[throwawayboise]

The oxygen generators in an aircraft are only designed to provide emergency oxygen if cabin presurization is lost, until the pilots can descend to breathable atmosphere (under 10,000 ft). I.e. maybe 10 minutes or so. There will be no dangerous accumulation of CO2 in that time, hence no need for scrubbers.

[a1369209993]

If cabin presurization is lost, there will be no dangerous accumulation of CO2 period, because it leaks out through the non-airtight seal between the oxygen mask and your face. The emergency oxygen is more about providing a breathable partial pressure of oxygen (~0.2 atmosphere IIRC) when the total pressure is much lower than sea level, by greatly increasing the concentration.