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Formaldehyde occurs naturally, and is "an essential intermediate in cellular metabolism in mammals and humans." According to the American Chemistry Council, "Formaldehyde is found in every living system—from plants to animals to humans. It metabolizes quickly in the body, breaks down rapidly, is not persistent and does not accumulate in the body."

The twelfth edition of NTP Report on Carcinogens notes that "food and water contain measureable concentrations of formaldehyde, but the significance of ingestion as a source of formaldehyde exposure for the general population is questionable." Food formaldehyde generally occurs in a bound form and formaldehyde is unstable in an aqueous solution.

In humans, ingestion of as little as 30 millilitres (1.0 US fl oz) of 37% formaldehyde solution can cause death. Other symptoms associated with ingesting such a solution include gastrointestinal damage (vomiting, abdominal pain), and systematic damage (dizziness). Testing for formaldehyde is by blood and/or urine by gas chromatography–mass spectrometry. Other methods include infrared detection, gas detector tubes, etc., of which high-performance liquid chromatography is the most sensitive.