Expert: Walter Hintz - 3/20/2007

Question
I've twice heard a story about how the Earth supposedly had far more oxygen in the air in the past(ie around the age of the dinosaurs), with a 35% oxygen ratio, whereas now it's only 21%(according to one report of this type; I'd always thought oxygen was nowadays only 19-20% of total air, but anyway). Anyway, it seems rather dodgy as this theory is mainly used by various sellers of "alternative-health-products" such as magentic water devices etc. - also an ancient Earth with 35% oxygen would, presumably, be constantly aflame, as a result, with not much prospect of life. So, is this an urban myth or the truth?

Geoff

Answer
Hi Geoff
I have never heard of this idea and I doubt that it is true.  Before green plants the Earth had no O2. Once photosynthesis began atmospheric O2 increased until it stabilized at 21%.  There is no reason to expect that it would increase above that amount.  O2 is toxic in large amounts.  It would have killed all aerobic organisms.