How effective are sunscreens?

In Australia two out of three people will develop some form of skin cancer during their lifetime. Often the best ways to protect skin from the sun are by use of clothing and shade. However, any remaining exposed skin should be protected by use of a sunscreen of at least SPF 15 and preferable SPF 30+. Remember that the purpose of using sunscreens is to reduce UVR exposure, not to extend the time spent outside in the sun. Sunscreen is used on the skin to stop harmful UVR from reaching the skin. There are two main types of sunscreens, chemical absorbers and physical blockers. A sunscreen that absorbs UVR is a chemical barrier and is the most common type of sunscreen available. Additionally, these sunscreens may be waterproof, non-greasy and contain a variety of water-soluble or oil soluble chemicals. Chemical sunscreens are usually easy to apply to the skin. A sunscreen that scatters or reflects UVR from the skin is called a physical blocker. Zinc cream, which contains zinc oxide, is this type of blocker but is generally only used on small areas of skin as it also prevents heat loss and perspiration from the skin. Titanium dioxide is also used in sunscreens due to its reflective properties. The physical blockers tend to mainly reflect UVR, however, they can also absorb UVR at specific wavelengths. The individual chemicals in sunscreens absorb UVR at specific wavelengths. Broad spectrum sunscreens contain several ingredients that each absorb at different wavelengths and so are effective over more of the UVR spectrum. The effectiveness of sunscreens is dependent upon many factors including how thickly the sunscreen is applied to the exposed skin. When considering how much sunscreen is adequate, it is internationally accepted that the application should be about two milligrams per square centimetre. This translates to about thirty millilitres (ml), which is approximately six teaspoons, of sunscreen lotion to protect the entire exposed skin of an adult male. Therefore a 120ml tube of sunscreen should only last four applications if used on the entire body of an average adult.

The sun protection factor (SPF) rating indicates the level of protection provided by a sunscreen against UVR. Sunscreens sold in Australia must be labeled with an SPF rating of at least 4, up to a maximum of 30+. Sunscreens of less than SPF 15 offer only moderate to low protection.

A sunscreen with a rating of SPF 15+ would provide a fair-skinned person with 15 times more protection for their exposed skin than if they didn't use a sunscreen. For example, if a fair skinned person reddens after 10 minutes of sun exposure, then correctly applying SPF 15 sunscreen will provide protection for up to 10 x 15 = 150 minutes. It is important to remember that after 150 minutes in the sun while wearing sunscreen this person would still have received the same UVR exposure as they would have received in 10 minutes if they had not been wearing sunscreen. In both cases their skin has received the same amount of UVR exposure.

Factors that may alter the effectiveness of sunscreen are the time of year, time of day, amount of surface reflection, cloud cover, water resistance and the person's skin type. Use of sunscreens with a higher SPF rating than SPF 30+ is not generally recommended as they may not provide much greater protection but require an increased amount of active chemicals which may irritate some sensitive skins.

The following table shows that a broad spectrum sunscreen of SPF 15 blocks approximately 93% of the UVR and one of SPF 30 blocks approximately 96% of the UVR. Simply looking at the ratio of the SPF value one can compare the amount of protection offered by SPF 15 and SPF 30 sunscreens. For any exposure time an SPF 30 sunscreen has double the protection of an SPF 15 sunscreen. In practice the amount of sunscreen applied and the evenness of the coverage can have a significant affect on the duration of protection offered by the sunscreen.

SPF rating	% UVR blocked
4	75
8	87
15	93
30	96

The values shown in the above table are approximate

Sunscreen is best applied to clean, dry skin. Sunscreen must be applied at least 15 minutes before going outside to all exposed areas of skin and reapplied every two hours to maintain the stated protection. Reapplication does not give additional protection but ensures that the stated protection is achieved. Application of sunscreen ineffectively or too sparingly may considerably reduce the level of protection for the wearer.

Remember that sunscreens do not block out all of the UVR so a person is not completely protected by sunscreen and may still sunburn.

The current Australian sunscreen standard (AS/NZS 2604:1998 'Sunscreen products - Evaluation and classification') limits the maximum protection claimed on labelling of sunscreen products to SPF 30+.