Sunscreen Application & UV Risk Guide

How SPF works

SPF (Sun Protection Factor) is based on the amount of ultraviolet radiation that causes sunburn with sunscreen compared with without it under standardized testing. It is not a timer and does not mean that SPF 30 permits 30 times longer in the sun. Solar intensity changes with time of day, altitude, latitude, reflection, cloud cover, and many other factors.

Choose a broad-spectrum sunscreen with an SPF of at least 15 and use it with shade and protective clothing. Broad spectrum means the label indicates protection against both UVA and UVB radiation; SPF primarily describes UVB sunburn protection.

SPF number guide

Correct application amount

The FDA advises using enough to cover all exposed skin. For an average-sized adult or child, that is about one ounce (30 mL, roughly a shot glass) for head-to-toe coverage. Coverage, rubbing, and water exposure all affect real-world performance.

• Full body: about 30 mL (1 oz, or a shot glass full).
• Apply first: put it on about 15 minutes before going outdoors.
• Check the label: directions can differ by product form, particularly sprays.

Reapplication rules

Reapply at least every 2 hours during outdoor activity and more often after swimming or sweating. There is no waterproof sunscreen: water-resistant labels specify 40 or 80 minutes of protection while swimming or sweating, so follow the interval on your product.