It's fairly difficult, but definitely possible. It's hard to recognize because what is killing you is the lack of oxygen. People generally don't notice they are becoming hypoxic.

But there are plenty of things to help you out here. First is that helium is very light. So if you pass out and fall down you will be in a lower concentration of helium (assuming you are not in a sealed room). (Labs with heavy gases have vents on the floor) If the room is reasonable ventilated you should be fine. It dissipates fairly quickly. Helium permeates through most things, but this is a slow process so probably won't save you.

I meant more that you don't feel like you are suffocating when you are inhaling helium, so it can be hard to realize something is going wrong until it is too late. Definitely easier to suffocate with heavier-than-air gasses though.

Multi-story building. The floor could be, given enough gas, under.

If helium is small enough to get in a watertight phone, it is certainly small enough to dissipate in a probably not so watertight building.

Quenching MRI magnets can suffocate people in the room they quench, and potentially the entire floor. I don’t know about a whole building, but just because something can dissipate doesn’t mean it can’t do damage in the meantime.

Not true - the whole point of burst discs etc in MRIs is so that if a quench happens, the patients and any technicians in the room survive

Venting even a full MRI's worth of LHe (~1500l) as most will make you a little lightheaded for a few minutes

The risk of death from quench events is effectively non-existent

If you fall over unconscious from helium then you'll probably die without immediate help. Your natural breathing reflex is triggered by the presence of CO2, not the lack of oxygen. If you inhale a lot of helium and pass out, your body doesn't start breathing again because there is no CO2, so you just suffocate instead of inhaling oxygen.

You're continuously producing CO2 whenever you're alive. If you fall down and land in a normal atmosphere then your normal breathing reflex will save you. Low oxygen atmospheres don't stop you from breathing; they're dangerous because you continue breathing as normal. The CO2 is carried away from your lungs with each breath, so there's no feeling of suffocation.

> If you inhale a lot of helium and pass out, your body doesn't start breathing again because there is no CO2, so you just suffocate instead of inhaling oxygen.

Other posters have pointed out that this is false(you will continue producing CO2 while unconscious just fine, chemoreceptors will notice). I would like to add that I have personally witnessed people falling unconscious from helium. They recovered just fine.

Really, the greatest danger is the fall.

you don't breathe because there is CO2 in the air, you breathe because there is CO2 in your blood

If that were true then divers would have to bring canisters of CO2 down with them.

Well helium rises. So falling down is a good thing, as most vents are above you. If you go to a lab with heavy gases you'll notice vents on the floor for exactly this reason.