Admittedly, the hazard is supposed to be a gas, but is the lab that single purpose?
If the lab has storage cabinets, lab sinks and a hood, you never know exactly what might transpire that might require an emergency shower.
1980 - I had an office next to an electrochemical lab. Screaming alerted me to an emergency next door. I ran over to find one of the lab guys clutching his face, screaming "my eyes, my eyes". His lady lab tech was shrieking along with him, but unharmed.
I grabbed him and lead/dragged him out to the hallway where the emergency shower was, pulled the ring and got lots of water flowing on him. He lucked out, ending up with only 2nd degree burns on his eyelids, nose, cheeks and forehead. His eyes weren't damaged. A solution of fairly dilute acid he was heating over a bunsen burner had flashed/boiled up onto his face and eyes (he was not wearing safety glasses or face shield).
That lab typically prepared and ran experiments on small volume solutions in the 20-30 ml range, unlike the liter of solution he was brewing.
Others in the company couldn't recall when, if ever the emergency shower had ever been used previously.
I've never actually seen an eye wash station in operation, so I'm not sure what kind of water volume is available, but that guy was darn glad that a high volume shower was right outside his lab door to cool and dilute the acid all over his face.
Dan