Sun protection factor (SPF): What is the best sunscreen?

Sunscreen is an important way of protecting the skin against the sun’s harmful rays. With such a wide range to choose from, how can we know which is the best option?

Sun protection products are sold because they protect against cancer and other health conditions. For this reason, the United States Food and Drug Administration (FDA) consider them drugs and regulate their sales and labeling.

By law, a sunscreen product’s label must include a sun protection factor (SPF) number to indicate the level of protection from UVB rays, and state whether or not it has passed a broad-spectrum test against the sun’s ultraviolet (UVA) radiation. The sunscreen innovation act (SIA) governs how this must be done.

Read on to find out what SPF and broad-spectrum mean, and get some tips on how to use sunscreen, how much to use, when to use it, and how other factors affect the need for protection.

What is sun protection factor (SPF)?


Sunscreen should have an appropriate SPF number and be labelled “broad spectrum.”

When we spend time in the sun, we are exposed to two potentially harmful types of ray: UVA and UVB.

Sun protection factor (SPF) is a number, for example, SPF15. It indicates how much protection a product offers against UVB light.

A product with a higher SPF number will offer greater protection. All products sold in the U.S. must show this value. They must also show whether the sunscreen has passed a broad-spectrum test.

According to the FDA, the use of a sunscreen with an SPF of 15 or above, combined with other measures, such as wearing sunglasses and avoiding the midday sun, can help prevent skin cancer.

In some places, protection levels are expressed as follows:

  • Low protection: SPF is below 15
  • Medium protection: SPF is 15 to 29
  • High protection: SPF is 30 to 49
  • Very high protection: SPF is over 50

This system is not used on labels in the U.S.

Apart from an SPF number, in the U.S., the label must show that the product has passed a broad-spectrum test used by the FDA. The level of UVA protection must increase proportionally with the SPF value that indicates protection from UVB.

No product that has an SPF of under 15 can claim to offer broad-spectrum protection.

All products sold in the U.S. below this factor must include the following warning on the label:

”This product has been shown only to help prevent sunburn, not skin cancer or early skin aging.”

What does SPF mean?

SPF is a scientific measure. It gives an idea of how much lower the risk of skin damage is due to the use of a sunscreen.

It focuses on the time it takes for UVB rays to get through a sunscreen and cause the skin to go red, compared with the time this takes when there is no sunscreen.

The factor is calculated by dividing the sun radiation dose needed to cause skin reddening with the dose needed to cause reddening without sunscreen.

SPF = sunburn radiation dose with sunscreen / sunburn radiation dose without sunscreen

This calculation is based on the application of 2 milligrams (mg) of sunscreen for each square centimeter (cm) of skin surface.

If it takes 15 times longer to burn the skin with a sunscreen on than it does with no sunscreen applied, the SPF is 15.

In theory, if, under certain UV conditions, it would take 10 minutes for unprotected skin to start going red, an SPF 30 sunscreen would prevent this for 300 minutes, or 5 hours, which is 30 times longer.

It is wrong to think, however, that with a higher SPF, a person can spend longer in the sun.

Other factors have an impact.

These include:

  • weather conditions
  • time of day
  • skin type
  • how the lotion is applied
  • how much is used
  • other environmental and individual factors

Most people, for example, only use 25 to 50 percent of the recommended amount.

In addition, the blocking effect wears off after a maximum of 2 hours. After this, the lotion will need to be reapplied.

What percentage of UV rays are blocked by SPF?

The protection offered by sunscreens of different SPFs is as follows:

  • SPF 15 blocks about 93 percent of all UVB rays
  • SPF 30 filters out 97percent
  • SPF 50 is an almost complete UVB block, at 98 percent

These percentages show that no sunscreen blocks all UVB. It also shows that an apparently large increase in SPF will boost the blocking power by only a small percentage.

What is broad spectrum?

Sunscreens help protect the skin from potentially harmful UVB and UVA rays. UVB causes the skin to go red, and UVA does not.

However, UVA can lead to photoaging, including wrinkles. Both UVA and UVB increase the risk of skin cancer.

If a product with a specific SPF effectively blocks UVB rays, this does not guarantee UVA protection.

For this reason, it is important to look for a suitable SPF, but also a broad-spectrum or full-spectrum lotion.

Broad-spectrum sunscreens block UVA rays in the same proportions that SPF blocks UVB.


Sunscreen labeling set down by the FDA. Look for “broad spectrum” and an SPF of at least 15 to reduce sun risk. Products must also show a drug facts box and cannot claim to be waterproof, only water-resistant.
Image credit: FDA

Buying and applying sunscreen

People are advised to use a sunscreen that:

  • has an SPF of 15 or above
  • offers broad-spectrum protection

The Environmental Working Group (EWG) is a non-profit organization in the U. S. that assesses, among other things, sunscreen quality.

They evaluate the following:

  • Hazards: Do the listed ingredients pose any hazards, for example, toxicity?
  • Effectiveness: The EWG checks the claimed levels of protection and notes the balance between them
  • Stability: How quickly does an ingredient break down in the sun?

Before buying a sunscreen, you can check the product according to the EWG’s assessment.

A wide range of sunscreen products are available for purchase online.

Tips for using sunscreen

Other important factors will affect how well a person is protected. Application is important, as are a number of measures that are not related to sunscreen use.

Here are some examples:

  • The amount of sunscreen used: One ounce of sunscreen — equivalent to a shot glass — is considered a suitable amount for a single application for the average body size
  • The use of other measures: Clothing, sunglasses, and hats are also important for reducing skin exposure to UV light
  • How uniformly the sunscreen is spread: This is difficult when some parts of the body rub or sweat more than others
  • How much is used: One ounce of sunscreen is considered a suitable amount for the average body size
  • The time of day: It is best to avoid the sun when it is high, for example, between 10.00 a.m. and 4.00 p.m. The rays are stronger at this time.

According to the FDA, the amount of solar energy that reaches us in 15 minutes at 1.00 p.m. is the same as we would receive in one hour at 9.00 a.m. In other words, when the sun is high, exposure is four times as intense.

There is no evidence that any sunscreen can offer whole-day protection or that any product is completely waterproof.

How often and how much?

Sunscreen should be applied around 15 minutes before going out into the sun and reapplied at least once every 2 hours. In water, it should be reapplied every 40 minutes, or more often if the instructions say so.

The amount of sunscreen used in tests to produce the SPF number is 2 mg of lotion on every square centimeter of exposed skin, or around 6 full teaspoons to cover the body of an average adult.

This is roughly equivalent to half a teaspoon of sunscreen for each arm, and for the face and neck and ears, and just over one teaspoon for each leg, the front, and the back of the trunk.

The amount needed will also depend on its formulation, whether lotions, creams, mousses, sprays, or gels.

Am I safe out of the direct sunlight?

It is not only direct sunlight that poses a risk.

  • When UV rays hit snow, water, metal, and certain surfaces, they can reflect back on the skin, increasing exposure.
  • UV rays can penetrate up to 1 meter under water. People who are swimming can still burn. Water-resistant sunscreen protects for up to 40 minutes in water.
  • We are still exposed to UV rays when we are in the shade, under a beach umbrella, and during cloudy weather.
  • Densely wooded areas offer good protection.

Skin types

The risk of skin damage depends to some extent on the type of skin and the activity.

  • Very fair skin: There is a high risk of sun damage because the melanin content is different.
  • Fair-skin: This absorbs more solar energy than dark-skin under the same conditions.
  • Darker skin: This is also susceptible to the harms of UV light, but to a lesser extent, as it contains more melanin, one of the biological absorbers of UV light.

High-risk activities

Activities that can increase the need for more frequent application include:

  • swimming
  • anything that increases sweating
  • physical activity that causes sunscreen to be rubbed off
  • skiing and other high-altitude activities, because less UV light is absorbed by the atmosphere

The UV index

The risk of exposure also depends on environmental factors, such as pollution, location, the time of year, and altitude.

The UV index (UVI) is designed to give warning of the protective action to take when UV levels are likely to be high.

Weather announcers and news outlets often refer to it. The Environmental Protection Agency (EPA) provides a search tool for checking the UVI of your local area.

Or try this tool, also provided by the EPA:

Levels 1 to 2 are color-coded green, as they pose the least danger. Levels 8, 9 and 10 are coded red. Extreme UV conditions, at level 11, are coded purple. At this level, unprotected skin and eyes can burn in minutes.

The following actions are recommended at each level:

  • 0 to 2: No special precautions are needed
  • 3 to 7: Seek shade during midday hours, use sunscreen and wear long-sleeved clothing and a hat
  • 8: Seek shade and avoid being outside during midday hours and seek shade. Long sleeves, sunscreen, and a hat are essential.

Protective clothing


Other sun protection tips include wearing a hat with a brim, having long sleeves, and avoiding the midday sun.

The sun’s rays can pass through some fabrics, and clothing can be chosen for the protection it offers.

Fabrics that absorb UV rays or prevent them from passing through are best.

For example:

  • thick and tight-weave fabric, such as denim, prevent the rays from passing through to the skin more than loose-weave fabric
  • fabric that is treated with UV absorber, either purchased with a UPF label or treated at home
  • dark colors, bright colors, and unbleached fabric, as they absorb more UV

Wide-brimmed hats also offer more protection than hats without a brim, and wraparound glasses protect the eyes more fully than small lenses.

Darker fabrics get warmer under the sun than lighter ones, but lighter fabrics allow more UV rays to pass through.

Special, sun-protective clothing is available for purchase online.

Does SPF block vitamin D?

Using sunscreen and avoiding the sun will reduce the risk of skin damage, but it can also reduce levels of vitamin D.

The skin produces 90 to 95 percent of our vitamin D when it is exposed to UVB light. The remaining 10 percent comes from dietary sources.

Vitamin D deficiency is associated with osteoporosis, rickets, and other health conditions. It is more common among people who do not spend time in the sun.

Overall, a small amount of sun exposure every day is probably the best option.

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Final word

The FDA recommend taking measures to prevent sunburn and other sun-related health problems.

They advise:

  • using a broad-spectrum sunscreen with an SPF value of 15 or more
  • following other sun-protective measures, including staying out of the sun at peak times and seeking shade
  • wearing protective clothing, hats, and sunglasses

They continue to monitor the sale and science of sunscreens to ensure protection for consumers.

Research published in 2007 about the pros and cons of sunscreen use warned that “Sunscreens should not be abused in an attempt to increase time in the sun to a maximum.”

The research team recommended “covering the body with clothing and avoiding intense sunlight” to prevent overexposure to the sun’s harmful rays.

By Marcus McGill

Source: Sun protection factor (SPF): What is the best sunscreen?

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