Any cheap sensor (electrochemical? single electrode or even dual) sold for amateur use is unlikely to provide any useful measurement, especially if you want to draw any conclusion about concentrations exceeding safety limits like the author is doing. They are a pain to calibrate, the absolute values you get cannot be trusted, the most you can use them for is to say something about trends and spikes. "Look the gas stove is on, there's a NO2 and CO spike!", now let's get a chemoluminescence spectrophotometer to see it we should worry.