Bryan Johnson· Author
When the UV index is >4 , wear a hat (with a brim circumference of roughly 3 feet) or use a UV umbrella.
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The headline is broadly defensible, but the qualifications matter. Effect sizes vary by population, the strongest claims rest on shorter trials, and credible voices push back on how it's typically framed.
When the UV index is >4 , wear a hat (with a brim circumference of roughly 3 feet) or use a UV umbrella.
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in times where the UV index is above three you can use a hat sunscreen or a UV umbrella what you're trying to avoid is excessive sun exposure on unprotected skin during High UV times of the day