HONG KONG — The Malaysian authorities said on Friday that Kim Jong-nam, the half brother of North Korea’s leader, had been killed by VX nerve agent applied to his face. The substance, listed as a chemical weapon, was kept for decades in the arsenals of many militaries, including that of the United States. But since the Chemical Weapons Convention came into force in 1997, most stockpiles globally have been destroyed.

What is a nerve agent?

A nerve agent, sometimes called nerve gas, acts on the nervous system of an organism and prevents muscles from functioning properly. The substance is derived from organophosphate pesticides, and military officials had envisioned dispersing it over a wide area — using a specialized artillery shell or bomb, for instance — to kill or incapacitate enemy forces and make the affected area impossible to safely move through.

How does it kill you?

The VX molecule interferes with the way glands and muscles function by blocking an enzyme that allows them to relax. That causes muscles to clench uncontrollably and, eventually, prevents a victim from being able to breathe. The lethal dose for VX ranges from about 10 milligrams via skin contact to 25 to 30 milligrams if inhaled. Early symptoms can include pinprick pupils, runny nose, wheezing and muscle twitching. Death can occur anywhere from within a few minutes to hours, depending on the dose and the method of contact.