“Sola dosis facit venenum” (only dose determines the poison) - Paracelsus

Paracelsus (1493 - 1541) was a famous alchemist, physician, astrologer, and general occultist. Born Theophrast von Hohenheim (full name Theophrastus Philippus Aureolus Bombastus von Hohenheim), he took the name Paracelsus later in life, meaning "superior to Celsus", an early Roman physician.

The chief criterion regarding the toxicity of a chemical is the dose, i.e. the amount of exposure to the substance. It is safe to say that all substances are toxic under the right conditions, including molecular oxygen. Inhalation of 100% molecular oxygen causes extensive damage to the lung, while the lack of molecular oxygen reaching the bodies' tissues is not usually compatible with life.