Argon as an inert habitat gas

I was worried about the potential side effects of even an 'inert' gas like argon if we used it in Martian habitats. As in replacing oxygen, which is how I imagine nitrogen narcosis works... We 'need' some nitrogen if we want the atmosphere in the habitat to behave more or less like Earth, and there's a lot of argon available, half as much as nitrogen by proportion but easily accessible everywhere so therefore it'd be easier and cheaper than mining for nitrates or nitrites or whatever. I imagine digging a trench, plopping in a gasbag tube for the habitat pressure vessel, insulating and  roofing that over and burying it for radiation shielding then piping in sunlight with glass fiber?

(Turns out we don’t want to do this because Argon-40 gives you cancer, so… yet another way Mars is trying to kill you! Why do we want to go there?! 8-P)

We could use pure nitrogen gasbags for the lifting cells of airships in the atmosphere of Venus, and habitat volumes at the cooler half atmosphere level could be 40/60 oxygen/nitrogen, 200 mb O2 and 300 mb N2.