Hydrogen cyanide (HCN), is a chemical compound with the chemical formula HCN.
It is a colourless, extremely poisonous and flammable liquid that is volatile (turns to gas) at slightly above room temperature, at 25.6°C (78.1°F). (Wikipedia)
A solution of hydrogen cyanide in water is called hydrocyanic acid, or prussic acid. (Encyclopedia Britannica).
Where does Hydrogen Cyanide Occur?
Hydrogen Cyanide is occurs naturally, in several edible plants, in the form of cyanogenic glycosides. The best example of cyanide containing plant is the cassava root. Other good examples are Lima beans and flax (Linseed) (ncbi). In order for the cyanide to be released from the plant, the cyanogenic glycoside must be broken down by an enzyme, in the presence of water. Most of these plants do not contain toxic levels of cyanide.
When is it toxic to Human Beings?
Humans should not consume more than 1mg hydrogen cyanide per kg body weight per day. For example if you weigh 50kg then you should not ingest more than 50mg of hydrogen cyanide per day. This means that children are more vulnerable to poisoning because their body weight is lower. (For more information)
Some bitter cassava root varieties can contain 10-490mg hydrogen cyanide per kg (2.2Ib) in the pith (middle portion of the root), so cassava must be thoroughly washed and cooked and there are methods employed by people who use it as a staple food for reducing cyanide levels.
Below is a photo of Cassava
Below is a photo of Lima Bean and Flax (Linseed)
What are cyanogenic glycosides?
Cyanogenic glycosides are chemical compounds contained in foods that release hydrogen cyanide when chewed or digested. The process of chewing or digestion causes the compounds to come in contact with water which allows enzymes to breakdown the cyanogenic glycosides to release Hydrogen Cyanide.
More than 75 different cyanogenic glycosides have been reported from at least 2650 plants in 130 families, including Fabaceae (leguminosae)-bean family, Rosaceae (Rose family, Linaceae (flax family) (ncbi), and Compositae (Asteraceae)-sunflower family. (Science Direct). They occur in varying amounts.
They can be found in the pits (seeds) of apples, apricots, cherries, almonds, peaches, plums and quinces. They are also present in cassava, sorghum, bamboo shoots, linseed (flaxseed), lima beans, chickpeas and cashews, in varying amounts, just to mention a few. (intechopen.com)
Industrial use of Hydrogen Cyanide
Industrially, hydrogen cyanide is produced and used in the manufacture of dyes, fumigants, synthetic rubber, acrylic fibres, and plastics.(Encyclopedia Britannica).
Below is a photo of cassava roots (Manioc, Yucca Wax)
myfavouritepastime.com
My Favourite Pastime 
The name sounds scary. Crazy that it’s safe to eat. I love flax. Well it seems to be good for me anyway. 😉
I eat a tablespoonful of flax in my oatmeal every morning. The amount is negligible.
Liz