What is in a sunscreen spray, and why?

This is a guest post by Gary Neudahl who is currently Product Application Manager, Personal Care Ingredients, for the HallStar Company and is based at the HallStar Manufacturing and Technical Center in Bedford Park IL USA.

In this post, we move from general hand & body care to sun protection. With the issuance of a new FDA Final Rule that goes into effect later this year, there has been a heightened interest in the OTC Sunscreen Drug Products category. This rule requires sun protection products for the U.S. marketplace to move a step closer to the quality of protection provided in the EU, Japan, Australia and most of the rest of the world. And with rising instances of skin cancer in young adults, it is none too soon.

Sunscreen UVB history

For many years, sunscreens marketed in the U.S. have provided protection against UVB radiation (290 — 320 nm), the rays that are responsible for sunburn, that is comparable to that of similarly rated products around the world. A sunscreen’s sun protection factor (SPF) is the ratio of the minimum erythemal dose (MED) — the amount of UV radiation it takes to turn the skin slightly red — of skin protected with sunscreen to the MED of unprotected skin. Higher SPFs mean greater protection against burning (and, presumably, skin cancer). Sounds good, but UVB is not all the sun delivers!

UVA protection

Sunscreens marketed in the U.S. must soon also provide protection against UVA radiation (320 — 400 nm), the rays that are commonly known as the “tanning rays,” to avoid sporting a warning label. That’s because the longer wavelengths of UVA rays allow them to penetrate more deeply into the skin than UVB rays – through the outer layer, the epidermis, all the way to the dermis. The only visible short-term effect of UVA irradiance is skin tanning, and short wavelength UVA exposure actually has a beneficial effect in children, converting 7-dehydrocholesterol into vitamin D, thereby reducing the incidence of rickets, a bone-softening disease.

But as with UVB irradiation, long-term UVA exposure has detrimental effects. In the dermis, it degrades collagen, a critical structural component, resulting in sagging and wrinkling of skin. And UVA radiation contributes to the incidence of basal cell and squamous cell carcinomas, as well as malignant melanoma. Like UVB radiation, it decreases the number of Langerhans cells, which lowers resistance to infection.

Limiting the sun’s effects

So there are good reasons to limit sun tanning, and one way to do that is to apply a broad-spectrum sunscreen (one that significantly reduces the amount of both UVB and UVA rays that reach the skin). A sunscreen’s UVA Protection Factor (PFA) is determined by measuring persistent pigment darkening (PPD), the appearance of brownish skin pigmentation within three hours of exposure to UVA. Products with higher PFA provide better resistance to persistent pigment darkening. Rather than using this in vivo measure of UVA protection, the FDA chose to use the critical wavelength — the wavelength at which the integral of spectral absorbance curve reaches 90 percent of the integral over the UV spectrum from 290 up to 400 nm.

Therefore, the higher the critical wavelength, the greater the level of UVA protection. The FDA’s new Final Rule requires a critical wavelength of at least 370 nm (and an SPF of at least 15) for a sunscreen to be called “broad spectrum” and to avoid a “Sun Alert” warning that includes the statement “This product has been shown only to help prevent sunburn, not skin cancer or early skin aging.”

Formulation

Sun protection includes not only classic “beach wear” sunscreens, but also the “daily wear” sun protection that is built into lip balms, color cosmetics and general skin care products. Consumer product forms from which sun protection may be delivered include creams, lotions, gels, sprays, and sticks. Because we looked at emulsion-based lotions last month, we’ll now examine sunscreen sprays, with the following typical composition:

Volatile Carrier: 50 — 80%
Humectants: 0 — 4%
Sunscreen Active Ingredients: 2 — 40%
Fragrance: 0 — 0.5%
Water-Resistance Polymer: 0.8 — 2%
“Label Copy” Ingredients: 0 — 0.5%
Sunscreen Solvents: 0 — 10%
Other Active Ingredients: 0 — 0.5%
Photostabilizers: 0 — 6%

The most common volatile carrier for sunscreen sprays is anhydrous ethyl alcohol, which produces a solution rather than an emulsion, so fewer ingredients are required. The alcohol must be essentially anhydrous to promote sunscreen active ingredient solubility.

FDA Sunscreen Monograph

The sunscreen active ingredients are selected from the FDA monograph, using authorized levels and combinations (e.g., Homosalate and Octisalate or Octinoxate with Avobenzone). Generally speaking, the higher the level of sun protection, the greater the quantity and number of sunscreen active ingredients that will be utilized. Zinc Oxide and Titanium Dioxide, although approved for use, are unsuitable in alcohol-based sprays because they are not soluble in alcohol, and their high densities would result in their rapid settling during storage.

Sunscreen ingredients

A water resistant polymer, such as Acrylates/Octylacrylamide Copolymer, is required for two reasons. First, it helps retain the sunscreen on the skin and, second, it promotes uniform deposition on the skin, preventing the sunscreen solution from pooling in crevices, which results in disproportionate protection of the crevices and a lower than expected overall level of sun protection. A uniform layer of sunscreen produces the maximum possible sun protection ratings.

As the levels of solid organic sunscreen active ingredients (e.g., Avobenzone and Oxybenzone) are increased, the liquid sunscreen active ingredients may prove inadequate to keep them in solution, particularly at low temperature. If these solid sunscreen active ingredients crystallize on the skin after the alcohol has evaporated, a reduction in sun protection level and an increased potential for skin irritation will result. Consequently, addition of sunscreen solvents (e.g., Butyloctyl Salicylate, Dimethyl Capramide and/or Diisobutyl Adipate) may be required to ensure adequate solid organic sunscreen active ingredient solubility.

Photostabilizers, the next ingredient category, are particularly important for any sunscreen utilizing Avobenzone as a sunscreen active ingredient. This is because, although Avobenzone strongly absorbs UVA and UVB radiation, it is prone to degrading as a result, markedly reducing its UV absorbing capabilities, especially when Octinoxate, a strong UVB absorber, is also present. While solvent polarity optimization can be used to some effect (by using sunscreen solvents of higher than usual polarity, such as those listed in the previous paragraph), triplet state quenchers such as Polyester-8 and Undecylcrylene Dimethicone are frequently more effective. When Octinoxate and Avobenzone are used together, the photostabilizer of choice is singlet state quenching Ethylhexyl Methoxycrylene, which can remove the existed state energy from Avobenzone perhaps a thousand times faster than even triplet state quenchers, allowing substantial photostabilization of this notoriously photolabile combination.

Humectants are substance that have an affinity for water and so increase the water-holding capacity of skin. They help to counter the otherwise drying effect of the alcohol used as a volatile carrier in sunscreen sprays.

Fragrance (parfum) is sometimes added, if for no other reason than to mask the base “sunscreen” odor of the product. When a signature fragrance is part of the market positioning, up to about 2% fragrance may be present. When hypoallergenic and/or “no fragrance” claims are made for the product, carefully selected essential oils, botanical extracts and/or individual synthetic chemicals may be added to control product odor without needing to use the word “fragrance” on the label.

We’ve covered “label copy” ingredients before. These are ingredients, often added at vanishingly low levels, that are included not because they are functional, but because they are useful as marketing “hooks” to catch consumer interest and garner initial product trial.

Last on our list are other active ingredients. These are ingredients, such as Vitamin E derivatives, that are added for functional effect, but not necessarily to enhance sun protection properties. If anti-aging ingredients such as Reservatrol, Retinol, Retinyl Palmitate, Ubiquinone and/or Cholecalciferol are included, all of which are photolabile (not photostable), they can be substantially photostabilized by the incorporation of the aforementioned singlet state photostabilizer Ethylhexyl Methoxycrylene.

If you have additional questions about sunscreen sprays, or about sunscreens in general, you may contact the author at gneudahl@hallstar.com.

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