Firstly, life is a terminal disease.
Secondly, walking outside is carcinogenic with all the vehicle fumes. The benzene in Unleaded fuel is 10 times more carcinogenic than the leaded fuel it replaced.
Resin flux is generally a pine resin with additives. In its liquid form it is safe. It is only when it is heated or burnt it becomes dangeous. Any form of smoke is carcinogenic so as the others have said if you burn the flux and produce smoke then it MAY be dangerous.
I have been sniffing flux fumes for 30 years and it hasn't affected me.
heheheh hohohohho hahahahahh :

:-\
See. Hasn't affected me at all.