Reading about the Litvinenko inquiry is chilling for many reasons: not least the suffering of the ex-spy and the potential after-effect of his poisoning on international relations. But using a radioactive substance as poison is particularly frightening because it is undetectable by human senses.
The part of the brain which responds to threat is called the amygdala. To keep us alive, it has evolved to be super-sensitive to potential perils and is linked directly to our eyes, nose, tongue and ears. These are called subcortical pathways. They bypass the rest of the brain, allowing us to pick up threats subliminally and respond to them before we even realise there’s a problem. So, for example, a smell of burning can make us feel alarmed even when it’s below the level we can detect consciously, and while we may not hear sounds below the perceptual threshold, they can nonetheless make us uneasy.
Radioactivity undermines this system, as it is invisible, noiseless and tasteless. So it’s less immediately threatening than cyanide with its almond smell, fire, or even a gun. But that makes it much more scary.
Dr Daniel Glaser is director of Science Gallery at King’s College London
• This article was amended on 1 February 2016. An earlier version referred to the almond smell of arsenic, rather than cyanide.
