Did you know that many people who apply sunscreen are only getting HALF (or less!) of the SPF they see advertised on the bottle? That’s astounding, and it’s a huge problem for your skin! So why is this?It’s all about the dose! When a sunscreen is developed and tested for efficacy, a term known as sun protection factor or “SPF” is used as a measure of how well it blocks UV radiation.
During this testing, sunscreen is applied in the dose of 2mg/cm2 to the body. At this amount of 2mg/cm2, research shows that a user can achieve the SPF as labeled on the bottle.
However, many users (25-75% of people) apply roughly half of this amount (or 1mg/cm2). Therefore, the effective SPF of your sunscreen is being reduced by about 50%. This means that if your sunscreen bottle has an SPF of 15, you are only getting the benefit of an SPF 7.5. And some of you are applying even less (0.5mg/cm2) while thinking you are achieving an adequate level of sun protection.
So how much is 2mg/cm2 actually? Have you ever been told in PRACTICAL TERMS exactly what this means? Likely not. So this is the crucial part! This is your take-home message. Let’s break it down by body part to give you a more concrete understanding of the correct dose of sunscreen to use for adequate protection.
The correct dose of sunscreen:
There are two essential concepts in the field of dermatology (the rule of 9’s and the fingertip unit) that help us prescribe the correct amount of topical medication to treat a patient’s condition. These two concepts can be extrapolated to determine your correct dose of sunscreen.
The “rule of 9’s” is typically used to estimate the total body surface area affected by burns or other skin diseases. The rule also happens to be very helpful when deciding how much sunscreen to apply to which part of the body to get adequate protection. For the rule of 9’s, the body is broken up into 11 individual parts with each part representing roughly 9% of our total body surface area.
The 11 body parts are as follows:
- Head, neck, and face
- Left arm
- Right arm
- Upper back
- Lower back
- Upper front torso
- Lower front torso
- Left upper leg and thigh
- Right upper leg and thigh
- Left lower leg and foot
- Right lower leg and foot
The second concept is the “fingertip unit,” which is measured as a line of cream spanning from the distal crease of the index finger to the tip of the finger. In the case of sunscreen application, we are going to expand the concept of the fingertip unit to involve the entire index finger from the palmar crease to the tip of the finger. Now let’s apply these two concepts to determine your correct dose of sunscreen.
For each body part (which is roughly 9% of your total body surface area), you need TWO strips of sunscreen squeezed out onto the index and middle fingers from the palmar crease to the fingertips. If this seems like an overwhelming amount, I suggest applying half and waiting for 30 minutes before applying the second half. In these measurement terms, that means applying one index finger length, waiting 30 minutes for it to absorb, and then applying the middle finger length amount. This separate application will also help decrease your chance of getting skip areas where you miss spots from uneven application.
For the face and neck, this equals about 1/2 teaspoon. For the entire average-sized adult body, this is roughly 1 shot glass full of sunscreen or 2-3 tablespoons. Does this sound like a lot to you? Where do you fall on the spectrum of the amount of sunscreen applied? These amounts described in practical terms can be extremely useful to assure you are getting the full SPF out of your bottle of sunscreen.
Tips to avoid common sunscreen mistakes:
Now that we are clear on how much sunscreen to apply, let’s address other common missteps that cause many people to under-protect their skin.
First, sunscreen must be applied BEFORE getting dressed for outdoor activity. It is a misconception that all clothing and swimwear are equal when blocking the harmful effects of UV radiation. Not all clothing has an adequate ultraviolet protection factor or “UPF” to take the place of sunscreen safely. For example, a typical white t-shirt has the equivalent of an SPF 7 and drops to an SPF 3 when wet. Therefore, it is crucial to apply sunscreen in the correct dose before putting on your outdoor wear.
Next, sunscreen should be applied about 15-20 minutes BEFORE going outdoors. This is more true for chemical sunscreens compared to physical sunscreens. It takes approximately 15 minutes for the chemical sunscreen to be absorbed into your skin and become activated to provide full protection. So if you wait until you are in the sun to apply it, that leaves you about 15 minutes where you are under-protected. This short amount of time is certainly long enough to get a sunburn. Physical blocking sunscreens, on the other hand, work by creating a barrier that sits on top of your skin. This works more immediately and can be applied directly before going outdoors.
The last common misstep that is contributing to most people under-protecting their skin is the lack of re-application often enough (or at all!). The textbook recommendation is that you should re-apply sunscreen every 2 hours. However, if you are heavily sweating, you should re-apply more often. And if you are getting out of the water, you should re-apply immediately.
Tips to help you achieve proper application:
Apply two layers to prevent “skip areas.”
- Measure out the correct dose of sunscreen using my guidelines above so you can know how much 2mg/cm2 is in real-life use.
- Set a timer to remind yourself to wait 15-20 minutes before going outdoors when using chemical sunscreens.
- Buy a higher SPF sunscreen to give yourself an extra boost in the level of protection in case you fall short in applying the correct dose.
You’re now empowered with the facts for how to properly apply sunscreen. Awesome! This is a major factor in your overall skin health and one that should not be overlooked or minimized.
In this next post, I delve into the answer for the next big question I know you are already asking…
“What sunscreen do I buy?”
In a world where there is no shortage of options at the drug store (and online), I will decode the sunscreen lingo for you. After reading this next post, you will know how to pick out a sunscreen like a dermatologist with no stress or confusion!
- Levy, SB. (2013). Sunscreens in S.E. Wolverton (Ed.), Comprehensive Dermatologic Drug Therapy (pp 551-562). Elseveir, Inc.
- Runger, TM. (2012). Ultraviolet Light in J.L. Bolognia, J.L. Jorizzo, and J.S. Schaffer (Eds.), Dermatology. Elsevier Limited.
- Williams JD, Maitra P, Atillasoy E, et al. SPF 100+ sunscreen is more protective against sunburn than SPF 50+ in actual use: Results of a randomized, double-blind, split-face, natural sunlight exposure clinical trial. J Am Acad Dermatol. 2018 May;78(5):902-910.e2.
- Young AR, Claveau J, et al. “Ultraviolet radiation and the skin: Photobiology and sunscreen photoprotection.” J Am Acad Dermatol 2017;76:S100-9.