Titan II Propellant Transfer Operations (1980) (youtube.com)

At 10:00, it becomes obvious why the suits are needed. I think the cloud of brown vapor (!) is Dinitrogen tetroxide. I found the whole process interesting in their use of checklists and rigid procedures, as one does in the military.

https://youtu.be/Ejap_EFlriA?t=600

https://en.wikipedia.org/wiki/Dinitrogen_tetroxide

https://en.wikipedia.org/wiki/Titan_(rocket_family)#Titan_II...