Sunlight causes premature aging of the skin, immunosuppression and skin cancer. There is a direct relationship between the amount of ultraviolet radiation that reaches the skin during our lifetime and the number and extent of some skin cancers.
UV radiation is one of the factors, along with genetic mutations, skin color type and geographic area, that affect the number and type of cancers of the skin.
Skin cancers can be extremely aggressive, such as some malignant melanomas, or be indolent and locally destructive like basal and squamous cell carcinomas.
Sunscreens prevent the majority of the UV radiation from reaching the skin. The ideal sunscreen should protect against UV-A and UV-B and should not cause irritation or allergies and should be cosmetically acceptable.
How much and how well UV radiation is prevented from reaching the skin is expressed by the SPF index (Sunlight Protection Factor). That, however, is an index applicable only for the UV-B radiation.
Simply speaking: The SPF describes how long the skin is protected when in the sun with sunscreen compared to when it is not protected.
For example, if a fair person gets a light sunburn after 10 minutes of sun exposure, a sunscreen with SPF 5 would prevent sunburn for up to 50 minutes. An SPF factor of 10 filters approximately 90 percent of UV-B radiation while an SPF of 30 filters out 96.67 percent of the same radiation and an SPF of 50 protects against 98 percent of UV-B.
Sunscreens can be chemical or physical. Physical sunscreens contain particles of zinc oxide and or titanium dioxide that reflect and scatter the light including the UV radiation. Chemical sunscreens absorb the radiation and neutralize it with chemical molecular changes.
Physical sunscreens used to look chalky but these days it has been possible to reduce the size of the particles; consequently the creams and lotions containing them are more cosmetically elegant.
In conclusion, we should use sunscreens with SPF 25-30 containing filters for both UV-A and UV-B and reapply them every two hours when in the sun for prolonged periods. The sunscreen should not irritate and should feel good when applied to the skin.
Dr. Francesco D'Allesandro works in dermatology at the Walla Walla Clinic.