Real Life Experiment – Indoor vs Outdoor Air Quality

There are countless times that I stretch the importance of monitoring and managing indoor air quality by keeping an eye on the outdoor air quality/conditions.

In this experiment, I want to demonstrate that it is hard to manage indoor air. I want to state that indoor spaces should be subject to 24/7 monitoring and not a periodic inspection.

The graph below demonstrated that indoor AQ gets influenced by outdoor air and on top of that, the polluted air lingers for a longer time indoors unless there is a mitigation plan (aka filtration) in place.

The graph shows PM2.5 data during a period of 48h hours of continuous monitoring with an indoor and outdoor air quality monitor. The house uses natural ventilation to keep CO2 and VOC concentrations low. During poor air quality events, windows and doors were kept closed.

Once the neighbor started a domestic fire (for an unknown reason) both indoor and outdoor environments were equally influenced. Unfortunately, as automation wasn’t active, it was too late to stop outdoor air from coming indoors. Outdoor air improved faster due to winds and faster dilution of the pollutants, but the indoor air was above WHO AQGs recommended levels (5 μg/m3) for the rest of the day.

Then another fire event of a bigger magnitude occurred, and once more it influenced indoor air in a great extent. The overall 48h average PM2.5 values for the outdoor and indoor air were 8.6 μg/m3 and 13 μg/m3, respectively.


All in all, my exposure to fine particulates was higher indoors, even though the source was outside the house (about 60 meters away). Air purifiers or a central ventilation system with high-quality HEPA filters are recommended. I have found from past experiments that a positive pressure solution is more effective (in most cases) as it keeps VOCs and CO2 at low levels indoors.


2 thoughts on “Real Life Experiment – Indoor vs Outdoor Air Quality

  1. Hi Sotirios,

    Interesting post. How are you sure that the indoor levels are not from an indoor source considering outdoors is lower than Indoors most of it?

    Thanks as always.


    • Hi Rohit, there were some indoor sources of PM but not of a great importance, like normal friction of textile. My mistake was that I didn’t open the windows after the outdoor incident finished because I didn’t look closely the AQ monitors and because I was skeptical about the actions of the neighbor. I didn’t know whether he will continue the malpractices or not.


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