To get the most from your sunscreen, follow the label instructions.
When to apply: 15 minutes before sun exposure begins. “That allows it to bind to the outer layer of the skin. Once it’s bound it’s going to last longer and be harder for it to get rubbed off or washed away.”

How much to use Be generous: To cover all exposed skin, a common guideline is a golf ball-size blob. Skin-care pros are adamant: Don’t skimp. Applying sunscreen too thinly weakens its effectiveness. A too-thin layer of SPF 30 may deliver only SPF 5 or SPF 10 results.
When to reapply:  At least every 2 hours.

Skin Conditions and Skin Types
What if you have sensitive skin, skin conditions or skin allergies? Your better sunscreen choice is likely to be a mineral sunscreen (containing zinc oxide or titanium dioxide). You may also want to steer clear of formulas that include alcohol or fragrances.

Sun exposure affects every type of skin. People with fair skin (e.g., Scandinavians) must employ extra vigilance to shield their skin.
Other factors that influence the level of protection you need from UV rays:

• Altitude: The higher you ascend, the thinner the atmosphere (and the UV-inhibiting gases contained in it). It is estimated that UV radiation increases 4% per 1,000 feet of elevation gain. Thus at higher elevations, apply and reapply sunscreen with greater diligence.
• Latitude: Sun intensity becomes greater near the equator and both poles.
• Sweat: Heavy perspiration due to temperature, humidity or exertion is likely to weaken your sunscreen.
• Abrasion: Bushwhacking or scrambling typically means your skin will be brushing against other surfaces. Towelling off after a swim also compromises protection. Reapply regularly.

Sunscreen Shelf Life and Storage
Manufacturers sometimes stamp a “best by” date on the product. If not, write the year you purchased the sunscreen on the cap or label. It is estimated that sunscreen is good for 3-4 years after purchase, perhaps longer if it is a lotion that retains a creamy texture.

Store sunscreen in a cool, dry place. Heat degrades the active ingredients in sunscreen. Avoid leaving it stashed in a glove box or car boot during warm months.