Surprisingly, the sun’s UV rays are harmful during this time of year because the ozone layer is at its thinnest. So aging and skin-cancer causing UVA rays are actually at its highest (even though sunburn rays caused by UVB rays are at its lowest during this time).
Staying indoors may block most UVB rays, which cause sunburn, but allows more UVA rays through windows. UVA exposure has been linked to skin cancer, photoaging, and pigmentation issues. Therefore, being indoors near windows may not provide adequate protection against harmful UV rays.