Why is the ratio of SPF important to know when using sun block?

The ratio of SPF, or Sun Protection Factor, is crucial because it determines the duration of protection a sunscreen provides against UVB rays, which are primarily responsible for sunburn and contributing to skin cancer. The SPF number indicates how long you can theoretically stay in the sun without getting sunburned compared to wearing no protection at all. For instance, if a person normally burns after 10 minutes of sun exposure, using a sunscreen with an SPF of 30 could allow them to stay in the sun for about 300 minutes without burning, assuming proper application and reapplication is followed.

Understanding the SPF ratio helps individuals make informed choices about sun protection, especially when considering factors like skin type, the intensity of sun exposure, and activities that may affect sunscreen efficacy, such as swimming or sweating. The other options do not relate to the protective capabilities of the sunscreen against harmful rays.