Generally a minimum SPF of 15-20 is necessary. As the intensity of the exposure increases, a higher SPF is indicated and blocking agents and clothing should be considered. This is another topic. There are some other special considerations. Reflected UVR can cause burns in unusual areas.
Some people have reactions to the sun while they are on particular medications.
There are different kinds of reactions, but they are all essentially UVA mediated. A very small percentage of people are sensitive to PABA-containing products and they should avoid them. Likewise, anyone with an allergy to sulfa-containing products or benzocaine should not use a PABA screen. Sunglasses and eyes are a topic for another discussion. There were some articles that appeared in the literature within the last three years that suggested that the use of sunscreens increased the incidence of melanomas in children.