Letter published in the New Zealand Listener, November 13-19th 2010, p.8. Dear Sir, People writing about sunscreens (Health, Oct.30), could pay some attention to teasing out what the sun protection factor really implies. A little simple arithmetic can clear away decades of misinformation. SPF has no dimensions; it is the ratio of two other numbers. When we see where these have come from, we are on the way to a better understanding. The standard SPF test involves using a UV lamp to produce a mild burn on a volunteer's skin. (Even a mild burn has been shown to produce precancerous damage). For example, if it took two minutes to burn skin that had been coated with a measured amount of sunscreen, and four seconds to produce the same burn on uncoated skin, the SPF would be 120/4 ie. 30. Human skin, depending on the skin type and the time of day, takes about fifteen minutes in the sun to become red. We are often told that we can multiply the SPF by this time to calculate how long we can safely stay out in the sun, eg. 30 x 15 minutes, or seven-and-a-half hours. But, as Joanne Black told us (The Black page, January 10, 2009), if using the SPF 30+ product she named, we should reapply it, not every seven-and-a-half hours but every two hours. What has gone wrong? Why the discrepancy? The UV lamp, being so much more powerful than the sun, required only two minutes; not much of a rehearsal for hours out on the sports field. That is the explanation; as far as sunscreens are concerned, real life and the laboratory are very different places. In other words the SPF test doesn't tells us how much protection we will still be getting during actual use, moving and sweating. Some products stay on the skin better than others. Dr Brian Wilkins 92 Kelburn Parade Wellington 6012 Ph 04-9773975