Range: ±0.006 volume %
||Duursma EK, Boisson MPRM, Global oceanic and atmospheric oxygen stability considered in relation to the carbon-cycle and to different time scales. Oceanologica Acta (0399-1784) (Gauthier-Villars), 1994 , Vol. 17 , N. 2 , P. 117-141 link p.117 3rd paragraph and p.119 left column bottom paragraph
||Machta L, Hughes E. Atmospheric oxygen in 1967 to 1970. Science. 1970 Jun 26 168(3939):1582-4.PubMed ID5420541
||P.117 3rd paragraph: "The present global atmospheric oxygen level remains remarkably constant at a level of 20.946 ± 0.006 vol %, with a slight decrease of 0.0004 O2 vol %/yr (4 ppmv/yr) which is counter-correlated to CO2 produced by fossil-fuel and biomass burning." P.119 left column bottom paragraph: "The level of atmospheric oxygen also seems very stable
when measured over a number of years, as was deterrnined by Machta and Hughes (primary source). Between 1967 and 1970, the atmospheric oxygen between 50 ° and 60 °S had an almost constant value of 20.946 ± 0.006 vol %."