Nathan is on the right track. False positives are very real with these types of instrument. Most of the common detectors are electrolytic in nature or a semiconductor cell that is sensitive ions. These devices usually DO NOT DISTINGUISH between different species of gas molecules.
Industrial safety people will frequently use MORE THAN ONE TYPE of detector, a detector that senses and differentiates multiple gases or some combination thereof. I like having more than one detector, so as to get a "consensus" of readings. Sometimes these machines belong "in the shop" or the trash can.
Safety officers also have their machines calibrated, tested or use "sources" or "reference standards" to verify their device's operation and level of sensitivity. A periodic review of the characteristics of your device is as important as having it in the first place.
SENSITIVE does not mean it is accurate; also TOO MUCH SENSITIVITY is a fault and causes false positives or other manifestations .
The detectors that Nathan mentioned that use transmissivity or spectrographic effects are better, preferred and less often fooled. However, they are generally less used because of the higher expense.
But, ANY concentration of gases that DISPLACES the normal breathing mixture is dangerous. That includes gases that are not normally considered dangerous, such as nitrogen or CO2. While not directly lethal, these do not support life. Exposed to such mixtures, you might be overcome and asphyxiated (PERMANENTLY) after you pass out.
On the positive side: In any event, better safe than sorry. That means take the precautions even if the detector is wrong!!!
Mustang
Attends safety classes Class