There's also something deeply unsettling about the smell of O3 in any concentration where you can truly smell it, not like getting punched in the lungs that ammonia bicarbonate is, and less irritating than than NOx compounds, but the almost sweet and sickly pleasant smell to it carries a sense of dread.

Speaking of which NOx is another good one; N2 is (essentially) inert and makes up most of our atmosphere, O2 has already been explained, but run a high frequency (white) electric arc in our atmosphere and you cause the N2 and O2 to combine in varying quantities producing NOx compounds.

NOx compounds are effectively gaseous nitric acid, it has a brown/red colour, and on constant with water it forms nitric acid. It doesn't matter whether that water is in a beaker with the NOx being bubbled through it, or the water that exists in your sinuses, lungs and eyes.

You lose your sense of smell pretty quickly when exposed to it and concentrations can build up quicker than you expected, if you've been overexposed to it, you'll wake up the next day or even a few hours later, feeling like you've got the worst sore throat in your life, like someone stamped on your throat then tried to scrape it clean... assuming you do wake up.

1/10 do not recommend.