Interesting Reads: About High SPF Sunscreens and Under-application of Sunscreen

This website is really informative! It gives you all you need to know about sunscreens.

Below is an excerpt about the under-application of sunscreen:

http://www.ewg.org/2010sunscreen/full-report/high-spf-and-other-misleading-claims/

SPF factors are based on two-to-five times more sunscreen than people actually use

In the real world, people get far less protection than the bottle advertises.

Sunscreen makers establish a product’s SPF by testing their products on volunteers. The testers coat the volunteers’ backs with 2 milligrams of sunscreen per square centimeter of skin (mg/cm2), the amount stipulated in FDA’s draft sunscreen regulations (FDA 2007), and then expose them to sunlight-simulating UV radiation until a burn appears. The time needed to burn, divided by the time it takes to burn the volunteers’ unprotected skin, is the SPF.

In real life, people apply one-half to one-fifth the amount of sunscreen used in the laboratory SPF tests (Autier 2003, Azurdia 2001, Reich 2009). Because of the physics of sunlight, that cuts the protection factor not by a factor of just 2 to 5, but by between the square root and the fifth root of the SPF. That’s a much steeper “exponential” cut in protection (Faurschou 2007, Schalka 2009, Kim 2010, Playtex 2007). For example, this means that someone who applies one-fourth as much sunscreen as in the SPF test gets just SPF 2.3 protection from an SPF 30 product. SPF 100 becomes just SPF 3.2.

How under-application of sunscreen cuts effective SPF
(based on applying one-fourth the recommended amounts)

SPF on label	Average SPF of users at (0.5 mg/cm2)	% UV transmission (amount reaching skin)
15	2	50%
30	2.3	43%
50	2.6	38%
100	3.2	31%

Source: EWG analysis of sunscreen efficacy based on a typical sunscreen application rate of one-fourth of 2 milligrams per square centimeter of skin (the SPF testing application amount).

A number of studies have confirmed people’s tendency to apply less sunscreen than is used in SPF testing. One study of 124 students found that the average application rate was one-fifth (0.39 mg/cm2) the testing amount (Autier 2003). Another study found that ten female volunteers applied a median thickness of sunscreen that was one-fourth (0.5 mg/cm2) the amount used in testing (Azurdia 2001). Even when researchers instructed volunteers on the proper amount to use, they applied too little: A study of 52 subjects found that uninstructed volunteers applied 34 percent of the recommended thickness, and even those who were instructed on how much to apply used only 43 percent of the recommended amount (Reich 2009).

The fact that people use less sunscreen than recommended is not an argument for using even higher SPF products to compensate. Higher SPF products produce small increases in real-world SPF. But even this small change allows sunbathers to stay in the sun longer – and absorb more overall radiation – before a sunburn sends them indoors. In the process, the substantially greater amounts of sunscreen chemicals in higher SPF products can penetrate the skin and lead to much higher internal exposures to potentially hazardous compounds. The user is left with a burn and a significantly higher “body burden” of sunscreen chemicals.