Our early Homo sapiens ancestors in Africa were probably dark-skinned, and had the benefit of natural melanin to avoid sunburn. With the slow migration towards the north, circa 60 000 year B.C., those original inhabitants of the Earth found a different climate, colder and with lower solar radiation.
So they began to clothe themselves with the skin of the animals they hunted for food.
An additional benefit obtained was sun protection, and man slowly lost the natural skin pigmentation that acted as a sunscreen.
Later on, ancient populations in the Middle East and Egypt learnt how to knit vegetable fibers as linen and tailored their clothing.
We can observe in the chart adapted from Ke (see Fig. 1; ) the different migrations in a timescale, together with a map of radiation received from the sun at different latitudes.
Still the ancient Greeks did many of their activities (Olympic Games) nude, but otherwise used robes to protect their bodies. Basically the Roman Empire continued with these same mores, becoming in any case more sophisticated and adapting to their geographical climatic conditions in Britannia, Normandy or Constantinople.
But the western way of life changed a lot during the Middle Ages, with Christianity and the extreme religious views of the Inquisition. The human body became sinful, and was totally covered. Even paintings had to be obliterated by linen. And thus protected from radiation! And this situation continued with little change up to the XIXth century and the Victorian age.
And so we arrived at the XXth century, and World War I. When the war finished, white skin was no longer attractive . A new era arrived, wishing to live in peace, experience new sensations and enjoy life to the full. Youth, free time and tanning were fashionable and synonymous with good health. Skin was still unprotected by any kind of UV filters, only helped by re-hydration with emulsions, the first being Nivea Crème .
Clearly those messages of happiness, youth and health were seen in advertising (Fig. 2a,b).
About 1930, oils appeared as cosmetics to “protect” the body, but still without any real protection to solar radiation. While navigating between Brehat and Dinard in his boat L’Edelweiss, Eugene Schueler (founder of L’Oreal) discovered the happiness of sunbaths . He tested the oils already existing in the market at that time, and found that none satisfied him. He formulated the first “filtering” oil, Ambre Solaire, replacing old home-made based recipes based on olive oil and iodine tincture . Other brands also began producing similar products.
The first widely used sunscreen was produced by Benjamin Green in 1944 during the height of WWII, when it was likely that the hazards of sun overexposure were becoming apparent to soldiers in the Pacific. The product had limited effectiveness, working as a physical blocker of UV radiation. It was a red, sticky substance similar to petroleum jelly. Sales of this product boomed when Coppertone acquired the patent and marketed the substance .
The first effective sunscreen may have been developed by the chemist Franz Greiter in 1946 . It was called Gletscher Créme and became the basis of the company Piz Buin, which still exists as a marketer of sunscreen products. Schulze  is credited with introducing the concept of SPF in 1956, measuring the effectiveness of sunscreen when applied at an even rate of 2 mg/cm2, and the original Greiter’s cream was rated later on at SPF 2 .
Facts and figures after WWII
A Consumer Report study from the late 1940s showed that, of 61 sun-care products, five gave excellent protection, 13 gave good protection, and 31 gave no protection . It is interesting to know what was considered at the time for “good”: Maison G. de Navarre said that , for a sunscreen to be effective it should give a minimum of 2 hours protection. In the article cited from the same magazine I.R. Hollenberg said in 1955: “The underlying principle in suntan formulation is the development of a product which, when applied to skin, will form a continuous, water and sweat resistant film that will absorb UV rays which cause sunburn while permitting the tanning rays of higher wavelength to reach the skin surface.” No distinction between UVB and UVA radiation was noted.
This kind of thinking begun slowly to change in the 1960s. In 1965, Smith and Finlayson  presented an abstract to the SCC stating: “The changes commonly believed to be due to aging in human (Caucasian) skin represented primarily the effects of prolonged repeated damage to the skin from the sun.” One year later (1966) an article published by C&T reported that “sunscreen should screen the radiation producing erythema (UVB rays 280–315 nm) only to the point where the skin is protected from injury by light, while the radiation exerting a therapeutic and tanning effect (UVA rays 315–400 nm) must not be excluded from absorption!” .
By about 1967 formulators were trying to develop water-resistant sunscreens. By 1976, para-aminobenzoic acid (PABA) and its derivatives were the most used UV filters. In the 1980s it became obvious that they had a sensitizing potential and possibly a nitrosamine content. So PABA and its esters were almost abandoned.
p-amino benzoic acid & esters
All UV absorbers used in sunscreens possess aromatic moieties. The substitutions at the aromatic ring are of great importance, the increase in the number of resonance structures stabilizes the excited state, leading to stronger absorption at longer wavelengths. Most efficient are di-substituted systems in the para-position. The absorbed UV is normally released as thermal energy .
During the early 1980s Australia first, and then almost all the other countries, accepted the SPF definition as “the ratio of UV energy needed to produce a minimal erythemal dosis on protected to unprotected skin,” and the standard to test sunscreen formulations. No experimental details will be given in this paper, because the issue was covered by other authors at the syposium. Just to mention the several limitations it has, beginning with the usual solar simulators, that emit too much UVB and have a deficit of UVA compared to sunlight (Fig. 3).
And also there are questions about latitude, dose compliance, skin type and uniform application of products over the skin.
My personal notes of 1982, for the launches of Vichy and Biotherm sun protectors, still showed SPF factors ranged between 2 and 6.
Towards the end of the XXth century, the SPF of sunscreens had already migrated to values normally between 15 and 30, even if some products were still in the low range. Normally they were formulated only with a few UV filters, the most common were octylmethoxicinnamate (Fig. 4) in the UVB range and avobenzone (Fig. 5) in the UVA range. As avobenzone was not totally photo-stable, normally octyltriazone (Fig. 6) was added to increase photo-stability.
New cosmetic forms were developed, as nano and micro-emulsions dispersible by mechanical pumps, which were normally obtained by phase inversion emulsification techniques (PIT).
The idea still prevailed that between SPF 15 and 30, there was only a difference of 3% in the absorption of UV radiation due to the non-linear scale. In fact the important thing is the difference of transmittance, not of absorption, as shown in Fig. 7 .
To overcome the filtering and photo-stability limitations L’Oreal developed and patented two new filters, one was a liphopilic benzotriazole of “broad-spectrum,” called Mexoryl XL™ (Fig. 8), with absorption peaks at 303 nm (UVB) and 344 nm (UVA). The other filter in the UVA range, called Mexoryl SX™ (Fig. 9), water soluble, is a benzilidene camphor derivative, known for its excellent photo-stability. The patents are held by L’Oreal and the products are exclusively used in their products. The first is still not approved by the FDA to be used in the USA [3, 9].
For sure, the number of filters approved in the USA by the FDA continues to be fairly poor in relation to the EC, Japan and even Australia .
The UVA concern
Sun exposure can have acute and chronic consequences for the skin. Making up to 95% of UV radiation, UVA (320–400 nm) can penetrate the epidermis and cause indirect DNA damage as a result of ROS (reactive oxygen species) and RNS (reactive nitrogen species), dermal damage and matrix remodeling, including elastin fibers and collagen degradation (photoaging). UVB (290–320 nm) is more cytotoxic and mutagenic, directly inducing DNA damage. But 95% of radiation is filtered by the stratosphere . Exposed to an excess of UV radiation, cells can die directly by necrosis or, if there is damage of DNA, uncontrolled cell proliferation and cancerous diseases will take place .
Towards the beginning of the XXIst century it was already clear that UVA radiation was important in photo-aging and skin cancer, and it seemed prudent therefore to develop sunscreens with uniform protection in the whole UV range. The classic sunscreens at the time preferentially protected in the UVB. I have tested several formulas produced and sold by a world-wide first brand in the early 2000 with the BASF solar simulator . Some results are shown in Table 1.
The simulation showed a calculated SPF of 27, which is fair because the simulator only gives an estimation of SPF, but clearly the formulation did not meet any of the actual standards of “broad spectrum” protection and UVA protection. Instead, once tested an actual “broad spectrum” formula, it meets the different UVA standards used in Europe, UK and Japan, but still the labels must clarify in the USA that “ it is not proven to prevent cancer” because it does not meet a critical wavelength of at least 370 nm, as demanded by FDA Final Rule  Table 2.
Conclusions and future
Many things happened during the XXth and XXIst centuries related to sunscreens and sun protection. Most concepts changed, even those related to fashion and lifestyle. Today we understand much better the effects of radiation on the skin, mainly the need to control the longer wavelenghts and more penetrating UVA rays. The need for more and better cosmetic applications, long lasting, with good sensorial properties, that allow dose compliance and a more even application on skin surface became more apparent.
Still we find differences on the set of UV filters allowed for usage in different parts of the world, and also on the testing methods and scales to classify the different UVB/UVA protection balance requisites [14–17].
There is a trend towards higher molecular weight UV absorbers as cited by drometrizole trisiloxane and new filters introduced by BASF (as bis-ethylhexyloxyphenol methoxyphenyl triazine) with more than 500 Daltons molar mass [18, 19]. Higher molecular weight implies less risk of penetration through the skin.
The latest development is “transformer materials (TMs)”: non UV absorbing precursors that are converted to UV absorbers upon natural UV irradiation. Normally they are dibenzoyl methanes such as avobenzone, but with different substitution patterns that allow adapting the final absorbance to specific situations and need. So sunscreen formulations containing TMs show higher UV absorption after sun exposure than before .
How could we define then an “optimal sunscreen” ?
Protects against UVB and UVA.
Does reduce the quantity of the UV radiation that reaches the skin.
Does not change the quality of the UV radiation that hits the skin.
As Gavin Greenoak stated , the sun and UV radiation are not the problem, too much of them is the problem!
The author thanks to Dr. Uli Osterwalder for the permission to reproduce some of the illustrations in this paper.
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