cancer in a dose related fashion.

Evidence suggests that sunburn substantially increases the risk, but I don't believe that it is assumed that getting burned is a requirement; just about any dermatologist will tell you that even tanning should be viewed as an indication of damage to the skin.

 

[Steve]

Bump.

I asked the OP question two years ago. There was some interesting and reasonable speculation on the answer but nothing definite. The recent sunny spring weather has made me get out the sunscreen again and, with the influx of new posters over the past two years, I thought I would bring it up again to see if there are any new ideas.

 

[Miss_Kitt]

BTW, if you live in one of these low-sun areas (I'm Pac NW also, dogguy), it's a good idea to get about 20 minutes of sun on your neck and arms daily to assist with Vitamin-D levels. Just don't forget to sunblock after your brief 'get some sun' stroll, okay?

ETA -- Too bad your hockey team bounced out of the playoffs so quickly!

 

[El Greco]

Sunscreens based on inorganic compounds (like Zinc Oxide & Titanium Dioxide) work immediately since they just deflect sunlight. Sunscreens based on organic compounds (like PABA & Oxybenzone) have to be applied a little earlier because they work by absorbing sunlight (mainly UVB) and the wavelength they absorb depends on the sunscreen's absorbance through the skin.

 

[Steve]

BTW, if you live in one of these low-sun areas (I'm Pac NW also, dogguy), it's a good idea to get about 20 minutes of sun on your neck and arms daily to assist with Vitamin-D levels. Just don't forget to sunblock after your brief 'get some sun' stroll, okay?

ETA -- Too bad your hockey team bounced out of the playoffs so quickly!

Yeah, I know about the vitamin D thing. I probably get a lot more unprotected sun than I should considering my fair skin and the red hair I had when I was younger. This is part of the reason I asked this question. I regularly go outside and about half an hour later think "I should put some sunscreen on".

The Canucks looked so good against St Louis and so ordinary against Chicago. I don't think the long layoff between the series helped. Chicago do look like a pretty good team so it was no disgrace being beaten by them.

 

[Steve]

Sunscreens based on inorganic compounds (like Zinc Oxide & Titanium Dioxide) work immediately since they just deflect sunlight. Sunscreens based on organic compounds (like PABA & Oxybenzone) have to be applied a little earlier because they work by absorbing sunlight (mainly UVB) and the wavelength they absorb depends on the sunscreen's absorbance through the skin.

This makes some sense.

The label on the sunscreen I have on hand lists the active ingredients as:

Octyl Methoxycinnamate 6.0% w/w
Zinc Oxide 6.0% w/w

I also states that it is "PABA free", and the instructions say "apply 15-30 minutes before sun exposure". It seems that at least some manufacturers require advance application even for zinc oxide. I wonder if there is some "cover their butt" component to this.

 

[Cuddles]

I wonder if there is some "cover their butt" component to this.

I suspect this may be the most important point. If a single small, badly done study once suggested that a few minutes may be needed before the full effects kick in, it's quite likely to be judged cheaper to put in instructions telling you to put it on earlier than to potentially be subject to lawsuits. Especially given that printing such instructions is likely to cost approximately nothing.

 

[El Greco]

This makes some sense.

The label on the sunscreen I have on hand lists the active ingredients as:

Octyl Methoxycinnamate 6.0% w/w
Zinc Oxide 6.0% w/w

I also states that it is "PABA free", and the instructions say "apply 15-30 minutes before sun exposure". It seems that at least some manufacturers require advance application even for zinc oxide. I wonder if there is some "cover their butt" component to this.

Octyl Methoxycinnamate is an organic sunscreen, hence the suggestion to apply some time before exposure.

But I don't doubt this guideline may appear as well in purely inorganic sunscreens. I suppose in those cases they're simply doing it out of habit.

 

[Steve]

Octyl Methoxycinnamate is an organic sunscreen, hence the suggestion to apply some time before exposure.

But I don't doubt this guideline may appear as well in purely inorganic sunscreens. I suppose in those cases they're simply doing it out of habit.

Thanks for your input, El Greco. I think you have answered my question in as much detail as I need. Much appreciated.

 

[ZirconBlue]

Well 2 years later, you finally got your answer!

(Note that 2 years ago, I too must have been interested in the answer, since I apparently subscribed to this thread.)

 

[Soapy Sam]

What is this "sun" of which you speak?