SPF 40 means that if your skin burns within five minutes of sun exposure, Super Serum Skin Tint SPF 40 will offer you 40X that time in the sun before burning. 40 x 5 minutes = 3+ hours in the sun without burning. To reapply protection, we suggest using the Radiant Translucent Powder SPF 20 instead of layering the Super Serum Skin Tint SPF 40.