When inhaled these two molecules have virtually no effect. Propylene glycol can however be an irritant, whose intensity depends on the individual. Irritation is usually stronger during initial exposure, but then decreases as the respiratory tract becomes accustomed to it. When they are heated, propylene glycol and glycerol tend to become fragmented and therefore to break down into smaller molecules, mainly aldehydes. Aldehydes are part of a molecular family known for its toxicity.