oxygen

Almost everything in this world is linked and related
Such as green plants need Carbondioxide and sun light to produce Oxygen, if the content of CO2 is lesser, Green plants will then slow down its "photothesis", and all animals are needing O2 to live, then when O2 production is slowing down, our O2 intake will not slow down, therefore the O2 content will become reduce and CO2 will become increase... it then will lastly become a standard level...
but biologically, too much O2 is not a good things, it will speed up our aging problem which may speed up our duration of life...

有，如果地球有太多氧氣，环境变得易燃性。如果有森林大火，氧氣就会助燃，火场的范圍就会越来越大了！