Effective absorption cross sections applicable to total signal from Lyman &agr; ion chambers at all altitudes are presented as a function of normalized signal as well as altitude (and solar zenith angle) for four solar H Lyman &agr; flux conditions (2, 3, 4, and 6 ergs cm-2 s-1) using Carver et al.'s recent &sgr;02 data (over Lyman &agr;) at 195 K and 294 K. The total effective cross sections (which increase to about 2 orders of magnitude over Ly &agr; at ∞) allow the analysis of (generally neglected) Lyman &agr; ion chamber signal at the upper altitudes (greater than or equal to about 95 km where non-Lyman &agr; radiation contributes in the detector signal) in a straightforward manner. The effective cross sections for 'Lyman &agr; absorption only' using a theoretically developed photoabsorption profile representing average mesosphere are also presented. O2 densities derived from Lyman &agr; ion chamber data at upper altitudes from several rocket flights using the total effective cross sections were found to be in good agreement with the O2 density data derived from quartz-triethylamine ion chambers flown in some of the flights and in general agreement with the Cira (1972) model.