[dredmorbius]

That sounds like it might have been carbon monoxide poisoning.

https://www.mayoclinic.org/diseases-conditions/carbon-monoxi...

Though symptoms are quite similar to those of CO2 poisoning:

https://www.emedicinehealth.com/wilderness_carbon_dioxide_to...

One of the symptoms of CO poisoning I've heard mentioned is of a sense of doom or dread.

[bjoli]

Nope, that was co2. It is similar to CO, but in animals at least it gives a more aversive reaction.