It isn’t safe, there is risk involved. Noble gases are mostly inert, meaning that they do not interact chemically with anything (except in extreme cases). Which means they’re probably not going to interfere with any biochemical processes.
But anything you are breathing that isn’t oxygen, well, isn’t oxygen, and this does poses a non-zero amount of risk. Clearly if you fail to breath oxygen for long enough you will die and people have indeed died this way (a couple entered a large tent-like balloon that was being inflated with helium. It’s interior was almost entirely helium – no oxygen – and they passed out and died).
It should be thought of as slightly riskier than holding your breath for the same amount of time. If it is in short periods, not much risk. I say riskier because holding your breath for too long triggers physiological responses that force you to start breathing which wouldn’t happen here.
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