[Sun Care] Is it really necessary to wear sunscreen in winter/during times where venturing outdoors isn't really an option? +More questions in thread

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As the title reads, I'm not really sure if wearing sunscreen during winter is that necessary. I understand that it protects against the UV rays that come through the clouds, but I really don't go out much during winter. Other than lunch breaks at school, which my friends and I usually spend in the library to get away from the cold and rain. Also, is vitamin D deficiency a real issue? Because in the summertime, I reapply and apply sunscreen all over me in order to go to the beach, and I have never had any problems.
Girls Blog 2015
Submitted by spunkypotato