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I know that formaldehyde naturally occurs as a metabolite and can be found in our own and other animals' bodies, plants, and unprocessed food.

But formaldehyde is also registered as a carcinogenic VOC and is considered bad when it's in plywood and cosmetics.

Is this judgement based on dosage, form (e.g. gas/liquid), or amount released?

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  • $\begingroup$ Well, dosage, yeah. But it’s also not seen well as a metabolite and one of the causes of a hangover after having drunk too much. $\endgroup$
    – Jan
    Aug 23, 2016 at 9:47
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    $\begingroup$ @Jan I think you thought about acetaldehyde. If you drank CH3OH hangover would be your last problem ;) $\endgroup$
    – Mithoron
    Aug 23, 2016 at 13:02
  • $\begingroup$ @Mithoron I thought there was some small-scale side-metabolism that created a small amount of formaldehyde, too, but it was on TV and it was a long time ago ;) $\endgroup$
    – Jan
    Aug 23, 2016 at 13:04
  • $\begingroup$ As for why it is dangerous - formaldehyde is widely in biology used to fix cells as it causes crosslinks between proteins to other proteins and to DNA. $\endgroup$
    – Bitwise
    Aug 23, 2016 at 13:59

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The toxicity of formaldehyde is dose-dependent. The famous saying "the dose makes the poison" is one of the rules of thumb about toxicology. Your body can effectively detoxify small amounts of formaldehyde in a given amount of time, however if you are exposed to concentrations that result in you ingesting/inhaling more than what your body can metabolize, then toxicity will begin to occur.

Most consumer products with detectable levels of formaldehyde (I'm thinking of shampoos and body washes particularly) do not directly have formaldehyde added to them. As formaldehyde is a fairly reactive gas that will polymerize & evaporate easily, formulation chemists will use compounds that decompose slowly to formaldehyde plus other mostly inert material. What this does is provide a constant, low level of formaldehyde present to halt microbial growth. (Formaldehyde is a wonderful disinfectant - few things if any will survive treatment with concentrated formaldehyde - and it turns out you need a LOT of formaldehyde to kill a human as compared to a microbe!)

Formaldehyde is also a natural trace compound formed in fermentation/decomposition of other organics, so it's detectable in the biosphere if you want to go looking for it - I'm sure you can find detectable amounts in fruits, wood products, even the air around a garden.

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  • $\begingroup$ Why would formaldehyde be more poisonous to microbes than to the average mammal, after scaling? Concentrated H2CO kills everything, why would a level low enough to be totally harmless to humans effectively stop microbial growth? $\endgroup$
    – Karl
    Aug 23, 2016 at 15:44
  • $\begingroup$ @Karl that's an insightful comment. There's 2 important reasons why microbes are more susceptible to formaldehyde poisoning by shampoo than humans. One is that microbes absorb everything through their cell walls while mammals have thick skin. The other is that microbes are literally swimming in the toxins. Try drinking a bottle of shampoo and it'll do you no good at all. Similarly, nicotine is used as insecticide. An insect on a plant covered in nicotine spray will easily ingest the mg/kg dose that would be lethal to a human The lethal per kilo doses do obviously vary between organisms though $\endgroup$
    – Level River St
    Aug 23, 2016 at 19:52
  • $\begingroup$ Formaldehyde will be absorbed by the skin pretty well, as are other, similar substances, eg. alcohol. I take the point that the microbe is immersed in the shampoo or whatever for it's lifetime, while a human has it on his skin for a minute or two per day. Different timescale. $\endgroup$
    – Karl
    Aug 24, 2016 at 7:59

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