COSMETICS AND BATHROOM PRODUCTS:SUNSCREENS.

SUNSCREENS

In 7500 BC, Egyptian shepherds and hunters in the Nile valley used oil crushed from castor beans to protect their skin from sun exposure. An Australian chemist experimented with sunburn cream formulations in the 1930s, and a French chemist invented the first sunscreen in 1936. By the 1940s, a Florida scientist developed a suntan cream formulation, and this basic chemical composition is still marketed by a major manufacturer today.

The skin is an organ basically composed of two general tissue layers,the inner dermis and outer epidermis. Of the four different cell layers of the epidermis, it is the outermost, nonliving keratinized layer called the stratum corneum that provides a protective physical barrier against environmental factors, including UV radiation. UV light is irradiated from the sun to the surface of the earth in the wavelength range from approximately 290 nanometers. This UV wavelength range includes two different types of UV, UV-B (290 to 320 nanometers) and UV-A (320 to 400 nanometers). Although UV-A is capable of penetrating into the dermis, UV-B only penetrates into the epidermis. Both of these UV types may elicit skin changes, but the shorter-wavelength UV-B is significantly more energetic than UV-A at inducing these changes, including vitamin D production, erythema (sunburn), melanogenesis (melanin production), DNA damage, and some types of cancer (e.g., melanoma and squamous cell carcinoma). The less energetic longer-wavelength zone of UV-A radiation tends to elicit melanogenesis slowly but also may enhance the tissue-destructive effect of UV-B radiation.

Tanning is a process that occurs within the skin and involves the production of a black-brown pigment polymer called melanin. Produced by cells called melanocytes within the epidermis, light-colored unoxidized melanin granules are changed to their dark brown-colored oxidized form after exposure to UV light. This dark melanin then physically prevents the deeper layers of skin from sustaining damage from further UV expo- sure. A more sustained melanin-based tan may be achieved via a second- ary tanning stage, which involves the production of new melanin pigment from the precursor amino acid tyrosine. Subsequent UV exposure may thus cause further melanin production and lengthening of the melanin polymer chains to produce darker skin coloration.

Commercial suntan and sunscreen products act by blocking UV-B selectively, allowing UV-A through to produce a slow tan, or by blocking both UV-B and UV-A and thus shielding the skin from the entire UV spectrum that reaches the earth’s surface. Sunscreen products are manufactured to be chemically and photochemically stable, because the UV absorption capabilities of organic sunscreen active ingredients change over time. Overall, various inorganic and organic active UV-absorbing chemicals provide the sunscreen (or skin) protection factor (SPF) prod- uct rating. SPF is a ratio based on the skin-burning potential of the con- sumer with the sunscreen applied relative to lacking any sunscreen within a specific duration of sun exposure. For example, a sunscreen applied with an SPF of 6 indicates that the consumer may remain exposed to the sun approximately six times longer than if a sunscreen was not applied and achieve the same effect on the skin. Generally, higher SPF values tend to protect the skin of the consumer better because SPF is a measure of sunscreen product efficacy. Formulas must be soluble in a cosmetic base but water and sweat resistant, since active sunscreen ingredients must maintain a high concentration in the stratum corneum for several hours to ensure product goals. For many years, the active ingredient in many sunscreen lotions and creams has been para-aminobenzoic acid (PABA) or one of its esters. A UV-B protector, PABA inhibits an optimum ratio of quick tanning transmission to sunburn protection, but also has been shown to elicit an allergic response in many consumers. As an alternative, other UV-B protection active sunscreen ingredients are often used, including octyl methoxycinnamate, octyl salicylate, and homosalate (salicylic acid 3,3,5-trimethylcyclohexyl ester). UV-A protection active ingredients used include oxybenzone (2-hydroxy-4-methoxybenzophe- none), dioxybenzone, or menthyl anthranilate. Inorganic metal oxides such as titanium dioxide and zinc oxide are commonly used as nonallergenic sunscreen active alternatives, as they both display absorbing properties throughout much of the UV-A and UV-B wavelength spectrum.

As the quality of the spreading agent is vital to consumer application ability (and thus protection potential), the agreeable cosmetic emulsion or solution should spread to form a continuous, coherent, and stable film on the skin. Thus, solvents (e.g., water, C12-15 alkyl benzoate, butyl octyl salicylate, isononyl isononanoate, alkyl salicylate, octyl dodecyl benzoate, propylene glycol benzoate), emulsifiers (e.g., cetearyl alcohol, ceteareth- 20, dimethicone copolyol, propylene glycol, polysorbate 60, PEG 40 stearate), agents imparting water resistance (e.g., acrylates, cetyl dimethi- cone, PVP eicosane copolymer, maleated soybean oil), preservatives (e.g., disodium EDTA, imidazolidinyl urea), and fragrance are often added.