They wore masks, had negative pressure ventilation suits on
I hope those were positive pressure suits, positive pressure helps to keep dust out of the suit. Negative pressure ventilation is used to help sick people breathe easier, like the iron lung for example.
I think those might actually be frosted glass cups.
Here is the best copy I could find, and the flared edges do not look like plastic cups - also the shiny inside and matte outside looks more like glass frosted on the outside.
PP was not in commercial use until 1957 (not sure when it was first used for cups), PET disposable cups weren't a thing until the 70s. Disposable cups in the 50s would have been wax paper.