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Sun Screening

Ultraviolet A (UVA) rays are the most abundant form of ultraviolet radiation in sunlight.  These rays are the longest and lowest energy form of ultraviolet radiation.  UVA rays pass through the atmosphere relatively easy.  Regardless of season or time of day, sunlight contains roughly the same amount of UVA rays.  These rays are also capable of penetrating the deeper layers of our skin, where it causes oxidative reactions.  This oxidative reaction causes the darkening of melanin, which most people recognize as tanning.  Unfortunately, oxidative reactions also cause photo-aging of the skin and increase the risk of melanoma, the deadliest form of skin cancer.  UVA does not cause sunburns.

Ultraviolet B (UVB) rays are the middle range of ultraviolet radiation.  Depending on the season and time of day, the atmosphere will absorb varying amounts of UVB.   Mid-day, during the summer months is the highest UVB exposure.  UVB rays do not travel as far into the skin, but unlike UVA, UVB rays can directly damage the DNA in our skin.  UVB rays are responsible for sunburns, and the most common forms of skin cancer.  UVB also stimulates the production of new melanin and vitamin D.

Sunscreens were originally developed to protect pilots from blistering sunburns during their flights.  As such, early sunscreens only protected against UVB, and to this day the majority of sunscreens only protect against UVB.  Sun Protection Factor or SPF is a multiplier for how much time an individual can be exposed to sunlight without developing a sunburn from UVB.  SPF 2 means that a person who would normally develop a sunburn in 20 minutes, can be outside for 40 minutes without developing a sunburn.  A sunscreen with SPF 10 means that same person could be outside for 200 minutes without a sunburn.  This multiplier starts to become less accurate around SPF 30-50. There is no corresponding scale for UVA protection, and unless specifically stated, users should assume that sunscreen does not protect against UVA.

UVB exposure is needed to make vitamin D, a crucial fat-soluble vitamin that plays a role in bone strength, immune response, mental health, and even cancer prevention.  Normal sun exposure is crucial for our bodies to make this vitamin.  Depending on skin tone, a person can produce 10,000 units of vitamin D with just 20 minutes of sun exposure.  However, sunscreens of SPF 8 and above prevent us from making any vitamin D.  To complicate matters, sunscreens of SPF 15 and higher are recommended for the prevention of the most common forms of skin cancer.

People with a personal or family history of melanoma should be sure to use a sunscreen that protects against both UVA and UVB, such as sunscreens containing titanium oxide or zinc dioxide.  Everyone who is going to get more than 20 minutes of sun exposure at a time, should use sunscreen with SPF 15 or higher to prevent the damaging effects of overexposure.  But for at least 20 minutes a day, let the sun soak in and enjoy the benefits of creating your very own vitamin D, with all of it’s health promoting effects.


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