Sun protection factor
A number assigned to a sunscreen that is the factor by which the time required for unprotected skin to become sunburned is increased when the sunscreen is used —abbreviation SPF
How does SPF work?
SPF is an indicator of how effective a sunscreen protects the skin from harmful ultraviolet (UV) rays. SPF value can range from as low as 8 to 100! Sunscreens with high SPF value allow for more protection against the UV rays. For example, sunscreen with SPF 50 offers more sun protection than sunscreen with SPF 15. Sunscreens with SPF 15 are ideal for minimal sun exposure and sunscreens greater than SPF 15 are more suitable for outdoor activities like a hike or swimming. Check out more information about sunscreen here!
The SPF value does NOT relate to the time of solar exposure, but the amount of solar exposure. There is more solar energy in the middle of the day compared to early morning or late evening because the sun is more intense in midday. Solar intensity is also related to geographic location which means there is more solar intensity at lower latitudes. Clouds also absorb solar energy, which is why there is more solar intensity on clear days than cloudy days.
The amount of sunscreen applied also impacts the amount of solar radiation absorbed because the frequency of reapplying sunscreen results in less solar energy absorption which decreases the chances of getting a sunburn.
One thing to note is that SPF does not inform how long consumers can be exposed in the sun without getting a sunburn. Sunscreens can wear or wash off depending on the activity level which becomes less effective over the course of application. Reapplication of sunscreen is important to limit the solar radiation absorption, especially if the activity involves water (e.g. swimming).
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— Written by Megan Enciso (MPH Candidate, USC). Reviewed by Adriana Bakhoum
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