Formaldehyde vs Acetaldehyde: what is the difference?

How they differ

Both are small aldehydes, both are reactive, and both are classified as carcinogenic to humans, but they arrive by different routes. Formaldehyde is the simplest of all aldehydes at 30.03, used in resins, embalming fluid and disinfectants, and it cross-links proteins on contact, which is precisely what preservation and what irritation both are. Acetaldehyde at 44.05 is what ethanol becomes as the body breaks it down, and it is responsible for much of a hangover and for the flushing reaction in people whose version of the clearing enzyme works slowly. Their reported oral LD50 values differ by more than sixfold, 100 mg/kg in rats for formaldehyde against 661 mg/kg for acetaldehyde. The body produces both in small amounts as a normal part of metabolism.

Side by side

Common questions

What is the difference between Formaldehyde and Acetaldehyde?

Both are small aldehydes, both are reactive, and both are classified as carcinogenic to humans, but they arrive by different routes. Formaldehyde is the simplest of all aldehydes at 30.03, used in resins, embalming fluid and disinfectants, and it cross-links proteins on contact, which is precisely what preservation and what irritation both are. Acetaldehyde at 44.05 is what ethanol becomes as the body breaks it down, and it is responsible for much of a hangover and for the flushing reaction in people whose version of the clearing enzyme works slowly.

Is Formaldehyde bigger than Acetaldehyde?

Acetaldehyde is the heavier molecule, 44.05 g/mol against 30.03 g/mol for Formaldehyde. Formaldehyde is CH2O and Acetaldehyde is C2H4O.

Which has the lower reported LD50, Formaldehyde or Acetaldehyde?

Formaldehyde, at 100 mg/kg (oral, rat), against 661 mg/kg (oral, rat) for Acetaldehyde. An LD50 is one acute animal measurement, and the two here were not necessarily measured in the same species, so it is a data point rather than a ranking of how dangerous either is to a person.