The Dose Makes The Poison
A fundamental principle of toxicology, is the adage that the dose makes the poison. Substances which, at very low levels, can be neutral or even beneficial to life, all have a level at which they will start to cause harm to the host. Even substances that we require to live such as water and oxygen can be toxic if too much is eaten, drunk, or absorbed. It is why upper limits for exposure are set by organisations in order to restrict damage to health and the environment. However, the line is not always linear. What can be happily absorbed at low doses, can, if exposure is prolonged, also cause health issues. Which is why some timed exposure levels are set - for instance the occasional X-Ray is unlikely to cause harm to individuals, however prolonged exposure by radiographers is controlled, and limits are set to protect their health.
Toxicologists measure the toxicology of a substance by calculating what a lethal dose would to kill half the members of a tested population after a specified test duration. This is given as the LD50 of a substance and is expressed in the mass of substance administered per unit mass of test subject, typically as milligrams of substance per kilogram of body mass depending on the amounts required. Water has an LD50 of 90, or 90,000 mg/kg and Botulinum toxin on the other hand is 1 ng/kg or 0.000000001. Other related measures are used depending on the substance, for instance LC (Lethal Concentration, usually in a body of water) LCt (lethal Concentration over time, useful when measuring radiation effects) and occasionally rather than measuring 50% deaths, toxicologists may measure LD99 or LD1.
The whole premise is often forgotten when discussing food and drug safety, especially by campaigners who are ideologically against things like vaccines or fluoridation. The levels of formaldehyde in one common or garden pear vastly exceeds the amount in vaccines, yet campaigners argue against its inclusion, and anti radio frequency radiation campaigners complain about a mobile phone mast putting out 50-watts of RFR, but ignore the TV mast on the hillside that has been emitting 400,000 watts of the same stuff for decades. As noted above, Botulinum toxin is the most lethal known substance, however, millions of people inject it into their face regularly in order to look beautiful.