THE dermatology group also recommends that anyone who spends 20 minutes or more outdoors should put on sunscreen in the morning. That doesn't satisfy hard-liners like Harvard's Mr Pathak. He thinks applying sunscreen should be as regular a morning habit as brushing teeth, even for office workers on weekdays. ''Even if you're just going across the parking lot to the office, you're getting sun exposure. It's short exposure, but it's a cumulative effect. It's like drops of water eventually filling a basin and overflowing.'' Dermatologist Mr Steven Price thinks such advice goes overboard and may risk turning off people from using sunscreen. Those who work outdoors all or part of the day certainly should apply sunscreen in the morning to any skin that won't be covered by clothing, Mr Price said. But someone who works inside all day doesn't need it. Mr Price's greater concern is that people may use sunscreens to stay in the sun longer than they would otherwise. ''We know sunscreens prevent burns. But there's no definite evidence they prevent skin cancer. If using sunscreens means they're spending six times more outdoors, it's conceivable that even though they're not burning, they're increasing their chance of getting skin cancer.'' For one thing, SPF ratings apply only to UVB rays, the main cause of sunburns, not UVA rays, which are less understood. What is known is that UVA rays penetrate deeper into the skin than UVB rays, and they cause long-term damage to the skin. UVA rays probably are at least a partial cause of skin cancer. Sunscreens with an SPF of 15 or above provide some protection against UVA rays, and many now tout this ability. Sunscreens that have ''broad screen'' on their labels contain the ingredient avobenzone, which has been accepted by the FDA as an effective partial screen against UVA rays. But Mr Pathak warned that many people find avobenzone irritating to their skin. He recommended using zinc oxide or titanium dioxide for protection against UVA rays. Titanium dioxide is getting much attention these days because, unlike zinc oxide, it can be formulated to be invisible on the skin. But no matter how good or high-tech, sunscreens are not 100 per cent effective. They don't do as good a job of screening the sun as tightly knit clothing or broad-brimmed hats, though clothes and hats also aren't perfect. Dermatologists say you're best off staying in the shade or indoors.