How sunscreens are tested

The method for determining a sunscreen's SPF uses at least 10 volunteers with skin type 1 (skin which burns and never tans) to skin type 3 (skin which burns and tans moderately). People with darker skin or skin that tans readily aren't used - the test would take too long.

As a first step, the lab works out how long it takes each volunteer's unprotected skin to begin reddening. It then applies sunscreen to a series of patches on each volunteer's back, using the thickness specified by the standard (2mg per square centimetre). This is much thicker than what most people use when they're applying sunscreen.

The patches are then exposed to a lamp which simulates the sun's rays (ordinary sunlight is just too variable). Each patch is exposed at increasing intervals - the first patch would have the shortest exposure and the last the longest.

When the testing is finished, the volunteers wear cover-up clothing until the patches are inspected the next day. On each volunteer, the lab identifies the patch that shows the first clearly visible sign of reddening. It then divides the length of time that patch was exposed to the lamp by the length of time taken for the volunteer's unprotected skin to redden. The resulting number is the volunteer's "protection factor". The SPF of the sunscreen is the average of all the volunteers' "protection factors".

When a sunscreen claims to be water resistant, the SPF is tested after the volunteers have been in water - a swimming pool or spa pool - for the length of time that the product claims water resistance. The volunteers dry off in the air (no towelling is allowed) and then the SPF is determined by exposing patches of skin to the lamp.

Ethical issues

Soaking in a pool may seem like a nice way to make a dollar, even if you do get a funny tan. But how ethical is it to test sunscreens on human subjects when it's known that over-exposure to ultraviolet radiation is harmful?

We're told that regulatory organisations are well aware of the issues involved and are working towards developing a laboratory (in vitro) test to determine SPF. But testing on humans (in vivo) is currently the best method because it takes into account real-life factors such as skin texture, absorption rates and sweating. No acceptable equivalent to human skin has yet been developed for laboratory testing.

There are measures to protect the human volunteers. Each volunteer must complete a questionnaire and give informed consent. Some people - such as pregnant or lactating women, people taking certain medications, and those with allergies - are excluded from testing. Someone who's taken part in a test must also wait at least two months before taking part in another.