Why do water-resistant sunscreens say 40 or 80 minutes rather than 'waterproof'?

Answer: Water, sweat, and rubbing gradually remove the sunscreen, so labels give a tested 40- or 80-minute window.

Water, sweat, and rubbing gradually remove the sunscreen, so labels give a tested 40- or 80-minute window. ✓ — Right: no sunscreen stays put forever, because water, sweat, and rubbing gradually remove or disrupt the film. FDA rules require a water-resistance claim to state a tested duration—40 or 80 minutes—rather than a permanent 'waterproof' promise. The American Academy of Dermatology notes that even water-resistant sunscreen needs reapplication after that window and about every two hours outdoors in dry conditions.

Water makes the sunscreen film stronger, so the minutes mark how long that protection lasts. — This reverses how water resistance works. The label does involve a film and a time limit, which makes the seal idea tempting, but water does not strengthen or seal that film. The minutes mark a tested period before reapplication, not the lifespan of a waterproof seal.

Regulators reject 'waterproof' as too absolute, though the sunscreen still lasts all day. — This gets the wording issue partly right: regulators avoid 'waterproof' because no sunscreen is truly permanent against water. But it wrongly assumes the product then lasts all day. The 40 or 80 minutes is the tested window before reapplication, so the label is a duration claim, not just a ban on one word.