Why isn't cabin air quality in airliners directly monitored with sensors?

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?

• At 8000' of altitude the effective oxygen concentration is about 15.4% (this takes into account the reduced volume of air). Also, the sensor used in that study was designed to operate at sea level, the notes say: "The literature includes a correction factor F = 1013hPa (14.692 psi) / actual atmospheric pressure - a factor that may be especially pertinent at high altitudes or in examining aircraft cabin conditions.". The standard deviation for the sensor is also +/- 10-15%, so I'm not sure how much faith I put in those being totally accurate. Dec 2, 2017 at 4:48
• @RonBeyer Please check your sources. From this site While the percentage of oxygen in the atmosphere remains fairly constant up to 70,000 feet, the available amount of oxygen to sustain mental and physical alertness decreases above 10,000 feet. The atmosphere is primarily nitrogen (78%) with oxygen comprising 20.9 percent of the atmosphere. There is 21% oxygen up to 70,000 ft, it's just that the body can use less and less of it as pressure decreases. A measurement will show 21% though. Dec 2, 2017 at 6:37
• If the percentage of oxygen has decreased, which component has increased? Related: EASA publishes two studies on cabin air quality --> "The results show that the cabin/cockpit air quality is similar or better than what is observed in normal indoor environments"
– mins
Dec 2, 2017 at 10:30
• The key issue for life is the partial pressure of O2, not the actual percentage. Dec 2, 2017 at 13:01
• Why is the O2 percentage so low, even on the control test, prior to takeoff? Dec 3, 2017 at 1:12