If, like me, you’re feeling a bit unhealthy after the Christmas and Hogmanay (New Year’s) celebrations and vaguely worrying about the toxins you’ve put into your body it seems like an appropriate time to have a wee look at how toxic a toxic substance really is. So, to continue on from the post on the Banana Equivalent Dose for radiation we started wondering about using other equivalency values to put contaminants into some sort of perspective.
Cyanide is a toxic compound with the chemical formula CN and as anyone who’s ever watched an old spy movie, where someone has resorted to ‘using their cyanide pills’ will know, cyanide can be rapidly fatal if consumed in sufficient quantities. In an environmental context it’s commonly encountered on sites where there have been historic gas works to generate coal gas because it was produced as a by product of gas production. If you’re on a site like this and you encounter big lumps of nasty looking blue stuff that smells of bitter almonds then treat it with a great deal of care and resist that temptation to pick it up and have a good sniff.
Cyanide in the form of substances which metabolise to release Hydrogen Cyanide (HCN) also commonly occurs at low concentrations in the seeds of the fruits mango, peach and apples. It is fairly unlikely that you’ll manage to eat a peach or mango seed without noticing or breaking a few teeth but many of us have managed to munch a few apple seeds without noticing. As a rough and rather unscientific way to put the current Generic Assessment Criteria (GAC) for cyanide into context we’ll compare it to a quantity of apple seeds.
The available information on expected concentrations of cyanide within apple seeds varies a bit depending on which source you look at but approximately 700mg/kg seems to be a common conclusion. A very rough average weight for one apple seed is 0.75g. So one apple seed contains about 0.525mg of cyanide in the form of HCN. This in turn equates to 0.505mg of CN- ion in each apple seed.
Cyanide can be fatal at concentrations of between about 1 and 1.5 mg per kilogram of body weight. So using the 1mg figure I would have to eat about 165 apple seeds with a total weight of 123.75g to poison myself. As an aside the seeds would have to be ground up and eaten immediately or very well chewed to actually release the full quantity of cyanide for digestion.
So how does this relate to an environmental context? The current Generic Assessment Criteria (GAC) for Cyanide is 800mg/kg for cyanide in soil with the primary exposure pathway being ingestion. One kilogram of soil with a concentration of Cyanide equal to this assessment criteria is therefore roughly equivalent to 1584 apple seeds! A fatal dose of this soil for me would be approximately 103.1g of soil consumed in a single sitting. The concentration of cyanide which would be of concern in a soil is unsurprisingly greater than that in the seeds of a common foodstuff but hopefully it’s a bit easier to visualise how difficult it would be to eat that pile of apple seeds.
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