Making Sense of Sunscreen Products

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Most people are understandably confused when it comes to choosing a sunscreen because of the baffling array of available choices. Common questions about sunscreens include the following:

  • What is SPF? How high should the SPF be?
  • Should a sunscreen block UVA or UVB radiation?
  • Does it matter whether it is a gel, cream, or spray?
  • Should it be water-resistant?

SPF stands for sun protection factor. These numbers refer to the product's ability to screen or block out the sun's burning rays. The SPF rating is calculated by comparing the amount of time needed to produce sunburn on protected skin to the amount of time needed to cause a sunburn on unprotected skin. The higher the SPF, the greater the sun protection. The U.S. FDA recommends that the maximum SPF value on sunscreen labels be "50 +" because many scientists believe that there is insufficient evidence to show that products with SPF values higher than 50 provide greater protection for users than products with SPF values of 50.

However, it is a common mistake to assume that the duration of effectiveness of a sunscreen can be calculated simply by multiplying the SPF by the length of time it takes for him or her to suffer a burn without sunscreen, because the amount of sun exposure a person receives is dependent upon more than just the length of time spent in the sun. The amount of sun exposure depends upon a number of factors, including the length of exposure, time of day, season, geographic location, and weather conditions.

The sun's rays contain different wavelengths of ultraviolet (UV) light. The two types of UV rays that pass through the earth's atmosphere and cause damage to the skin are UVB and UVA. UVB rays are the primary cause of sunburn and affect the outer layer of skin.

The strength of UVB radiation depends upon the time of day, season of the year, and geographic location. UVB rays are most intense from 10 a.m. to 2 p.m. and are stronger in summer, at higher altitudes, and closer to the equator.

Unlike UVB rays, which do not penetrate glass, UVA rays can travel through window glass and damage the deeper layers of the skin. Both UVA and UVB light contribute to age-related changes in the skin such as wrinkles, freckles, age spots, and prominent blood vessels. Both UVA and UVB exposure raise the risk of skin cancer. A major limitation of the SPF value is that these numbers measure protection against sunburn caused by ultraviolet B (UVB) radiation. Therefore, SPF values only indicate a sunscreen's UVB protection and do not provide any information on the product's effectiveness in blocking the UVA rays that contribute to the development of skin cancers.

Sunscreens can be broadly classified into two categories: chemical sunscreens and physical sunscreens. Chemical sunscreens absorb UV radiation while physical sunscreens act by physically blocking it. Chemical sunscreens can be UVA or UVB absorbers. Many sunscreens have a combination of ingredients and may contain both physical and chemical sunscreens.

Physical sunscreens are good blockers of both UVA and UVB radiation. The two most common physical blockers of UV radiation are titanium dioxide and zinc oxide. Examples of chemical sunscreens include the following:

  • PABA (para-aminobenzoic acid): Rarely found in modern preparations, PABA was an early chemical sunscreen that often induced sensitivity reactions.
  • PABA esters (glyceryl, padimate A, and padimate O): These newer preparations have fewer side effects than the original PABA.
  • Salicylates (homosalate, octyl salicylate)
  • Cinnamates (cinoxate, octyl methoxycinnamate, or octocrylene): Octocrylene is a cinnamate with both UVA- and UVB-absorbing properties.
  • Benzophenones: These can absorb both UVA and UVB rays.
  • Ecamsule (Mexoryl) is a potent UVA-blocking compound.

Since both UVA and UVB radiation can be damaging, an ideal sunscreen provides protection from both types of radiation. The SPF system measures only the degree of protection from UVB rays. No rating system exists that measures the degree of protection from UVA exposure.

Most experts recommend reading the list of ingredients in a sunscreen to see if it has broad-spectrum (UVA and UVB) coverage. FDA regulation states that only products that can protect against both UVA and UVB radiation can be labeled as offering broad-spectrum coverage. Formerly, many products labeled as "broad spectrum" or "UVA blockers" did not adequately block UVA rays. A good broad-spectrum sunscreen should contain avobenzone, ecamsule (Mexoryl), titanium dioxide, or zinc oxide for significant UVA protection.