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Summer 2008 Newsletter Don't Get Burned: The Science Behind Sun Protection

Summer is the time of year when we enjoy the outdoors with picnics, swimming, hiking and other fun activities. With increased time in the sun, though, we must remember to protect our skin from harmful UV rays.

Ultraviolet Radiation

Ultraviolet (UV) radiation that reaches the Earth's surface from the sun can be divided into two types: UV-A (320-400 nm) and the more energetic UV-B (290-320 nm). UV-A radiation only penetrates the top few layers of skin, while UV- B radiation penetrates more deeply and can damage the dermis, alter the immune system, and harm the skin's DNA. The use of sunscreens and sunblocks has proven effective in the prevention of some destructive effects of the sun's UV radiation on our bodies.

Physical vs. Chemical Sunscreen

There are two predominant mechanisms by which sunscreens are effective: they block the sun's UV radiation either by physical or chemical means. Sunscreens that contain metal oxides, such as zinc oxide, titanium dioxide, or talc work by physically blocking or reflecting the UV radiation. In other words, they are a physical barrier between the sun and your skin.

On the other hand, chemical sunscreens work by absorbing and chemically reacting with the sun's UV radiation. Most chemical sunscreens are aromatic, conjugated organic molecules with carbonyl groups. This family of chemicals absorbs highly energetic radiation from the sun, exciting the sunscreen chemicals into higher energy states. When the chemicals then nonradiatively relax to their ground states, the remaining energy is converted into lower energy radiation, which is not harmful to the skin. These types of sunscreens commonly include such chemicals as para-aminobenzoic acid (PABA), oxybenzone (benzophenone-3), and octyl salicylate.

Further, chemical sunscreens can be subdivided into the portion of the sun's radiation that they absorb most effectively. Most sunscreens typically contain chemicals that cover both the UV-A/UV-B spectra. Padimate O, homosalate, octylmethoxycinnamate, and benzophenone are used to block UV-B radiation, whereas oxybenzone and avobenzone are used to block UV-A radiation.

Sun Protection Factor (SPF)

The sun protection factor (SPF) of a sunscreen relates to the amount of time a person can spend in the sun without acquiring a sunburn. It's a simple calculation: if Jane can normally sit in direct sunlight for 10 minutes without getting sunburned and she used a broad-spectrum sunscreen with an SPF of 20, then she can be exposed to the sun for 200 minutes without getting sunburned.

A sunscreen's SPF is determined experimentally by using 2 mg of sunscreen per every 1 square centimeter of skin. This is a very generous dose of sunscreen. In the real world, sunscreen users' insufficient application means that the SPF of the sunscreen is most likely not fully realized. In other words, Jane may be using a sunscreen with SPF 20, but her slapdash application before running into the surf means that the effectiveness of her sunscreen is probably more in the range of a SPF 10. In addition, by not allowing time for the sunscreen to fully bind to the skin before heading into the ocean, she has further minimized the effectiveness of the sunscreen.

Zinc oxide has been long used as a sunblock and is safe and non-irritating. In the past, however, it has been opaque white in color when applied to the skin. Think back to those images of lifeguards with white stripes of sunscreen on their noses. In recent years, though, nanotechnology has found an answer to this problem: a nanometer-sized metal oxide particle to maximize UV protection while maintaining transparency upon application. Additionally, these micro- and nano-sized metal oxides provide more full-spectrum protection than most chemical sunscreens.

While there are a wide variety of different sunscreen formulations on the market, the formulation's base (the cream or lotion the sunscreen chemicals are mixed into) may have a strong effect on the power of the UV absorption achieved by the active ingredients. Solvents, emollients, fillers, emulsifiers, and other ingredients that impart the aesthetic characteristics of the sunscreen formulation may interfere with the UV-blocking ability of a sunscreen chemical. These effectiveness issues must be worked out during formulation and testing in order to verify the SPF claim.

Dual-Use Products

The increased awareness of the adverse effects of sunlight exposure has prompted manufacturers of other consumer products such as moisturizers, hair applications, insect-repellents, and cosmetics to incorporate UV protection. The home, health and beauty product formulators at Chemir enjoy the challenges of producing SPF-rated formulations that meet our customers' criteria for efficacy and aesthetics. Our staff has years of experience working with a diversity of matrices, and can partner with entrepreneurs or developers from concept to finished product.