What is sun protection factor (SPF)? What is the best sunscreen?

Source: What is sun protection factor (SPF)? What is the best sunscreen? – Medical News Today

The rules on the labeling of sun protection lotions sold to consumers mean that all sunscreen products are labeled with a sun protection factor (SPF) and whether or not there is broad-spectrum protection against the sun’s ultraviolet (UV) radiation.

Before the US Food and Drug Administration (FDA) simplified its regulation of over-the-counter (OTC) and other consumer sun lotions in June 2012, there was a confusing array of claims that could be made about the level of protection given against harmful UVA and UVB sunrays.

The simpler rules mean that the labels now show only an “SPF” number and whether this confers “broad spectrum” protection – but what do these terms mean exactly?

And what is the best way to use sunscreen to avoid the risks of UV light, which include sunburn and cancers of the skin such as carcinoma and melanoma?

How much sunscreen should I use? When should I wear suntan lotion? Does my skin color alter the level of sun protection? The straightforward answers to these questions and more are provided below.

Countries in the European Union have also been following new cosmetics legislation since July 2013, including requirements for all sunscreens to give protection against both UVA and UVB rays, and for clearer labeling.

The universal information about SPF and other advice given here applies to Europe and other parts of the world as well as to the US.

Fast facts on sunscreen

Here are some key points about sun protection. More detail is found in the article.

  • The wavelengths of light responsible for skin damage are in the ultraviolet (UV) range
  • UVA is responsible for photoaging of the skin and some of the cancer risk; UVB causes erythema and sunburn, and is also responsible for higher skin cancer risk
  • The SPF rating indicates the level of protection against UVB while “broad spectrum” marked on the sunscreen label indicates that the product also has an effect against UVA
  • The broad-spectrum properties of a sunscreen are marked in Europe by “UVA” appearing in a circle on the label, alongside a star rating for it in addition to the SPF number.
  • The higher the SPF number, the more resistant to damaging UV light the skin will be
  • No sunscreen blocks out all UV, and all products need regular reapplication.
  • No sun lotion is waterproof – labels can claim only that there is a level of water resistance
  • Sunscreen does not protect against all exposure to UV, and different environmental conditions affect the intensity of the radiation
  • Eyes are vulnerable to UV damage as well as the skin
  • Even with the use of sunscreen, the expert advice is still to avoid going out into the high sun in summer around the middle of the day when UV intensity can be extreme.

What is SPF? What does “broad spectrum” mean?

Sunscreen blocks UV light from reaching the skin – but only to a certain degree for a limited amount of time.

SPF is the acronym for “sun protection factor.” The numerical value of the factor must be shown on sunscreen labels as an indication of the level of protection afforded against ultraviolet light.

The higher the factor number, the higher the level of protection. All products sold in the US must show the value and, alongside it, the label also has to show when the sunscreen has passed the “broad spectrum” test.

This is because while SPF is a measure of protection against mainly UVB light (and the factor was labeled “UVB SPF” in the past), the SPF number does not give a clear measure of the protection against the slightly longer wavelength of UVA.

Products sold in Europe must also be labeled with the SPF, but there is no requirement to show broad-spectrum status. Instead, regulators say that no sunscreen should be sold if it fails to offer protection against both UVA and UVB. “UVA” marked inside a circle should also be shown on labels in addition to the SPF to indicate that the sunscreen covers against both.

A useful indication of what SPF figures mean for the level of protection comes from additional guidance in Europe that all sun protection lotions should be sold with one of the following descriptors:

Treating sunburn

What is the best way to soothe the pain, aid the healing and avoid the complications of sunburn?

  • Low protection – SPF below 15, marked either 6 or 10 (whatever the exact factor measured from 6 to 9.9 or 10 to 14.9)
  • Medium protection – SPF 15 and over, marked 15, 20 or 25 only (15-19.9, 20-24.9 or 25-29.9)
  • High protection – SPF 30 and over, marked either 30 or 50 (30-49.9 or 50-59.9)
  • Very high protection – SPF over 50 and marked 50+ (but has to be measured as at 60).

Conversely, the US has dropped the use of “low, medium, high and highest” – the broad-spectrum stamp is instead based on a simple pass-or-fail level of UVA protection.

To pass the broad-spectrum test used by the FDA, the level of UVA protection must increase proportionally with the SPF value that indicates protection from sunburning UVB.

The same is recommended in Europe, except that the existence of both protections is marked on labels by the use of “UVA” inside a circle in addition to the SPF, and a star rating of this wider protection is shown from 1-4.

Broad-spectrum protection cannot be claimed for any product that has an SPF below 15, and all products sold in the US 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.”

The scientific meaning of SPF

SPF is not an arbitrary measure of low-to-high sun protection; it is a scientific measure of how much lower the risk of skin damage is due to how much longer it takes for enough UVB to get through a sunscreen and cause sunburn, compared with the time this takes when there is no sunscreen applied.

The factor is simply the numerical result produced when the sun radiation dose needed to cause skin reddening (erythema) with sunscreen on is divided by the dose needed without sunscreen:

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

For sunburn radiation dose, the scientists use “MED” – short for minimal erythema dose, which is the threshold amount of sun radiation (mainly UVB) that produces sun-reddened skin.

The sun dose used in the ratio producing an SPF for a sunscreen is measured when 2 mg of the sunscreen has been applied for each square centimeter 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.

The amount of time taken to sunburn is, however, the potentially misleading feature of SPF. The radiation dose needed to cause sunburn is worked out from the extra time needed for the same amount of UV to get through a sunscreen, but this does not translate into advice that we should stay in the sun 15, 30 or 50 times longer according to the SPF number.

For one reason, that would mean staying in the sun to get the same amount of damaging sunburn as would happen without protection, albeit more quickly.

The purpose of sunscreen is to block UV light. Part of this light, UVB, causes skin reddening (erythema) and, worse, sunburn and skin peeling.

The idea is to avoid sunburn altogether, but by having the ability through sunscreen to also get some time in the sun before any erythema can occur. It is also important to remember that however much UV light is blocked by a sunscreen – however high the SPF – not all of the radiation is blocked, and the blocking effect wears off after a maximum of 2 hours, after which the lotion will need to be reapplied.

If the UV conditions outside mean that it would take just 10 minutes for unprotected skin to start going red, an SPF 30 sunscreen would theoretically prevent this for 300 minutes – 30 times longer at 5 hours. But the lotion would still need to be topped up at least every 2 hours to maintain this level of UV block.

Again, such a picture of UV-blocking power is just theoretical; as we will see on the next page about buying and applying sunscreen, the UV index varies widely in the forecast, the time of day, and so on, as does the effect of our activities on the effectiveness of a protective lotion.

Another interpretation of the scientific meaning of SPF is to see how much of the sun’s UVB rays are blocked out for each factor level:

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

These percentages show that no sunscreen blocks out absolutely all UVB, and also that it takes small increases in the percentage blocking power to have relatively large effects against the level of harmful exposure.

Or, to put it vice versa, allowing through relatively small extra amounts of UV radiation has a disproportionally larger effect on the risk of erythema.

Blocking out 93% of UVB at SPF 15, down from 97% for SPF 30, produces a much bigger jump in terms of solar damage to the skin – losing those four percentage points against UVB rays translates to a loss of a whole half in the protective power against sunburn.

The percentages of UVB rays blocked by the SPF are also not direct measures of the protection against UVA rays, hence the importance of selecting “broad-spectrum” lotion.

Broad-spectrum sunscreens should produce a power of UVA blocking that is proportional to their SPF power against UVB (and labeling requirements ensure this – “broad spectrum” is marked on products in the US while UVA star ratings are used in Europe).

UVB is the particular wavelength that produces skin reddening, but it is not alone in its ability to damage skin. While UVA is not the sunburning radiation, it is responsible for photoaging effects such as wrinkles, and it is a skin cancer risk as well as UVB.

Advice for buying and applying sunscreen

Advice for buying sunscreen is generally straightforward. Simply select a sun product with:

SPF 15 or higher (in Europe, SPF 15-30 is also marked “medium” protection, “high” accompanies SPFs of 30-50 and “very high” is 50+)
Broad-spectrum protection (European products should all be broad-spectrum, and labels show a UVA protection rating to indicate this, which should show 4 or 5 stars for good protection).

To be sure of the level of sun protection and other safety factors claimed by your chosen product, other concerns come into play, as does basic personal preference.

The Environmental Working Group (EWG), a non-profit organization in the US, offers help for assessing sunscreen quality. The EWG does research to evaluate the following:

Hazards – whether any health hazards are presented by any of a sunscreen’s listed ingredients, according to dozens of databases giving industry, academic, regulatory and toxicity information
Effectiveness – double-checking the claimed level of protection from the sun’s UVA and UVB waves, indicated by standard industry absorbance measures for UVA and the SPF rating for UVB, and noting the balance between these two
Stability – using an EWG database and other research to see how quickly a sunscreen ingredient breaks down in the sun.

A large number of recommended products have passed the tests (although hundreds of products did not meet the mark). See the 235 beach and sports sunscreens that meet the EWG criteria.

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.

Getting the best out of sunscreen

Any sunscreen – whatever the SPF, chosen brand or formulation – is only as good as its user.

Key points to remember about optimal protection include:

  1. Applying sunscreen is only one of a number of measures to reduce skin exposure to UV light
  2. Sunscreen may not cover the skin uniformly, even when applied liberally
  3. The FDA and others suggest that 1 oz of sunscreen – enough to fill a shot glass – is needed for the average body size
  4. Wider tips against too much sun (see below) apply to other exposure risks, too, such as to the eyes – overexposure to UV light can cause or exacerbate cataracts (UV plays a part in around 20% of cataract cases worldwide)
  5. Even with sunscreen applied, avoid being under the sun when it is highest in the sky, particularly in the summer – UV radiation is at its most intense during this time of day and in this season because the sun’s position means there is less of the earth’s UV-absorbing atmosphere between the sun and our skin
  6. When the UV index is raised, stay in the shade around the midday hours, between around 11 am and 3 pm, and certainly around 12 pm to 2 pm – the World Health Organization (WHO) advises us to “limit exposure to the sun” between 10 am and 4 pm

Information from the FDA helps to quantify the high UV intensity around noon – they say that the same amount of solar energy reaching us after just 15 minutes at 1 pm needs a whole hour of sun exposure at 9 am, making the exposure four times as intense when the sun is up fully.

All the scientific advice is that sunscreens do not have a long-lasting effect after application. Regulators have even prohibited any claims on labels that lotions can last all day or are completely waterproof.

In the US, for example, any claims of water resistance must indicate for how long water activity can be pursued before resistance is lost and protection against UV should be reapplied.

The advice on application – and reapplication – for all sunscreens is:

  • The more lotion used, the better – the minimum amount in tests used to produce the SPF number is 2 mg of lotion on every square centimeter of exposed skin – 2mg/cm2 is roughly equivalent to at least 6 full teaspoons to cover the body of an average adult (about 1 oz according to the FDA, or about 36 g according to British dermatologists)
  • Further translation means that this roughly equates to more than half a teaspoon of sunscreen for each arm, and for the face and neck (not forgetting the ears), and just over one teaspoon on each leg, with the same again for each of the front and back of the trunk
  • There are bound to be differences in the amount of product needed to be applied according to its formulation – users should follow the minimum requirements given on labels, whether for lotions, creams, mousses, sprays or gels
  • There is obviously no protection at all if going out into the sun before sunscreen is applied – it is recommended to put it on about 15 minutes before going out
  • All official recommendations tell us to reapply sunscreen at least every 2 hours
  • One review of the evidence(6) suggests this is not sufficient for the real-life use of products, concluding that reapplication should instead be every hour. Using the full 1 oz of SPF 15 or higher every hour is more practicable than expecting users to adhere to the proper application of the full amount of higher factors that tend to be thicker in consistency. The authors’ message adds that this strategy also helps to overcome the near inevitability that we do not properly cover all exposed skin with each application. They also concur with the advice to apply sunscreen before any sun exposure.

The UV index

High in the sky – when UV radiation from the sun reaches us at its greatest intensity.

Many weather forecasters and government agencies follow the UV index (UVI) established in a collaboration between the WHO, UN and an international weather science organization. This index is designed to give warning of the protective action to take when UV levels are likely to be high.

The scale runs from 1 and 2 (both color-coded green) to 8, 9 and 10 (all in red), and the index can go all the way to 11 – a purple warning “to take all precautions because unprotected skin and eyes can burn in minutes” under what are described as extreme UV conditions.

The US Environmental Protection Agency (EPA) have published a detailed set of actions that they advise people to take at different levels of UVI warning.

The WHO have simplified the UVI response in their advice on the scale:
“0 to 2 – you can safely enjoy being outside
3 to 7 – seek shade during midday hours. Slip on a shirt, slop on sunscreen and slap on a hat
8 – Avoid being outside during midday hours. Make sure you seek shade. Shirt, sunscreen and hat are a must.”

High-risk skin types and activities

Fair-skinned people are likely to absorb more solar energy than dark-skinned people under the same conditions.

Darker skin is not itself a complete protection from the harms of UV light, however – it is just that there is a lower risk with darker skin because it has more melanin, one of the biological absorbers of UV.

Conversely, people with red hair and freckles have a particularly high sun risk because the type of melanin in their skin is pheomelanin, as opposed to the eumelanin of people with more common skin types.

The British Association of Dermatologists, in 2013, published what it calls the “Skindex” – a chart of skin colors showing the correspondingly higher risks of UV exposure as skin gets lighter.

Effect of activities on sun exposure risk: as well as noting the maximum amount of time that a water-resistant product can offer protection before it must be reapplied by swimmers or people enjoying the sea, putting on a new dose of sunscreen earlier is also needed for other activities that are higher-risk in terms of sun exposure.

People engaged in a sweat-inducing activity, for example, are at higher risk simply because sunscreen dilutes away, and also gets rubbed away more with physical activity.

Other groups who should take more care are people doing work or leisure at higher altitudes, which includes the reflective risk of snow-covered landscapes as well as the lower amount of UV light getting absorbed by the atmosphere (see UV index).

Beyond sunscreen

Sunscreen lotions do not prevent all UV exposure in all conditions. There is a need to consider the wider ways in which we are exposed to harmful sunrays and the different conditions that affect our risk.
Environmental factors affecting UV exposure

The best time to avoid the sun is around noon and doing so is the best way to cut out high exposure to UV light.

There are ways in which environmental UV light exposure varies. It is important to think about other conditions beyond sunscreen and the time of day:

  • UV light gets through in almost all weathers, not just when it is sunny.
  • From the ultraviolet light that hits snow, sand and metal, up to 90% can be reflected, reaching us even when we are not directly under the sun
  • Waves and ripples can reflect up to 15% of the UV light that hits seawater; conversely, none is bounced back from still water surfaces, such as a very calm lake or pool
  • For up to 1 meter below the surface, UV transmits through water, and so swimming still exposes us to high levels
  • Being in the shade does not provide consistent protection, although the amount of exposure is cut significantly – between a half and nearly all of the direct UV light is stopped from reaching us when we find shelter from the sun
  • Sources of shade outdoors range from the most protective in densely wooded areas to the least protective under beach umbrellas (UV light both gets through umbrella materials and is also reflected by the beach)
  • UV light also gets through on cloudy days. Different cloud conditions have a wide range of effect in blocking ultraviolet, with levels reduced anywhere between just 10% and as much as 90% depending on factors such as fog, haze, clouds and pollutants. Follow forecasts of the UV index to get an indication of high UV levels in your area.

Clothing with higher UV protection

Clothing with the following qualities offer higher levels of UV protection (and clothes with the opposite qualities give lower protection):

  • Lighter-colored clothing may stay cooler, but darker fabrics offer better protection from the sun’s UV light.
  • Tight-weave fabric and washing that causes some shrinking (versus loose-weave fabric)
  • Denim, wool and synthetic fabrics (versus cotton, linen, acetate and rayon)
  • Thick fabric (versus thin)
  • Dry fabric (versus wet)
  • Treatment with UV absorber (versus washing with only water; some fabrics are sold pretreated)
  • Dark color (versus light fabric)
  • Unbleached fabric (rather than bleached)
  • Wide-brimmed hats (versus brimless).

Manufacturers who make sunblocking claims for their clothing are doing so only under voluntary codes, and regulations vary worldwide about the standard measures to be shown on labels. As a result, the above general tips about fabrics may be more useful than looking for specific claims.

Some of these factors may not be so obvious – for example, while darker fabrics get warmer under the sun than lighter ones, lighter fabrics are in fact not as good at blocking UV. However, summer garments are commonly white, or light in color.

Sunscreen and vitamin D

Guidance to almost completely cut out UV light exposure with sun avoidance and sunscreen presents a dilemma.

The advice is definitively proven to be a direct way of reducing risks such as sunburn and skin cancer, but it also cuts out the major source of vitamin D.

UVB-initiated production in the skin is the biggest source of our vitamin D, which is vital to health. An estimated 90% or more of our daily need gets produced this way, with the remainder coming from our diet.

Deficiency of vitamin D is associated with numerous disease risks, and this problem is much more prevalent in people living less sunny lives.

There is no question that sunburn is bad, and there is no need to get anywhere near this much exposure for the skin to produce enough vitamin D.

The risks of deficiency from too little UV radiation (UVR) may be just as important as the skin cancer risks of too much, as outlined by WHO, who use a simple graph to show a spectrum running from low UVR exposure to high exposure, plotted against the corresponding disease risks.

But blocking out exposure to UVB to the detriment of vitamin D, for the benefit of protection from skin DNA damage and other risks, is not considered controversial by those who argue that the answer can lie in dietary supplementation of the vitamin.

Such arguments also emphasize the fact that links between vitamin D deficiency and, for example, internal cancer, are not well-established, whereas the link between UV and skin cancer firmly is.

Rather than having to think about the cost and obligation of taking vitamin supplements while trying to hide completely from all the sunrays, however, could developments in sunscreen technology provide new answers?

The following development introduces a theory outlined by researchers who believe sunscreens could give our skin UV protection for time spent under the sun without affecting vitamin D production.

The researchers say that “an SPF of 30, properly applied, reduces the capacity of the skin to produce vitamin D by 97.5%,” but that sunscreen materials could be used in such a way as to overcome this problem.

Chemical compounds typically used in a sunscreen efficiently absorb “varying wavelengths of UVB radiation,” so certain compounds could be selected to produce a balance of sunburn protection versus vitamin D production.

The people behind the paper’s theory include a scientist working for a UV lamp company and another working for the chemicals giant BASF, although the authors also include an endocrinologist from Boston University Medical Center, MA. They conclude:

“In theory, the judicious choice of compounds that have [fewer] UV-absorbing properties at wavelengths near the peak action spectrum for previtamin D3 production, i.e. 295 nm, while retaining the ability to efficiently absorb the rest of the solar UVB radiation, would provide a sunscreen that still had its anti-erythemal [anti-reddening] properties while permitting selective UVB radiation to be transmitted into the skin to produce previtamin D3.”

The full UVB range of light wavelengths is 280-320 nm, and the authors show that the peak range for causing sunburn is around the 305-311 nm range.

They show that the peak range for previtamin D3 dose production happens at slightly shorter wavelengths and that, because a much smaller dose is needed, sufficient D3 can be gained at a still shorter point along the spectrum. So, if only the 295 nm UVB wavelength is allowed through by a sunscreen for the same amount of time that it is applied against sunburn effects, enough vitamin D can be made.

What is vitamin D?

Vitamin D supports the development and maintenance of bones and teeth
The richest food sources are oily and fatty fish, especially herring and salmon.

Ideas about encouraging safe sun exposure for vitamin D benefit may remain controversial, however, because of the tension between different sunbathing messages.

WHO, for example, state that most people get enough vitamin D without having to go out under the sun more.

While a “zero population exposure to UV would generate a substantial burden of disease through diseases of vitamin D deficiency,” most people, the WHO add, are “casually exposed to UV radiation such that extremely low vitamin D levels are rarely found.”

This overview of the present guidance leads us to the conclusion that the sensible sun advice outlined above probably is, on balance, about right, and the improved clarity given by sunscreen labels can only help with this.

Written by Markus MacGill


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