I'm five weeks pregnant. Is it safe to use hair dryer at this time?

You must choose. There is limited science regarding exposure to electromagnetic fields and human health. Short use of hair dryer likely far less emf issue than cell phone use, even if same duration. Sorry, but you must choose if the potential risk that might have increased asthma or other risks, and if you want to avoid them. Also in theory, but after 12 weeks (1st trimester) at least the organs are all formed.