Urea? It's used on foodcrops and you pee out 20-35g/day of it:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9161740/

It has a boiling point of 135C. Something else was (I gotta say it, likely) causing your sickness.

There’s a reason you pee it out.

I am not sure why you so casually dismissed this complaint without doing a quick search.

The whole reason why DEF is used[0] is because it reduces nitrous oxides, which are thought to be a pollutant.

Whether they actually are an effective greenhouse gas or not is probably a completely different thing we could argue about, but what is undisputed is the mechanism of action in the hot catalytic converter during engine operation, since it's the entire point of using the DEF in the first place.

In the catalytic converter, the urea thermally decomposes[0] to form Ammonia and Isocyanic acid[1], which is "a colourless, volatile and poisonous substance, with a boiling point of 23.5 °C".

23.5C is 74.3F. So, basically at room temperature the isocyanic acid becomes vapor. On a hot day, it vaporizes quickly and all that isocyanic acid has to go somewhere.

This is nasty stuff[2].

Anyway, the EPA probably figures that poisoning someone who's operating a diesel serves you right in order to reduce nitrous oxides. I've operated other diesels that are pre-DEF and never got sick like that.

0. https://en.wikipedia.org/wiki/Diesel_exhaust_fluid

1. https://en.wikipedia.org/wiki/Isocyanic_acid

2. https://www.osha.gov/isocyanates

There is no reason you should be exposed to volatile compounds from a normal parked rental truck. I would have asked for another truck or avoided U-Haul altogether.

Agreed. It was actually another national moving truck rental brand (not U-haul, I was just using it as a generic descriptor). I don't know why I'm not mentioning it since it seems like there's only a few national brands, but it's not their fault they have to use DEF and the truck was really great otherwise.