Question: What is the most harmful substance known to man?????????? ??

Angela Stokes answered on 12 Nov 2014:

Hi Adam

If by harmful, you mean poisonous in the smallest amount then we have to look at something called the LD50 – this is the dose of a substance that will kill 50% of a population…so if this is what we are looking for then…..

The botulinum toxin family of neurotoxins are the most deadly with LD50 values of about 5ng/kg – ng=nanogram and is one billionth of a gram. we do use botulinum toxin in some medical and cosmetic procedures eg Botox, but at minute concentrations!

Number 2 will be snake venoms which are deadly at about 1mg/kg – these are proteins and their ability to kill is due to the speed their activity, so in the case of some slow acting highly potent venoms their may be time to intervene with an anti-venom and prevent death.

Number 3 would be Arsenic which is deadly at about 13mg/kg.

There is also a substance called Polonium which is toxic at less than a billionth of a gram but its toxicity does not come from its chemistry, but from the fact that it emits radiation which destroys biological molecules like DNA. It takes about a month to die of this type of poisoning. There was a case of polonium poisoning in London not very many years ago Try googling Alexander Litvinenko for details.