[guitarbill]

Carbon monoxide might be easier because it's one specific molecule, and the concentration has to be fairly high (comparatively) to be life-threatening. The paper mentions VOC (volatile organic compounds) which are probably harder to quantify. Detectors aren't always great either. I've worked in labs that didn't use helium detectors, because - at the time maybe - they weren't super reliable, but having them usually meant people would rely on the detectors instead of paying attention to the symptoms of a helium leak.

Smell/organic chemistry is weird, too. Some molecules have similar smells, despite being sometimes quite different. Many molecules have quite different smells, despite being not that different [0], edit: the esters table maybe shows that better, [1].

[0] https://jameskennedymonash.wordpress.com/2014/01/04/table-of...

[1] https://jameskennedymonash.wordpress.com/2013/12/16/infograp...

[manifestsilence]

Oh yeah, I'd forgotten how weird smell in particular was. I seem to remember at the time that there were three competing theories for how smells are even perceived or what gives a substance a particular kind of smell, and each only explained a subset of smells.

For the helium leak, is one symptom a high squeaky voice, or is that at already really dangerous concentrations?

[guitarbill]

From what I remember, dizziness, headache, inability to concentrate, etc. You'd notice a big/quick release as the liquid helium boils of quite easily. A slow leak is more dangerous, in which case you might even get used to the higher pitch without noticing.

[fwip]

One symptom of a helium leak is that your iPhone might stop working: https://motherboard.vice.com/en_us/article/gye4aw/why-a-heli...

This happened also to the coworkers of a friend of mi e - occasionally everyone's iPhone in their office would stop working. Android phones still worked fine. Later (after this article was published) they found out there was an MRI machine downstairs.

[hinkley]

Isn't carbon monoxide also a ridiculously simple chemical? Detecting complex molecules has to be a lot more complicated, doesn't it?

Also noses self repair. You can foul them with another scent for a while but eventually the receptors will recover.