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Here's How Much Sunscreen You Should Apply

It's More than You Think. People Generally Use Only 1/4 of What Is Required.

Last updated: April 11, 2019

Article Summary

Sunscreens are tested using a thick coating on the skin, but the typical person puts on just a fraction of this. To make sure you are using enough, apply from ½ teaspoon to 1 teaspoon to your face and neck every 2 hours that you are in the sun, and a total of 6-9 teaspoons, or the equivalent of a shot glass, to your entire body.

Other factors, such as the time of day, intensity of the sun, and the time of year can change the amount of protection you get, so be extra conscious to use plenty of sunscreen during midday hours during the summer. 

The way most sunscreen protects is by absorbing to the sun's rays, and this reaction slowly reduces the ability of the sunscreen to do its job. After 2 hours of sun exposure, most sunscreens will lose their ability to protect the skin, so reapply sunscreen after 2 hours of sun exposure. And if you end up toweling off, and rub the sunscreen off your skin, you'll need to reapply immediately. 

Your skin is the largest organ in your body, and it is directly exposed to the sun's ultraviolet (UV) rays, so it is important to do what you can to shield against damage.1 Sunscreen can help, but for the most part, people do not apply enough--far less than they need to get the protection advertised on the label--and they forget to reapply. 

How Much To Use

The recommended dose of sunscreen is 2 milligrams per square centimeter of exposed skin.2 At this concentration, the average adult needs a little more than a ½ teaspoon (3mL) to cover their face and neck, and 6 or 7 teaspoons (33mL) to cover their entire body. 

The teaspoon rule

To get this coverage, you can follow the so-called teaspoon rule, which gives a rough idea of how much to put on each body part:2,3

  • Slightly more than ½ tsp. (3mL) on: 
    • Face and neck combined
    • Each arm 
  • Slightly more than 1 tsp. (6mL) on:
    • Chest and abdomen combined
    • Back
    • Each leg 

The problem is, most people do not follow this rule, and end up applying much less sunscreen.4 Studies have found that people typically use just 0.5mg/cm2, or ¼ of what they should.2,3,5

The modified teaspoon rule

As a result, healthcare providers have revised this rule to make it easier for people to remember. The modified teaspoon rule ups the volume to a full teaspoon on the face, and 9 teaspoons--roughly the contents of a shot glass or the size of a golf ball--across the entire body, distributed as follows:3

  • 1 tsp. on:
    • Face and neck combined
    • Each arm
  • 2 tsp. on:
    • Torso, front and back combined
    • Each leg

The Modified Teaspoon Rule

Why Too Little Sunscreen Is a Problem

Sun exposure is not all bad - our bodies use ultraviolet (UV) radiation to make vitamin D, a hormone important for absorbing calcium from the gut and keeping our bones strong.1 Vitamin D is also vital for wound healing, and since acne lesions are wounds, getting the proper amount of vitamin D could potentially help heal acne more quickly. Doctors also sometimes use UV radiation to treat people with vitamin D deficiencies, as well as people with a type of depression called seasonal affective disorder (SAD), and other diseases such as psoriasis.1

But getting too much sun has major downsides. A sunburn is skin damage, and anything that damages the skin can bite back with acne in the weeks following. This is why people often notice a breakout a week or two after getting a sunburn. Too much UV radiation can also cause skin cancer.1,6 Also, in rare cases, UV rays can weaken their immune systems, produce allergic reactions, or make certain skin symptoms worse, as in the case of lupus. 

There are three main types of UV radiation, only two of which you need to worry about:1

  1. UVA
  2. UVB
  3. UVC

UVA light is the longest wavelength light and penetrates the deepest into the skin. It is responsible for most of the sun-caused premature aging symptoms like sagging and wrinkling.

UVB light is responsible for most of the burning in a sunburn, and can cause hyperpigmentation (red/dark marks that linger after an acne lesion heals).

UVC light does not make it to Earth because it is blocked by the ozone layer, so it cannot hurt you.

Both UVA and UVB can damage DNA and chronic exposure from either type of light can lead to skin cancer.1,6

Because of these dangers, physicians recommend you always protect your skin from the sun, starting early in life. And with sunscreen, that means not skimping.

SPF Explained

SPF numbers are commonly advertised on sunscreen bottles, usually between SPF 15 and SPF 50. A SPF of 15, then, means that with sunscreen on, a person can be exposed to 15 times more UV radiation than normal before burning. Dermatologists normally recommend that people with acne use a sunscreen product with an SPF between 15-30. One of the reasons why doctors recommend such large sunscreen volumes is because they want to make sure you are hitting the 2mg/cm2 concentration, since that is what is used to determine a product's SPF, or sun protection factor. If you use less, then you will not be getting the promised protection.4

The SPF value comes from skin tests in which human subjects are exposed to UV radiation until they develop a slight sunburn. Scientists measure the amount of radiation it takes to create the sunburn when a person is wearing sunscreen, and compare that to the amount when the person is not.1,6 

Things to Be Aware of with SPF

An SPF number is a helpful guideline, but due to the following factors, protection can vary:

  • Time of day: Typically, the sun's rays are most intense at midday. For instance, 10 minutes of sun exposure at noon may be equivalent to 40 minutes of sun exposure at 9am.
  • Cloud cover: Clouds will reflect some of the UV radiation, so the same amount of sunscreen can protect you a bit longer on cloudy days.
  • Geographic location: Sun intensity increases the closer you are to the equator. For instance, a sunny day in Mexico will result in more UV exposure than a sunny day in Canada.
  • Skin tone: Fair-skinned people have less melanin in their skin and will naturally absorb more UV radiation than darker-skinned people who have more melanin, which naturally protects the skin from the sun. 
  • Sweating, swimming, or toweling off: Water or sweat can wash off sunscreen, and toweling off can rub it away. Any activity that reduces the amount of sunscreen on your skin will also increase your UV exposure,  which is why reapplying sunscreen is so important.Water resistant sunscreen formulations can help with the problem of sunscreens coming off. They stick to your skin more than traditional sunscreens, and provide 40 to 80 minutes of coverage if you are in the water or sweating. Completely waterproof sunscreens, though, do not exist.8

SPF does not tell you about UVA protection

Even if you use the recommended amount of sunscreen, you might not be getting all the protection you need. Because SPF only measures sunburn, SPF is primarily a measure of UVB rays, and says very little about UVA rays, which are also dangerous.1

To get full protection, you need to look past the SPF number and also check that the sunscreen is advertised as broad-spectrum, which means it blocks both UVA and UVB rays. Look at the active ingredients and make sure you see one of the following three broad-spectrum sunscreen actives:

  1. Avobenzone
  2. Zinc Oxide
  3. Titanium Dioxide

Higher SPF numbers can be misleading

You might think that a SPF 50 sunscreen is twice as good as one that is SPF 25. But the scale is not linear. In fact, a sunscreen with SPF 15 blocks about 93% of UVB rays, while a sunscreen with SPF 30 blocks only a little bit more, around 97%.1 The SPF 30 sunscreen is more effective, but not as much as you would think. 

Higher SPF numbers might mean too much active ingredients for acne-prone skin

There are countless anecdotal accounts of sunscreens clogging pores and breaking people out. Whether this comes from the active or inactive ingredients in a sunscreen, or a combination of the two, or whether the negative effects come from free radicals that are produced in the skin as the sunscreen absorbs UV rays, remains unknown. To be safe, if you are acne-prone, using a slightly more moderate SPF value sunscreen of between 15-30, as dermatologists recommend, may be smart.

Also, even though you cannot always believe marketing claims, choosing a sunscreen that is "for acne-prone skin" or at the very least claims in writing on the label to be "100% non-comedogenic (won't clog pores)" will at least ensure that you are buying from a company who has thought about the issue, and hopefully (fingers crossed) will mean you're buying from a company who has done comedogenicity testing on their products. 

Other Reasons Why You Might Not Be Protected, Even If You Use Enough Sunscreen

Even if you carefully use the full amount of sunscreen, there are still a couple of important things to consider to make sure you are protected.

Your sunscreen expired

Expired sunscreens may have degraded and might not provide the level of protection promised. The FDA recommends throwing out any expired bottles or tubes. If there is no expiration date, only use sunscreens that you purchased in the last three years.8

No sunscreen, perfectly applied, provides 100% protection. When the sun's rays are most intense, usually between 10am and 2pm, limit your time outdoors. And don't forget that clothing, hats, and sunglasses also provide great protection from the sun. They do not need to be reapplied, and never expire.8

Remember, sunscreen breaks down

If you're going to be outside in the sun all day long, remember that UV radiation can break down sunscreen ingredients, making the sunscreen less effective the longer you wear it. This is one reason why the FDA recommends reapplying sunscreen at least every 2 hours.8

Clothing, hats, and sunglasses protect from the sun

The Experts at Acne.org

Our team of medical doctors, biology & chemistry PhDs, and acne experts work hand-in-hand with Dan (Acne.org founder) to provide the most complete information on all things acne. If you find any errors in this article, kindly use this Feedback Form and let us know.

References:

  1. Dale, Wilson B., Moon, S., & Armstrong, F. (2012). Comprehensive review of ultraviolet radiation and the current status on sunscreens. The Journal of Clinical and Aesthetic Dermatology 5, 18-23.
  2. Kaimal, S., & Abraham, A. Sunscreens. (2011). Indian Journal of Dermatology, Venereology and Leprology 77, 238-43.
  3. Isedeh, P., Osterwalder, U., Lim, H. W. (2013). Teaspoon rule revisited: proper amount of sunscreen application. Photodermatology, Photoimmunology & Photomedicine 29, 55-6. 
  4. Mancebo, S. E., Hu, J. Y., & Wang, S. Q. (2014). Sunscreens: a review of health benefits, regulations, and controversies. Dermatologic Clinics 32, 427-38.
  5. Autier P1, Boniol M, Severi G, Doré JF; European Organizatin for Research and Treatment of Cancer Melanoma Co-operative Group (2001). Quantity of sunscreen used by European students. Br J Dermatol. 144(2):288-91.
  6. Latha, M. S., Martis, J., Shobha, V., Sham Shinde, R., Bangera, S., Krishnankutty, B., …. Naveen Kumar, B. R. (2013). Sunscreening agents: a review. The Journal of Clinical and Aesthetic Dermatology, 6, 16-26.
  7. Available from: https://www.fda.gov/aboutfda/centersoffices/officeofmedicalproductsandtobacco/cder/ucm106351.htm. Accessed on 25/09/2017.
  8. Available from: https://www.fda.gov/ForConsumers/ConsumerUpdates/ucm239463.htm Accessed on 25/09/2017.

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