How long does a sun block with SPF 20 provide protection?

The protection duration of a sunblock with SPF 20 can be calculated based on the understanding that SPF (Sun Protection Factor) indicates how much longer a person can stay in the sun without getting sunburned compared to wearing no sunscreen at all. Generally, the formula often cited is that the SPF number multiplied by 10 provides an estimate of the minutes of protection.

In the case of SPF 20, using this guideline: 20 (SPF) × 10 minutes = 200 minutes of protection. This means that if you would normally begin to burn after 20 minutes of sun exposure without sunscreen, with SPF 20, you could stay in the sun for a total of 200 minutes before reaching the same level of sunburn.

Choosing 400 minutes for a product with SPF 20 overestimates its effectiveness and does not align with the standard calculation used to understand SPF ratings. Therefore, it is crucial to recognize that while higher SPF levels do offer greater protection, the calculation leads us to a maximum protection time that is significantly lower than 400 minutes for SPF 20, making 200 minutes the more accurate response based on established guidelines.