How It Works:  Sunscreen

It’s at our side throughout the summer or else we pay dearly for it. Many of us have known for most of our lives that sunscreen prevents sunburns, which can cause skin cancer. But what do you really know about those tubes of ointment and how they work? Dr. Lawrence Mark, dermatologist at the IU Health Simon Cancer Center, breaks it down:

What is sunscreen?

Sunscreen is any product that helps to reduce or limit exposure to ultraviolet radiation. This can be in the form of a protective film, lotion, umbrella or clothing. 

What is SPF?

SPF stands for Sun Protection Factor.  This is a measure of how much ultraviolet B radiation is blocked from passing through the protective product.  An SPF of 2 blocks 50% of the radiation penetration – so it takes twice as long for the same amount of radiation to pass through with continued exposure.  An SPF of 15 blocks 93% of the radiation – that means it takes 15 times longer for the same amount of radiation to pass through this barrier than with no protection at all.  And an SPF of 70 blocks nearly 99% of the UVB radiation, thus taking 70 minutes for the same amount of UVB to penetrate as it would in 1 minute if there were no protection.  SPF does not define how well a product protects against ultraviolet A, another form of harmful radiation given off by the sun and tanning bed lamps.

Who should wear sunscreen and what type should they use?

Everyone should consider wearing sunscreen unless directed otherwise by their physician.  Some people develop allergies or irritancy to the compounds used in the sunscreen lotions. Usually inorganic physical blockers such as titanium dioxide and zinc oxide contained in  sunscreens are well tolerated by those with sensitivity to the organic mixtures of sun protective compounds.  Direct sun avoidance, appropriate protective clothing and seeking shade are generally better practices than use of sunscreen.

Is sunscreen harmful?

Sunscreens without broad spectrum coverage may be more harmful than no sunscreen at all. Only “broad spectrum” sunscreens, which protect against UVB and UVA, are recommended. Unopposed UVA exposures such as those found in tanning beds, sun through window glass and use of sunscreens lacking broad spectrum protection could theoretically increase skin cancer rates, because only UVB stimulates the skin cell’s natural repair enzymes. UVA does not appear to stimulate cellular repair to any measurable degree.

Does sun exposure cause skin cancer?

Chronic unprotected sun exposure is associated with increased rates of developing non-melanoma skin cancer and premature wrinkling/aging.  Intermittent high-dose UV exposures such as those one gets on vacations to sunny climates and using tanning beds is associated with increased rates of melanoma development.



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