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

MNT Knowledge Center

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 includesunburn 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.

Contents of this article:

  1. What is SPF? What does “broad spectrum” mean?
  2. Advice for buying and applying sunscreen
  3. Sunscreen and vitamin D

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?1-4

[suntan lotion applied in shape of sun]
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:

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  • 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 SPF1,3,5-7

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.

[sunburnt peeling skin]
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 thathowever 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.

 

[SOURCE :-medicalnewstoday]