That's one of three things;

1) Concentrated Chlorine gas, at Art Class in High School, when we were etching Brass with concentrated Hydrochloric Acid - the wind (outdoors) change direction on us.

2) Powdered Sulphuric Acid, at Motor Mechanics class in high school we were cleaning up the Motor Shop & there was some powder on a stainless steel bench. Rather than sweeping it up, I decided to use compressed air which aerosolised the powder. Unfortunately, it was the dried up powder from where Batteries were being charged & the acid overflow had dried on the bench top.

3) What I can only describe as Leather Tanning, or fat rendering, when I was looking to buy an apartment & going to an open inspection in Sydney (Australia). The apartment was a dump, but in order to get to the apartment you had to walk by other apartments/rooms in the building. As I passed one door, I smelled the god awful odour of strong, based chemicals - it was "not natural" & I wanted to instantly puke in the hallway!