现在的guideline就是这样，一google就找到了 http://www.babycenter.com/0_how-to-keep-your-baby-safe-in-the-s
因为晒太阳的伤害是累积效应，美国这边太阳又比较厉害，大家还是注意点了
“Keep your baby completely out of the sun as much as possible before 6
months of age. When you do go outdoors, be particularly careful in the early
months about taking all the following precautions.”
