An Australian sunscreen labeled SPF 50+ came back from the lab at SPF 4, and the number was never the hard constant the box implies.
SPF is measured on human skin under a solar simulator (ISO 24444), not read off a machine, so it swings lab to lab. In Australia the company selling the product picks its own unaccredited lab. Sent to eight of them, the same sunscreen scored 4, 10, 21, 26, 33, 60, 61, and 64. In CHOICE's test, 16 of 20 SPF 50 sunscreens failed their claim.
A low SPF is not toxic, only less than promised. Behavior beats the digit: enough product, actually reapplied, shade past noon. On most bottles the most honest number is still the net weight.
