When you buy a sunscreen check its SPF [sun protection factor] . The ingredients in the sunscreen protect the skin by blocking, absorbing or scattering the UV radiation. SPF is basically the amount of UVB protection a sunscreen gives and sunscreens are typically rated by the sun protection factor. The rating states how long a person can be in the sun. Multiply the SPF by 10 and you can figure out how long you can stay in the sun. For instance, the sunscreen is rated as SPF 15, a person can stay for around 150 minutes in the sun without burning. With SPF 30 you should be able to stay for 300 minutes before burning. It should be reapplied if you propose to stay in the sun for longer hours.
If you swim or participate in activities that cause you to sweat, assure you reapply the sunscreen immediately after the activity. These days many foundations or lip balms contain an SPF 15; this can give you added protection. When the ingredients of the sun is combined with antioxidant vitamins C and G, the protection against the sun is improved, providing more defense than that is found with an normal sunscreen.
Sunscreen tends to deteriorate over time. Hence, always check the expiry date on the tube. If you cannot find the date and if you feel that the product seems to be discolored or dried, discard it. Never take chances with your skin. To be on the safer side it is advisable to buy a new bottle every season. Avoid products that contain unnecessary chemicals or artificial fragrance as it can irritate your skin, this is especially true if you have a sensitive skin.
Last but not the least, remember most sunscreens are not effective until 30 minutes. Hence apply the product half an hour before you go out in the sun on the face, neck, hands and legs to ensure maximum protection.