The oxeygen content on our planet was 35% because of industrialisation, pollution?

we are right now at 18%, and as we all know the human species needs atleast 15% of oxygen in order to survive. Now how long do you think we have left till then. 40 years?. and if the earth's oxygen levels have been dwindling since the beginning, is there any way to stop it at all? will that be the end of life as we know it and the beginning of a new era of evolution?