Proper use of sunscreen can help prevent skin cancer

What SPF should you use and other frequently asked questions

May 23, 2014
CBP1012125

As a specialist in skin disorders and cancers, UC Irvine Health dermatologist Dr. Janellen Smith sees the damage the sun can do to the skin. She answers some frequent questions about sunscreen.

What SPF number is the most effective?

I recommend using a sunscreen with a skin protection factor (SPF) of 30, and one that contains avobenzone, titanium dioxide, zinc oxide or mexoryl, which are the best ingredients to protect your skin from the damage of the sun's ultraviolet rays. 

How often do I have to reapply sunscreen?

To make sure you have enough sunscreen protection, you need to apply about a shot glass full of sunscreen to your body. Sunscreen should be applied at least 20 minutes before you go out in the sun to make sure the key ingredients are absorbed into the skin.  Reapply the same amount every two hours. Sunscreen should also be reapplied after sweating a great deal, swimming or toweling off. 

What’s the difference between sunscreens labeled as “waterproof” and “water resistant?"

No sunscreen is truly waterproof as sunscreen eventually washes off. According to the FDA, sunscreens labeled as “water resistant” have to be effective for 40 minutes, or 80 minutes when swimming or sweating.  All sunscreens are required to provide directions on when to reapply.  To be on the safe side, reapply your sunscreen about 30 minutes after swimming.

What type of sun protection does my baby need?

Babies under the age of six month should not be exposed to any UV rays. The best protection for little ones is to keep them in the shade as much as possible.  They should also wear sun-protective clothing such as long sleeves, pants and a wide-brimmed hat. The best clothing options are dark colors and fabrics with tight weaves. Light colored clothing doesn’t provide as much protection. Sun-protective clothing is another good option. 

However, the effect of sunscreen on babies younger than six months of age is unknown, so it really shouldn’t be used on them. For infants and toddlers older than six months, sunscreen can be applied in a small dose only to exposed areas not covered by clothing.

Special sunscreens specifically made for infants and toddlers, or sunscreens with zinc oxide or titanium dioxide may cause less irritation to the skin of infants or toddlers, and they have no known adverse effects.

Does sunscreen expire?

It’s probably a good idea to check the expiration label on your bottle of sunscreen. The FDA requires that all sunscreen retain their original strength for at least three years. 

Since it’s recommended to use sunscreen daily, you should go through the bottle fairly quickly. If you’re concerned, here are some guidelines to follow:

  • Some sunscreens do have an expiration date. If the date has passed, toss it.
  • If there are changes in the odor or consistency of the lotion, it’s probably a good idea to buy new sunscreen.
  • Write the date you purchased the sunscreen on the bottle. That will help you determine whether it’s time to replace it.


May is Melanoma and Skin Cancer Detection and Prevention Month. UC Irvine Health wants to improve your chances of living a long and healthy life — know your risks and get checked out.

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