A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

How do they know what lethal gases smell like?

March 7, 1997

Dear Cecil:

I am a grad student in a certain engineering/science school in Cambridge, MA. In some of the labs I work in we use phosphine and arsine gases to grow semiconductor films. These gases are very toxic (like, a quarter of a breath will kill you). I read a list of toxic gases that said arsine had a "garlic odor" and phosphine had a "rotting fish odor." I've also heard these descriptions from other sources. My question is, how do they find out the smell of these extremely lethal gases? Would anyone survive a good sniff long enough to tell about it? How much did they get paid and what were the perks?

Cecil replies:

I'll handle the smart aleck remarks, if you don't mind. Though the lethal concentrations for the gases you mention are extremely low, the thresholds at which they become smellable are lower still. A few lucky souls were accidentally exposed to these lower levels and lived to tell about it, and it's they who reported what the smells are like. Deliberately exposing human subjects to deadly fumes would be a gross violation of medical ethics. Lest you find the survivability of toxic gases overly comforting, you should know that the margin between the "odor threshold" and the death threshold is very thin. For example, the "IDLH" (immediate danger to life and health) level for arsine is 6 parts per million, whereas the odor threshold is less than 1 ppm--a difference of five lousy parts. So if you smell garlic in a place where arsine is being used, don't stick around looking to see if someone's having Italian for lunch.

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