This articles is very misleading and makes conclusions based on a low cost sensor that in most cases will give you very wrong absolute readings. I would not trust the results at all. Unfortuntaley all too often, people do air quality measurements and just blindly believe what they see on the display as true figures.
It appears that the monitor in this article is a uHoo meter that can also measure NO2. The uHoo uses the SGX Sensortech MICS-2714 as NO2 sensors. It is a low cost metal oxide sensor and has a couple of significant issues. It is extremely suspectible to interfering gases like CO, Ethanol, Hydrogen, Amonia [1] and also gives highly different results based on humidity and temperature [2]. So just normal temperature and humidity fluctualtions will give greatly different results.
Like other TVOC sensors, often the measurements correlate somehow with the gases but the absolute measurements are often by magnitudes wrong. If absolute measurements -as highlighted in the article- are important than the sensor needs to be calibrated exactly for this environment. This calibration requirement is even mentioned in the official specs [2]:
IS CALIBRATION NEEDED, AND HOW OFTEN?
It depends on the application, and on the required precision. Several factors tend to reduce the measurement accuracy e.g. production dispersion on base resistance (Ro) and sensitivity (S), temperature, humidity, the presence of
interfering gases, ageing of the sensor, and on-time.
Calibration is needed when the required accuracy cannot be obtained due to one or more of the factors listed above. This is the case for most applications where an absolute gas measurement is needed.
This problem with absolute measurements on MOX sensors (most TVOC sensors) has been recognised by the large sensors manufacturers like Sensirion and Bosch and the newer generation of these sensors do NOT provide absolute measurements anymore but an INDEX which defines if air quality is improving or getting worse.
In our open source air quality kits [3], we offer the Sensirion SGP41 as an option which is a much more modern sensor and offers an Index for TVOC and NOx. It still suffers from some of above issues like cross-interference of other gases but at least it does not give false absolute values. It can be very useful for detecting spikes but as outlined above the results need to be really understood with the knowledge of the limitations of these sensors in mind.
[1] https://www.sgxsensortech.com/content/uploads/2014/08/0278_D...
[2] https://www.sgxsensortech.com/content/uploads/2014/08/AN2-%E...
[3] https://www.airgradient.com/open-airgradient/kits/