The front of a sunscreen bottle is a compressed instruction sheet. Reading four terms correctly makes the label much more useful.
Broad spectrum
Broad-spectrum products are tested for protection across ultraviolet A and ultraviolet B radiation. UVB is strongly associated with sunburn; both UVA and UVB contribute to skin damage. “Broad spectrum” is therefore not decorative language.
SPF
Sun Protection Factor is based on protection against sunburn under test conditions. It is not a timer and does not mean SPF 30 lets you remain in the sun thirty times longer. The amount applied, rubbing, sweating, water, and the intensity of exposure all affect real-world protection.
Water resistant
In the United States, a sunscreen may state water resistance for 40 or 80 minutes when it meets the applicable test. The label does not mean waterproof or all-day protection. Reapply after the stated water-resistance interval, after towel drying, and at least as often as the directions require.
Directions still matter
The FDA recommends broad-spectrum sunscreen with SPF 15 or higher, used regularly and as directed, as one part of sun protection. Protective clothing, shade, hats, and limiting intense exposure remain part of the plan. Apply enough to cover exposed skin and reapply at least every two hours, more often when swimming or sweating.
If you have a history of skin cancer, unusual skin reactions, or questions about sunscreen for an infant or a medical condition, ask a qualified clinician for individualized guidance.