Concentrations in the air of 200 parts per million (ppm) of hydrogen cyanide are lethal for animals, whilst concentrations as low as 0.1 milligrams per litre (mg/l) are lethal for sensitive aquatic species. Non-lethal concentrations affect reproductive systems in both animals and plants.

For humans, lethal doses are, in cases of ingestion, between 1 to 3 mg/kg of the person's body mass, in cases of absorption through the skin, between 100-300 mg/kg of the person's body mass and between 100-300 ppm if inhaled. This means that a portion of cyanide smaller than a grain of rice would be sufficient to kill an adult.

Long term exposure to non-lethal doses may cause headaches, loss of appetite, weakness, nausea, vertigo and irritation of the eyes and respiratory system. Much care must be taken when handling cyanide in order to prevent harmful contact on the mine workers part. However, according to the industry, there has not been a case of human fatality in mines that use the cyanide leaching process.

In view of this fact, frequently used as an argument by the mining companies, Philip Hocker (op. cit.) points out that: "limiting our worries about cyanide to the fact that there have been no reported human deaths is to fall in to the trap which biochemists call "dead bodies in the streets": this is the attitude that, if cadavers are not seen, everything is in order. Despite the lack of human cadavers, there is evidence that not everything is in order.

Mining workers often have contact with cyanide, most of all during the preparation of the cyanide solution and subsequently in the recovery of the gold from this solution. For the mine workers, risks are encountered in the cyanide dust, the cyanide vapours (HCN) in the air coming from the cyanide solution and the possibility of skin contact with the cyanide solution as well.