Airliner air conditioning systems refresh the cabin air volume every couple of minutes. A portion of it is filtered and re-circulated into the cabin, however AFAIK there are no direct cabin air quality sensors on board, only pressure and temperature sensors. This question and this question on this site are related.
When researching for an answer to the oxygen question, I came across the surprising research result from this site: carbon dioxide levels in the first column are pretty reasonable, but oxygen content in the second column is around 12%, a bit over half of the normal percentage.
A report on fumes and smoke events in Australia shows that these are rare, but that often there is not a direct cause found. Would the fumes be detected by human smell only?
Many fumes/smoke notifications to the ATSB are followed up by a maintenance inspection that does not find a potential source or defect causing the fume/smoke, in which case an Service Difficulty Report would not be submitted to CASA.
Airliners do have smoke detectors on board. A Google search reveals that there are cheap key hangers for sale that detect carbon monoxide. Oxygen and carbon dioxide percentages can be easily monitored. My question is: why do aircraft not contain sensors for direct monitoring of the quality of the cabin air that passengers breathe?