Are char16_t and char32_t part of the std namespace?

I recently learned that using size_t as opposed to std::size_t in C++ is non-standard, and had to change my code because of it. I've read in some other answers that char16_t and char32_t are also introduced as typedefs, however intellisense only recognizes them without std::, and I don't know which header they come from, except the C header <uchar.h>, according to cppreference.com.

Should I prefix std:: for these types? And if so, which C++ header are they defined in?