What does SPF mean?

All sunscreens list a sun protection factor (SPF) on their labels, and in recent years, these numbers have climbed as high as 100, prompting many to question exactly what they mean.

Here’s the lowdown so you're ready for summer.

Also read: Family sunscreen safety>

A version of this article appeared in our June 2013 issue with the headline "Family suncare essentials," pp. 36-38.

No matter how high the SPF, sunscreen needs to be reapplied at least every two hours to be effective — sweating and movement can cause it to slough off or absorb into the skin, rendering the SPF meaningless.

SPF only refers to UVB rays. To ensure UVA protection, make sure your sunscreen is labelled "broad spectrum."

SPF 100 sunscreen protects from 99 percent of UVB rays, while SPF 30 protects from 96.7 percent — not a big difference. The Canadian Cancer Society recommends a minimum SPF of 30.

Proper application of sunscreen — generously, every two hours — is as important as the lotion’s SPF.