| |
|
About Sunscreens (Updated: 20 November 2006)
A little history and an introduction
Sunlight
Sunscreen Types:
Sunscreens are Drugs (not cosmetics):
How do sunscreens work:
The “Broad Spectrum” claim:
The “Water Resistant” claim:
Other “issues”:
UV immunosuppression
Tanning
Shade
Nano Particles
The complementary approach
A little history and an introduction
The first commercial sunscreen was introduced in the US in 1928. In Australia sunscreen development began in the early 1930s in Queensland.
The desire for a tan, the beach culture, and increasing “outdoor” activities have brought with them an awareness of consequent skin damage and the demand for better photoprotection.
The discovery of the ozone hole over Antarctica, and global ozone depletion leading to an increase in terrestrial UV exposure and a shift to lower wavelengths of the ozone filter cut-off, has reinforced awareness of overexposure to sunlight.
However it cannot be emphasized enough that it is not sunlight that is the problem, but only too much sunlight.
If we consider that only the Aborigines are naturally adapted to the Australian sun, and most of us do not have the same degree of protective skin pigmentation, then we may better appreciate how much protection we have to make up by other means, and why most Australians get “too much”.
Australia remains the skin cancer capital of the world with estimates as high as two out of three Australians suffering some form of skin cancer in their lifetime. And costs to the community ranging from $200-400 million per year.
The ability of sunscreens to prevent and protect from skin cancer is hotly debated. Evidence that they do based on research with animals addresses only one form of skin cancer (squamous cell carcinoma) which is not the most commonly occurring. People based studies and epidemiology, have not been conclusive. One problem for people studies is that skin cancers often take a long time to develop (5-25 years). However, if we accept, as most do, that the ultraviolet (UV) energy which penetrates the skin and damages living cells is the most significant factor contributing to skin cancer, and then understand that a sunscreen absorbs this same energy to varying degrees before it reaches the skin, then it is not then unreasonable to conclude that the use of sunscreens will both prevent and protect from skin cancers produced by UV.
To ask whether sunscreens protect from skin cancer should be understood in the same way as asking whether seatbelts protect from terminal injury. Protection is dependent in both cases on the conditions and duration of impact. It is unreasonable to expect sunscreens to protect from skin cancer like umbrellas protect from rain.
Sunscreens are complex and sophisticated technologies which present many questions not all of which have been answered, not least because of the even more complex context in which they are used, but neither these questions or the answers to date, defeat the logic which follows from their ability to absorb UV and protect from sunburn. The remainder of this document will address these complexities.
Sunlight
The graph below shows a typical profile of the sun’s energy reaching the earth’s surface. UV (the shaded portion on the left) represents only 5% of the total spectrum which is responsible for sunburn and associated long term skin damage. [The lower the wavelength of energy the higher the biological effectiveness.] It is this UV (invisible) energy (290 – 400 nm) which sunscreens are designed to absorb, scatter or reflect.
Wavelengths below 290 nm are absorbed by the stratospheric ozone layer which is formed by the interaction of oxygen (produced by plants) and UV (03).
The UV spectrum of sunlight which is continuous, is divided somewhat arbitrarily into UVB (290 nm – 320 nm), and UVA (320 nm – 400 nm); and with this latter UVA portion further divided into UVA-II (320 nm – 340 nm) and UVA-I (340 nm – 400 nm). This last division followed experiments showing that if the effectiveness to produce sunburn was plotted against wavelength then this effectiveness decreased linearly from the shortest UVB wavelengths but with a shoulder at approximately 330-340 nm. This decrease in effectiveness is major as can be seen in the upper panel of the graphs below (plotted on a log scale). The lower panel shows the peak sunburning effectiveness of the UVB in sunlight.
Below is a summary Table of some of the facts figures about UVB and UVA:
UV facts and figures*
- UVB+A / total sunlight = 5%
- UVB / total sunlight = 0.16%
- UVB / total UV = 3%
- UVA 33x >UVB
- UVB + UVA-2 (320 – 340 nm) /total UV = 16%
- UVA-1(340 – 400 nm) to / total UV = 84%
- UVA-1from 360 nm (not in xenon arc standard solar simulator)/total UV = 63%
- UVA-1from 360 nm ERYTHEMAL EFFECTIVENESS = 3% contribution.
In sunlight at 380 nm there is 100x more energy than at 300 nm but 5000x less erythemal effectiveness. Therefore 50x the dose is required to induce "the same" erythemal effect. Thus if exposure to a single wavelength were possible, then at 380 nm, with the palest skin, and on the clearest hottest day of mid-summer, you would not get a sunburn if you were out there from dawn till dusk. At 300 nm on the same day, and at noon only you would have a sunburn in 10 minutes or less.
It should also be noted that when we simulate sunlight in the laboratory we leave out 63% of the UVA present in sunlight. And while as the table above states, this only contributes 3% of the total erythemal effectiveness of sunlight, this relationship changes with a sunscreen applied absorbing most efficiently in the UVB region. In addition, the action spectrum for skin cancer has a second peak in this UVA-I region.
* These data were calculated from the US Standard Sun and willl fluctuate somewhat when different sun spectra are used. If the conventions used by physicists for the definitions of UVB and UVA (cut-off at 315 nm) then this will also affect the proportionalities.
Sunlight in Australia is intense especially in summer, and many visitors from equally sunny climes remark upon it. Australia is blessed with relatively low pollution which permits a greater UV flux to reach the earth, and due to its position and the earth’s axis it comes a few degrees closer to the sun in summer.
Sunscreen Types:
Sunscreens are available in a variety of forms including, milks, creams, lotions, gels and sprays. By definition, all these types of sunscreen will include active ingredients which variably absorb, scatter and reflect UV energy which would otherwise enter the skin and beyond a beneficial threshold cause damage.
Sunscreens are Drugs (not cosmetics):
Sunscreens in Australia and in the US have therapeutic status. All the ingredients of a sunscreen formulation must be tested and approved for human use, and the way in which they are tested must conform to the Australian Standard (AS/NZS 2604:1998). Any skin care product which claims a Sun Protection Factor (SPF) has therapeutic status and is classified as a Primary Sunscreen. Such products must also be tested for stability and a “use by” date included on the label.
A Secondary Sunscreen is a cosmetic (like a moisturizer) which has some unspecified sun protective capability. This capability is indicated by such labeling as: “contains sunscreen”. There are questions arising from this Secondary Sunscreen status such as the completely unknown level of protection which might be conferred. However it has been well argued that it is just this question which prevents these secondary sunscreen products from being used as primary sunscreens. Recently (2006), legislation allows secondary sunscreen products like moisturisers to claim an SPF and retain a cosmetic status. This is a new departure, and it should be noted that if the product is used as moisturizer, which is to say, used “sparingly” then it will not deliver on the SPF. (Also see under “How a sunscreen should be used”.)
How do sunscreens work:
Sunscreens are filters, not “blocks”. All of them will let some UV through at varying rates indicated by the SPF number. An SPF 30 product will permit 1/30th of the sunburning energy through it in the same time as it would take to suffer a minimal sunburn without any protection at all. The whole of this definition is important to understanding how they work. It is because the sunburning dose accumulates at 1/30 of the rate that it takes 30 times as long to see the skin reddening sign that damage has occurred and repair and recovery is underway.
The graph below illustrates the above principle and how an SPF 30 product confers double the protection of an SPF 15 product (1/30 is half 1/15). The problem with using percentages and fractions is that they can be viewed statically, and it was because of this incorrect view that for many years it was said that there is only a 3% difference between SPF 15 and 30 and therefore hardly any extra benefit conferred by the latter. [This was concluded from such statements as: “An SPF 15 transmits ~ 6% of the sunburning dose and an SPF 30 only transmits 3%.” It was the dynamic and cumulative reality of such transmission that led to a false conclusion, and the graph below supplies the correcting context.]
The SPF number is derived from the time or dose required to produce a minimal reddening of the skin, divided into the time or dose required to produce the same degree of skin reddening with the product applied.
Time or dose to
minimal erythema on protected skin
SPF = ___________________________________________
Time or dose to
minimal erythema on unprotected skin
You will fairly often see the SPF number defined as how much longer you can stay in the sun without burning. Because sunscreens should be seen as a way of minimizing sun exposure, we prefer to say that the SPF number tells you how much longer it will take to accumulate the same amount of skin damage.
“Doing the numbers” (estimating your time to burn and multiplying by the SPF of the sunscreen to be used) is not encouraged. The SPF numbers should rather be seen as a way of ranking products in order of protective capability, together with other features like Water Resistance, and Broad Spectrum capability (described later).
How should a sunscreen be used (application):
The SPF of a sunscreen has to be determined on the back skin of human volunteers (in vivo). This approach is used worldwide, and will remain in place until adequate laboratory methods are developed (in vitro). The amount applied to the skin for this testing is 2 mg/cm2, and for the whole body of an adult exposed to the sun this will translate to around 35 ml (a cocktail glassful) which is much more than most people use. There is some evidence to show that people tend to use half this much, or less. Half the amount will approximately halve the protection conferred.
Sunscreens should be applied lavishly, and not worked into the skin if they are to meet the protection stated by the label claim. The maximum protection claimable in Australia is SPF 30+. This limit is not in place because SPF 30 is considered enough, but because there are limitations to the testing method which lead to errors in the determination above SPF 30. Nevertheless, estimated, or indicative SPF values are obtainable above SPF 30, and to counter the smaller amount generally used there are products on the market in Australia which while labeled SPF 30+ are in fact much higher than SPF 31 (minimum 30+). Doctors can obtain from the manufacturers the actual performance limits of such products.
A sunscreen should be applied BEFORE exposure (10-15 minutes) when possible. This will allow the product to properly settle and bind to the skin surface and afford optimum protection.
Packaging will include instructions for use such as “re-apply every 2 hours or after swimming or toweling”. Sunscreens are not like batteries which provide a complete block and then “run out” until recharged with reapplication. Sunscreens, as already stated, are filters, not blocks. While a good uniform film is maintained on the skin surface they will absorb the UV to the degree indicated by the SPF. Once a sunburning dose has been accumulated then no amount of reapplication will avoid the skin reddening or sunburn which will develop and peak some hours later.
The “Broad Spectrum” claim:
This claim addresses UVA protection.
You may often encounter statements that the SPF number is a measure of UVB protection only. This is not true. When we test a sunscreen, the solar simulators we use emit UV up to 360-370 nm which is well into the UVA-1 region and takes into account all of the UVA-II. The SPF number is a measure of protection from erythemally effective (sunburning) energy. Sunburn is the inflammatory skin response to a generalized tissue damage including DNA damage, and is the sign that repair and recovery is underway. UVA ( > 320 nm – 400 nm) is very much less erythemally effective than UVB but when a sunscreen contains mostly UVB filters then should a sunburn develop then the contribution to it by UVA will be significant. It has been said that for any product to perform above SPF 10 the UV filters must extend absorption into the UVA region. UVA does penetrate the skin more readily and may reach deeper. It has been implicated in both premature photoageing and skin cancer.
The Broad Spectrum claim current in Australia attests to a measure performed which indicates that there is some UVA filtering between 320 – 360 nm. Australia is the only country which includes such a test and claim in its mandatory Standard.
Much international debate is focused on this Broad Spectrum issue and which is the best test for it. Some believe that the test should be performed in vivo on human subjects like the SPF test but with skin pigmentation as the endpoint rather than skin reddening. The Japanese have adopted this approach. Others believe that the test should be in vitro to reduce the hazard to human test volunteers, and everyone in Australia is of this persuasion. It is probable that the label claim will remain “Broad Spectrum” but it will have more meaning than at present.
It should be noted that there is no Broad Spectrum, or UVA issue for clothing protection from UV. This is because they protect from all wavelengths equally. They reduce the amount of UV reaching the skin but do not change the quality, or shape, of the spectral curve. The degree to which a sunscreen changes the quality of UV reaching the skin can be measured using a method called the UVA Ratio. This author argues for this approach as the best since it complements the SPF measurement of quantity without making any assumptions about biological end-points. It is also the simplest, most robust and reproducible of the many methods proposed for adoption.
The “Water Resistant” claim:
A water resistance capability of sunscreens is one increasingly sought after. In Australia, it must be followed by a time period limited from 20 minutes to 4 hours. Importantly, when a sunscreen is labeled Water Resistant then the SPF number will refer to protection after this period of water resistance.
While the test supporting this claim is performed in a spa pool or swimming pool, the claim does also indicate a greater general ability of the product to adhere to the skin (substantivity).
Water is not a good sunscreen and UV will penetrate to the depth of 1 meter.
Other “issues”:
Are sunscreens safe?
The author of this paper has been doing research investigating sunscreens and skin cancer for some 24 years, and commercially testing them for 20 years. The therapeutic status of sunscreens underscores their importance for Australians and as for most drugs indicates a risk, and a benefit. The door is never closed on safety issues. Things change, and we often have to wait for time to tell that which we cannot otherwise find out with any certainty. Yet we can be fairly confident that sunscreens on the market in Australia meet all the requirements of the legislation and will do no harm if used properly, with other sun protective modalities where possible, and not used to prolong sun exposure. The primary purpose of a sunscreen is to protect from sunburn, and the risks associated with a sunburn far outweigh any known risk of using a sunscreen. As a technology sunscreens are still in the process of development in a brisk competitive marketplace.
UV immunosuppression
It is well established that UV suppresses the immune system to some extent. Many believe that this is a predisposing, state for the outgrowth of skin cancers which would otherwise be recognized as antigenic and removed. This view is consistent with a long standing belief that carcinogenic transformation changes the “self-identity” of cells into “non-self” or foreign cells which then become visible to the immune system. Thus an immunosuppression is considered a “permissive state” and prerequisite for the formation of a cancerous tumour. This is often referred to as the “sneak through” theory of carcinogenesis within an immunological context of general “immunosurveillance”. This view is certainly the dominant one, and has driven research very positively to understand the processes underlying it. However, a small but increasing body of opinion, informed not least by an understanding of the molecular genetic regulation of cellular function are challenging this prevailing view. They argue that the immunosuppressive response to UV anticipates sunburn and the possibility of potentially life threatening auto-immunity. Down regulation of immunological competence is then in order and of a transient nature. Restoration occurs once the threat is past. This of course is a radical view with far reaching implications for our understanding of cancer. And the contest here is mirrored by the sunscreen industry where only some would like to see an Immune Protection Factor (IPF) become at least an option if not a mandatory requirement.
Tanning
Tanning some say is not protective of the skin but only an index of how much UV you’ve had and thence also an index of sun damage. Dedicated tanning behaviour is not a good idea either in the sun or solaria, but incidental tanning during that “must have” exposure will confer some protection. Oils without UV filters render the skin surface non-scattering and thence facilitate UV penetration, - not a good idea.
The schema below is an attempt to put the question of “how much?” into perspective. “Balance” is the thing to aim for, or as Solon first said, “Moderation in all things”, even moderation. Too much of anything will kill you, and so will too little. (And it was Aristotle who laid the emphasis that the middle road was also the most difficult.)
Australians have come to both enjoy and respect the dangerous oceans which demonstrate many always hazardous moods on varied shores. And we might come to the same relationship with our sunlight.
It has long been thought that a tan was only the sign of how much UV damage your skin has received, and while this in itself is a view rather skewed to the alarmist side of opinion, it followed more from an understanding that DNA damage was necessary for tanning to occur. Very recent evidence (2006) topples this view, showing that cells in the upper layers of the epidermis (the keratinocytes, or squamous cells) are provoked by UV to activate an important enzymatic pathway known as Cyclic AMP (cAMP) this in turn activates the melanocytes in the basal layer of the epidermis to produce melanin pigment which is then transferred to keratinocytes. The melanin is not scattered but forms tiny umbrellas in every cell to protect the nucleus. This is an important finding.
Shade
Shade is good. But its protection from direct sunlight can be deceptive. A significant proportion of UV comes from the blue sky (albedo) and is scattered through clouds. Snow, water, grass and sand will also reflect UV. Shade on its own will often provide less than adequate protection but if the protection factor of shade is say SPF 5 and you have rather poorly applied a sunscreen conferring an SPF of only 10, then together you will have SPF 50 protection!
Nano particles
Are very small. And there continues to be concern regarding the possibility of their entrance into the blood stream, and subsequent accumulation and adverse effects. To date, there is no evidence to support a problem. And the Therapeutic Goods Administration in Australia has cleared nano titanium and zinc for use in sunscreens based on data provided to them. It is important to understand that behaviour of particles in a sunscreen will be dependent on the formulation in which they are dispersed. For example, a formulation can be readily made which binds the particles in such a way that they stay upon the surface of the skin with no chance of penetration. I say readily, but only for a well informed formulation chemist. Other formulation systems may not achieve this skin surface limitation, but that a problem will then arise has not been shown.
The complementary approach
It is good advice to try and stay out of the mid-day sun; wear a broad-brimmed hat and clothing, together-with, the proper use of a sunscreen.
| |
|
