By Dr. Denis Dudley
As consumers still remain confused about all things related to sunscreens, facts need to be separated from the myths and fiction. Dr. Dudley would like to dedicate this interview to Nikiforos Kollias PhD (biophysicist, photobiologist, medical physicist, bioengineer, Professor of Dermatology, Harvard and UBC). He spent the last 20 years of his life trying to dispel the first myth - that there is a difference in the way “mineral or natural” and so called “chemical” sunscreens protect against UV radiation.
The words “natural” versus “chemical” are used deceptively to imply that some sunscreens are not chemicals, and that a natural or mineral product might be preferable because it acts as a physical barrier that bends or reflects light compared to chemical sunscreens that absorb light. This absolute nonsense has misinformed the consumer since the last century. It is repeated ad nauseam that “physical or barrier” sunscreens reflect or block UV radiation and “chemical” products absorbed harmful rays. This is a myth.
All sunscreens are chemicals - it is fundamental Chemistry. A chemical is a substance in any form : ionic, molecular, organic or inorganic - that is generated by or utilized in a chemical process. Some are organic - meaning carbon based with complex carbon chains and rings in their structure. Mineral UV filters are inorganic compounds (contain no carbon atoms) like zinc oxide, titanium dioxide, iron oxide, and others. Their individual atoms all occur on the Table of Chemical Elements.
The labels physical and chemical as applied to sunscreens are inappropriate (Professor N. Kollias, Archives in Dermatology, Feb 1999). Minerals like titanium dioxide (TiO2), zinc oxide (ZnO), and others, remain as particles in a sunscreen because of low solubility. These substances are ‘physical’ since they have a predetermined particulate size but are chemicals by any definition. Even soluble organic filters will form physical crystals as the carrier base evaporates. Consumers are blitzed with the fallacy that “natural” (mineral) filters reflect or bend light like a barrier- whereas the so-called chemical agents absorb light in a chemical reaction.
Photoprotection from scattering or reflection of light occurs only if a very thick optical barrier prevents light from passing through to the skin, similar to a thick coat of paint not seen in commercially available sunscreens. This would not be acceptable to any consumer. A thicker mineral based make-up will achieve some reflection or a “barrier” effect but all sunscreens absorb photons of light in reducing sun damage. Modern mineral sunscreens using smaller particles – micronised or nanoscale – only reflect or back scatter 5% of the UV light passing through the sunscreen.
New and more efficient micronized and/or nanosized mineral filters only scatter < 10% of incident light. Even older pigment grade forms of zinc oxide and titanium dioxide with larger molecules over 1 micron in size did not reflect more than 15% of the UV rays. All mineral or insoluble UV filters act as semi-conductors and absorb photons with electron shifts to a different valence band, so a harmful wavelength is converted to a less harmful or innocuous wavelength.
The word chemical is a misnomer as ALL UV FILTERS ARE CHEMICALS. Our product developers and expert panel at Cyberder avoid formulating with small Molecular Weight soluble organic filters that reach blood and tissue. Organic does not mean natural or safe – only carbon based in the chemistry sense of the word. Many of these synthetic filters contain the 6-carbon benzene ring so inimical to humans and the environment. The mineral filters zinc oxide and titanium dioxide are still chemicals- albeit inorganic ones made by a geologic system (ie. Mother Earth) - hence the tendency to think of them as natural. Mineral filters are now so processed, highly refined, milled, doped and coated that although they are really “naturally derived” they are so altered that they are semi-synthetic.
With these facts in mind, the only accurate classification of UV filters is soluble versus insoluble and definitely not natural versus chemical. Both types mostly act by absorbing photons. The mechanism of action in mineral UV filters involves the use of photon energy to excite electrons. For example, rutile TiO2, has a band gap energy of 3.06 eV corresponding to a wavelength of 412.5 nm. Light at or below this wavelength will have enough energy to excite electrons from the valence band to the conduction band. Any photon with a wavelength longer than the band gap will not be absorbed by the sunscreen. Each substance has its unique semiconductor properties and band gap, accounting for the filtering activity at different wavelengths. This means that specific UV filters will only work against certain types of UV light. Zinc oxide remains the only mineral filter that adequately absorbs UV light through both the UVB and UVA wavelengths. Titanium dioxide absorbs UV light mostly in the UVB range and somewhat in the short wave UVA range depending on particle size. Most soluble organic filters in the US absorb UV light in the UVB range with the exception of Avobenzone that filters long wave UVA.
Most filters available in the US would be defined as soluble organic filters, examples of which include Oxybenzone, Octinoxate, Avobenzone, Homosalate, Octisalate, Octocrylene. That is not an exhaustive list but an easy way to remember would be that all North American filters are soluble organic with the exception of zinc oxide and titanium dioxide. Filters are safer and preferable not because of “natural” versus “chemical” but insoluble inorganic filters like zinc oxide (inorganic) or Tinosorb M, Tinosorb S, or Tinosorb A2B (organic) are safer since their large size prevents entry through skin into blood, avoiding all issues with hormone disruption and adverse effects. They are not photocontact allergens like the soluble organic filters. The best UVA filters belong to this group and provide the broad spectrum protection and high UVA shielding required to prevent skin cancer and photoaging.
Mineral/inorganic insoluble filters provide the best quality protection in terms of efficacy versus currently available soluble organic filters if formulated correctly. There was a previous Consumer Report that suggested that SPF retesting done by them shows which brand name sunscreens are better. They recommended as best products, several with soluble organic UV filters like avobenzone and oxybenzone, since their tests showed better agreement with the label value for SPF. The same report suggested that mineral based sunscreens like zinc oxide with or without titanium dioxide were not to be recommended as the retest SPF did not meet label claims. This is a dangerous recommendation as it may influence consumers not to use mineral sunscreens. In the right concentrations mineral products are the safest sunscreens for humans and the environment, and are the most effective in preventing sunburn, skin cancer, and photoaging.
The current method for testing SPF uses a solar lamp that emits a limited light spectrum that fails to adequately replicate the full breadth of real life sunlight. This method is ineffective at predicting how a sunscreen performs in Real Life sunlight for many reasons. Studies confirm products labelled SPF 50-100 tested in sunlight are actually SPF10-15. Professor Brian Diffey – a physicist – showed that based on laws of biometrics and physics - most sunscreens cannot achieve a SPF above 25. FDA and Health Canada have been advised by scientists that SPF lab tests and others are accurate for lotion formulations using soluble organic filters, but are erroneous for products containing particulates like zinc oxide or titanium dioxide. The tests require modifications to accurately assess the true performance of these and other particulates. Ask any consumer if they have ever had a sunburn with mineral sunscreens using proper concentrations like 22-25% zinc oxide or 15-20% zinc oxide with a filter like titanium dioxide 7.5% or special particle dispersions. Yet 70% of fair skinned consumers may return from vacation with sunburn after applying high SPF brand name sunscreens using soluble organic filters, despite re-application every 2-3 hours and label claims of water resistance. The SPF is a fallacious test and bears no relation to outdoor performance in actual sunlight. The Consumer Reports repeated this same methodology and is subject to the same types of errors.
This Consumer Report suggesting that the best sunscreens are those using soluble organic filters is untrue and detrimental to the consumer. The ability to prevent sunburn is one thing and the SPF may be a rough guide, but the prevention of skin cancer and photoaging depends on UVA shielding. The soluble small sized filters all enter your blood, including oxybenzone. The entire group are suspected hormone disruptors and may be linked to some cancers like thyroid and prostate. Worse, they give UVB-BIASED protection where the sunscreen transmits 10 times or more UVA than UVB radiation. The UVA filter used in most sunscreens is avobenzone. It is not photostable, has a similar structure to the Hawaii banned oxybenzone - likely has similar effects - and at 3-4% in a SPF 30-50 sunscreen has a UVA-PF < 10 and is inadequate to prevent sun damage. Proper protection approaching indoor shade and dense textiles comes from the degree of UVA protection. More UVA is better and high UVA shielding usually means high UVB as well, but not vice versa. Using a zinc oxide based sunscreen in the right concentrations remains the current best option for consumers in North America.
Safety is the prudent the most important criteria when selecting a sunscreen. Use ONLY combinations of these filters where available :
North America: Zinc oxide, titanium dioxide, encapsulated octinoxate, ecamsule (Mexoryl SX™), bemotrizinol (Tinosorb S™, Canada only), bisoctrizole (Tinosorb M™, Canada only)
Globally: polysilicone -15 (Parsol SLX™), iscotrizinol, octyl triazone, and bisdisulizole disodium (Neo Heliopan AP™)
None of these are hormone disruptors and only octyl triazone is a photoallergen.
Avoid sunscreens that use the following filters in any concentration: oxybenzone, avobenzone, homosalate, octisalate, octocrylene, regular octinoxate (not encapsulated), and 4-methyl benzylidene camphor
These are all small molecular weight filters that pass into blood, are suspected hormone disruptors, photocontact allergens, and likely degrade coral through hormone disruption.
Adequate filter levels is the second criteria when choosing a good sunscreen to ensure sufficient UVA protection and truly BROAD SPECTRUM protection. Zinc oxide is the only safe and effective UVA filter widely available in N. America. Look for 20-25% with or without UVB filters like encapsulated octinoxate or titanium dioxide. Any sunscreen with < 15% zinc oxide hardly achieves the UVA shielding needed to prevent skin cancer and photoaging.
About the Author: Denis K. Dudley MD, Certified specialist in obstetrics and gynecology with postgraduate qualifications in Canada, Great Britain, and the USA. Subspecialty practice was in Maternal Fetal Medicine and Reproductive Endocrinology. Faculty position at the University of Ottawa, as Director of the MFM Unit between 1979 and 1991. Married to a photobiologist/dermatologist physician, Dr. Sharyn Laughlin, a Canadian pioneer and international luminary in cutaneous laser surgery and medicine. Co-founder of The Sunscreen Company.