The number on the sunscreen bottle is the SPF, which stands for Sun Protection Factor. SPF is not a percentage; rather, it is a multiplier. It tells you the number of times longer you can stay in the sun with sunscreen and get the same amount of UV light exposure as if you were not wearing sunscreen. For example, if you are wearing SPF 15 sunscreen, you can spend 15 minutes in the sun and receive the same amount of UV exposure as if you had spent 1 minute in the sun without sunscreen.

Another way to say this is that SPF tells you the number of times by which your UV light exposure is reduced. What does this mean? As the name "sunscreen" suggests, it does not totally block sunlight, but simply screens some of it out. It’s kind of like a kitchen strainer that you’d use to drain rice or pasta, or the insect screen on the windows of your house. Sometimes small pieces of food can slip through the strainer and into the sink, or small insects can wriggle their way through a hole in the window screen. Similarly, sunscreen only catches some of the UV rays, while others still reach your skin.

So SPF 50 sunscreen, for example, reduces your UV exposure by 50 times, which means that it allows 2% of the UV light through (100% / 2% = 50). Even SPF 100 sunscreen allows 1% of the UV light through. That’s why there is no 100% effective sunscreen. That’s important to remember, since UV light can cause skin damage and cancer, as I’m sure you know.

Another thing to consider is that sunscreen does not last forever. It can rub off your skin, break down in the sun or heat, or be affected by your sweat. It’s like how a screen door gets rips and tears in it over time; at some point, it has so many holes that it doesn’t even stop any insects at all. That’s why it’s recommended that you reapply sunscreen every hour to make sure that the “screen” is intact.

But don’t let the sun stop you from being outside! Some daily exposure to daylight is important for good health, but the more you can limit your skin’s exposure to UV light, the lower your risk of skin disease will be.

The number you mean is probably the SPF, or Sun Protection Factor. There are two main kinds of ultraviolet (UV) light that can cause skin damage. The SPF number refers to one type (B), but a “broad spectrum” sunscreen will give protection for the second kind (A) as well. SPF is supposed to mean that a person wearing it properly can stay in the sun longer. For example, if the SPF is 30, a person should be able to stay in the sun 30 times longer than they could if they weren’t wearing sunscreen. But there are several things to keep in mind.

Most people don’t wear enough of it. Experts say to wear at least 2 tablespoons full and to put it on 30 minutes before going outside. Sweat, swimming, and such can wash it off. Water-resistant sunscreen will last longer, but you’d still want to apply it after you get out of the water. If you’re not swimming, re-apply it every two hours.

Products with a higher SPF give more protection. Using protective clothing, such as a hat and long sleeves gives more protection. Of course, limiting your time in the sun, particularly on sand or snow, is a good idea.

Clouds reduce the damage from sun, but a person can get sunburned on a cloudy day. Darker skin protects a person more than lighter skin, but even people with dark skin can get sunburned and have long-term sun damage. Getting outside to enjoy the natural world is great, just use a little care.

I’m glad to hear that you’re interested in protecting yourself from the sun. Can you think of ways to convince other people your age to protect themselves?

Thanks for the question! I’m not entirely sure which percentages you mean, but I’m guessing you’re talking about the percentages listed under the “Drug Facts” part of the label. This is where manufacturers list what are called the active ingredients. The active ingredients are the ones that provide the drug’s effect, and the U.S. Food and Drug Administration (USFDA) requires this label to be on all FDA approved drugs.

Sunscreen is regulated as a drug in the U.S., so it has this label. Under the heading “active ingredients” it should list a handful of chemical names. These chemicals are sunscreen filters, chemicals that reduce the amount of UV radiation that hits your skin, generally by absorbing and/or reflecting UV light. Some common ones found in American sunscreens are Avobenzone, Homosalate, Octisalate, Octocrylene, Titanium Dioxide, Tin Oxide, and Oxybenzone. The percentage listed next to these ingredients is the percent of the product (it’s ambiguous to me as to whether that’s percent by volume or percent by mass – perhaps another scientist knows) that is that ingredient. So for example, the sunscreen on my desk has a label as follows:

Avobenzone 3.0%
Homosalate 10.0%
Octisalate 5.0%
Octocrylene 4.0%

That means that the product in this bottle is a mixture of many ingredients, and 3.0% of the mixture is Avobenzone, 10.0% is Homosalate, etc. The reason there are multiple sunscreen filters is because generally speaking each filter only blocks out UV radiation at a certain wavelength. In order to protect skin against the wide range of radiation wavelengths that come from the sun, multiple filters are needed.

The sun protection factor rating (SPF) label is the one that tells you how much protection you are getting in practice, not the active ingredient percentages. SPF is tested in a laboratory on human volunteers. It is essentially the multiplicative factor of time between burning without the sunscreen, and burning with the sunscreen.

For example, if a human volunteer took 300 seconds to burn with sunscreen applied and 10 seconds to burn without it, the SPF factor would be 300 divided by 10, which is 30. Stated another way, and SPF of 30 means that it will take the average person 30 times longer to burn in the sun with the sunscreen on than without it. SPF labels are usually printed on the front of the bottle. However, SPF only accounts for exposure to one type of UV rays, called UVB rays. These rays generally cause sunburn. However, we now have evidence that the shorter wavelengths of UV light that fall under the category of UVA rays also contribute to skin damage and increased risk of skin cancer, despite the fact that they do not cause burning. Unfortunately in the U.S. there is no measurement for protection against UVA rays. The label “broad spectrum” can be applied to sunscreens which provide UVA protection which is “proportional” to the SPF (the amount of protection you get against UVB rays) under new USFDA regulations (see Q1 at this FDA webpage ).

You are correct, we should be using sunscreen as often as possible. According to the American Academy of Dermatology, one in five Americans will develop skin cancer in their lifetime. The Skin Cancer Foundation recommends wearing a broad spectrum SPF of 15 or higher daily, and a water-resistant broad spectrum SPF of 30 or higher on days when you will be out in the sun (See their FAQ on sunscreen here ). The American Academy of Dermatology recommends wearing a broad spectrum SPF of 30 or higher (see their FAQ on sunscreen here ) any day you will be outside, even if it is cloudy. I highly recommend you read the FAQs above for more information about sunscreen, how much to apply and when to reapply, why we should be wearing it, and other methods of sun protection besides sunscreen.

The sunscreen number is the ratio of the time that you can expose yourself to the sun without being burned while wearing the sunscreen to the time you could expose yourself to the sun without being burned.

So, for example, suppose you go out into the sun that would burn you in 15 minutes with no sunscreen. Now, suppose you put on sunscreen 40. Now, you can go out into the sun and it would take 40 x 15 = 600 minutes (ten hours!) before you would burn.

Congratulations for wanting to know more about the sunscreen you put on your skin.

SPF stands for Sun Protection Factor. SPF is the measure of how well sunscreen protects you from sunburn - how much more solar energy you can get with sunscreen, compared with no sunscreen. SPF isn't %.

Here's the idea behind the SPF numbers:

Ideally, with SPF 30 it would take you 30 times longer to burn than if you weren't using sunscreen. With SPF 30 sunscreen, about 3% of the sun's ultraviolet rays hit your skin.

With SPF 50 sunscreen, it should take about 50 times longer to burn, and about 2% of the sun's UV rays hit your skin.

Wikipedia has LOTS of info about sunscreen, and I recommend
it.

Depending on the mode of action, sunscreens can be classified into physical sunscreens (i.e., zinc oxide and titanium dioxide, which stay on the surface of the skin and mainly deflect[citation needed][7] the UV light) or chemical sunscreens (i.e., UV organic filters, which absorb the UV light).

I have found on the Internet different answers about whether physical or chemical sunscreens are better. I guess they're both ok.

Also, there are 2 forms of UV light, with different wavelengths: UVA and UVB. UV [ultraviolet] light has a shorter wavelength than violet [purple] light.

UVA light has a longer wavelength than UVB, and it gets through the ozone layer. UVA light is the closest to violet light; it's between violet light and UVB light in this diagram. I also recommend you to read here.

SPF is a measure of how much solar energy (UV radiation) is required to produce sunburn on protected skin (i.e., in the presence of sunscreen) relative to the amount of solar energy required to produce sunburn on unprotected skin. As the SPF value increases, sunburn protection increases.

Sun protection factor.