You should generally reapply sunscreen every two hours to maintain effective protection from UV rays

. However, if you are swimming, sweating, or towel drying, you need to reapply more frequently-potentially every 40 to 80 minutes-to ensure the sunscreen remains effective

. Even if you use a high SPF sunscreen, reapplication every two hours is necessary because no sunscreen provides continuous protection for eight hours

. If you are indoors away from windows, reapplication may not be needed as often, possibly every 4 to 6 hours, since UV exposure is lower in that environment

. But if you spend time near windows or go outside intermittently, you should follow the two-hour rule from the time of sun exposure

. In summary:

• Reapply every 2 hours during regular outdoor activity.
• Reapply every 40 to 80 minutes if swimming, sweating, or towel drying.
• Reapply less frequently (every 4-6 hours) if indoors away from windows.
• Use enough sunscreen (about one ounce for the body) and cover all exposed areas thoroughly

This routine helps prevent sunburn, skin damage, and reduces the risk of skin cancer.