When scientists want to understand how dangerous a toxic substance is, they use something called the "median lethal dose" or LD50. This tells us how much of the substance it takes to be deadly.
The LD50 is the amount that would be fatal to half the test group. In other words, if you gathered 100 test subjects and gave them this exact dose, about 50 would survive and 50 would not.
The lower this number is, the more toxic the substance. For example, if substance A has an LD50 of 1 gram per kilogram of body weight and substance B has an LD50 of 100 grams per kilogram of body weight, substance A is much more dangerous because it takes far less of it to be lethal. As Swiss physician Paracelsus noted in 1538, "The dose makes the poison."
According to this infographic provided by CEUFast, a continuing education service for nurses, the LD50 ranges from relatively benign substances like water (90g/kg) to extremely toxic ones like botulinum toxin (0.000000001 g/kg). Botulinum, which is used in highly diluted form in Botox treatments, is one of the most potent toxins known — even more deadly than polonium-210, the substance found in some Russian teas.
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