與你所在地區緯度,紫外線輻射強度有關,看MAP陰影部分,幾乎涵蓋了美國2/3。。
Latitude and vitamin D production in the skin
Except during the summer months, the skin makes little if any vitamin D from the sun at latitudes above 37 degrees north (in the United States, the shaded region in the map) or below 37 degrees south of the equator. People who live in these areas are at relatively greater risk for vitamin D deficiency.