Does a SPF 30 absorb twice as much as a SPF 15?
SPF ratings conform to the principle of diminishing returns. In other words, the larger the numbers are, the smaller the gain in protection. This point can be illustrated by an analogy of Farmer Joe’s experience fertilizing his field. The first time Farmer Joe fertilizes his crops his production doubles. After seeing this increase in production Farmer Joe decides to double the fertilizer to double his production again. However, this time his production only increases by 25%. As he doubles his fertilizer time and time again, the percent of increase in production is minimal. The moral of the story is that more isn’t always better. A SPF 15 absorbs 64.6% of UVB radiation; a SPF 30 absorbs 97.0%, which is a 50% increase in protection. SPF 30 absorbs 97.0% and a SPF 45 absorbs almost 98%, this is less than a one percent increase in protection.
