Inhalation of toxins....mixing bathroom cleaners killed my father-in-law. Horrible experience. Fluid on lungs from chemical enduced pneumonia was cause of death.
I put bleach into a solution of nitrogen salts when I was a fairly new graduate student. I wasn't thinking about the fact that nitrogen salts are ammonia salts... I haven't forgotten that fact since.
So sorry about your father-in-law, that was terribly unfortunate.