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Sunscreen's effectiveness is measured through a Sun Protection Factor - SPF - scale. This measure has become almost the de facto measurement known worldwide today and was developed originally by the chemist Franz Greiter, in 1962. Today, the Sun Protection Factor used to describe sunscreen's effectiveness against sunburn is defined according to international standards. They correspond to the relation between the amount of UVB (Ultra Violet B) radiation that will cause sunburn on unprotected skin and the degree of UVB radiation that will cause sunburn on sunscreen protected skin.

The important aspect to know here is that the SPF measures the UVB rays burn ratio and not the UVA rays.

There are many questions asked about SPF and what it actually is, how it works, and importantly, how best one can choose which factor is right for you and your circumstances.

Research shows that people are mostly quite confused as to what protection they are actually buying when looking at the detail on the sunscreen labels, never mind what the technical terms mean, or what the difference is between UVA, UVB and SPF.

Common confusions

  • Many people mistakenly believe that the higher the Sun Protection Factor rating, the stronger the UV protection, as opposed to longer protection
  • The concept of "broad-spectrum" protection is even more confusing

Official organizations like the U.S. Food and Drug Administration (FDA) is urging sunscreen manufacturers to clarify the claims made on their products so that the public can make more informed purchases.

The following details are an attempt to clarify some of these misunderstood or confusing elements:

  • UVA (ultraviolet-A): long-wave solar rays of 320-400 nanometers (billionths of a meter). Although less likely than UVB to cause sunburn, UVA penetrates the skin more deeply, and is considered the chief culprit behind wrinkling, leathering, and other aspects of "photo-aging", as well as the main cause behind melanomas
  • UVB (ultraviolet-B): short-wave solar rays of 290-320 nanometers. More potent than UVA in producing sunburn

So UVB, being the shorter in wave-length, will burn the skin at the surface, resulting in the visible sunburn we are familiar with. But UVA will cause far deeper damage. Protecting against both UVA and UVB becomes of paramount importance. This is known as 'broad spectrum' protection.

Sun Protection Factor: measures the length of time a product protects against skin reddening from UVB rays, compared to how long the skin takes to redden without any sunscreen protection. Thus, if it takes 20 minutes without protection to begin reddening, using an SPF 15 sunscreen theoretically prevents reddening 15 times longer - about 5 hours. (Actually, it may sometimes take up to 24 hours after exposure to the sun for skin redness to become visible.) However, in order to maintain the SPF level, one is usually required to reapply the sunscreen every two hours or immediately after swimming or heavy sweating.

The Skin Cancer Foundation recommends SPF's of at least 15, with SPF's higher than SPF 30 for sun-sensitive individuals, skin cancer patients, and people at high risk of developing skin cancer. They also allow some margin for error if too little sunscreen is applied.

While SPF is the universal measurement of UVB protection, no comparable standard yet exists for UVA. Scientists worldwide are working to develop a standardized testing and certification method to measure UVA protection.

Broad-spectrum protection

The phrase indicates that a sunscreen product shields against UVA as well as UVB. It is very important to note that it does not guarantee protection against all the UVA wavelengths, however. Most broad-spectrum sunscreens with a Sun Protection Factor of 15 or higher do a good job against UVB and short UVA rays, but if they contain avobenzone, zinc oxide, or titanium dioxide, they are most likely to be effective against the entire UVA spectrum as well as UVB.

Even with the 'ideal' sunscreen, some UV rays will get through to your skin and cause damage. Sunscreen should be just one part of your sun protection program, along with sun-protective clothing, sunglasses, and shades.

How to decide on which SPF to use

By using a product with an SPF of 15, as an example, the skin's ability to protect itself from sunburn is multiplied by 15. These factors are established under laboratory conditions with a given amount of sunscreen. Applying this in the outdoors or 'real life', the sunscreen chosen should also consider your skin type, the natural conditions and should be reapplied frequently.

The first key to identifying what Sun Protection Factor to use is determining what your skin type is. The following are what are considered to be six main skin types:

  • Skin always burns easily, never tans, extremely sun sensitive, pale skin. Includes red heads, freckle faced individuals
  • Skin burns easily, tans minimally, and is sun sensitive. Includes pale skinned, fair haired people. Many Northern Europeans fall into this group
  • Skin occasionally burns, tans gradually to light brown, some sun sensitivity. Common among darker haired Caucasians
  • Skin burns minimally, tans to moderate brown, and virtually no sun sensitivity
  • Skin rarely burns, tans well, sun-insensitive skin. Includes Middle Eastern People, some Hispanics, some African Americans, and some native Americans
  • Skin never burns, deeply pigmented dark skinned, sun and extreme weather insensitive

Once you know your skin type, there are natural factors to consider:

Geographic location. The closer to the equator, the stronger the sun will be. The higher your latitude, the less extreme the sun's rays are.

Elevation. The higher the altitude, the less atmospheric filtering occurs and you are more exposed to both UVA and UVB rays. Some of the UVB rays may not reach you at low altitudes as they have been filtered out already. The UVA rays however, will most likely all reach the earth, even when there is a lot of cloud cover.

Time of year. Living on or near the Equator may not provide much variance in weather through the different seasons, but in many parts of the globe, summer is vastly different to winter! In winter our health can actually suffer from not enough sunshine, but in summer we need to be more diligent during the very hot days about how much exposure our skin can endure without getting sun-burnt.

Time of day. Taking in the above factors, the time of day to watch out for as potentially the most damaging, is when the sun's rays are most direct. In another measurement that can be used wherever you are in the world, this is when one's shadow is shorter than you are. This can be from 11am - 3pm in some places, or 12 noon - 2pm in others.

Weather. When the sun shines we go outdoors and enjoy the sun's energy and feel its heat and it is easy to know one needs some sunscreen protection. When the day is clouded however, people often make the mistake of not wearing any sunscreen protection. This is when sun burn can be it's most damaging as some of the UVB rays will penetrate the cloud cover and the UVA rays will almost all penetrate and so the damage to one's skin is potentially greater than when there is no cloud cover. In this case UVA sunscreen protection becomes more important than UVB.

Amount of tanning already achieved. If you tan gradually without incurring any sunburn, this allows your body to create increased levels of melanin in the skin. Effectively you will allow your body's own natural sun protection ability to manage the health of your skin effectively. However, once you are tanned, you move your skin type up a notch or two, which in turn, enables you to stay longer in the sun without getting burnt.

Length of time in the sun. Remember that the Sun Protection Factor level you choose to use is related to the length of time of protection, not the strength of protection.

For example, let's assume you're at sea level, have fair skin and no prior tan. From personal experience, you know that after about 5 minutes of sun, you start to burn. If you plan to be in the sun for 3 hours, you need protection equivalent to 180 minutes divided by 5 = 36.

So, if you chose to use a Sun Protection Factor of 50, multiply the SPF number by the "time it takes to burn" = 250 minutes (4 hours+) protection. Add to this the 15 minutes it takes to burn, and you have 265 minutes (4 hours 15 minutes) before you start to burn. This would allow for some added time or perhaps extra protection, especially if playing sport or such.

If you are at a higher altitude, increase the SPF level. If at a higher latitude, decrease the SPF level. If already tanned, decrease the SPF level. If a clouded day, make sure to have broad spectrum, or at least UVA cover.

SPF test process

It is very important to note that a particular SPF (sun protection factor) in a sunscreen is not achieved just by adding a certain amount of one of the sunscreen ingredients that are approved by international authorities, such as the FDA. This SPF value must be determined and then certified by official testing in a recognized laboratory, equipped to test the product's effectiveness. This is a necessary process before any such claims on a SPF level can be made.

The highest SPF values are achieved through combinations of ingredients that include the physical sunscreen ingredients zinc oxide and / or titanium dioxide, but will usually include some of the chemical sunscreen agents as well.

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