Have you ever experienced that in the morning, your eyes, nose and/or throat got irritated? You felt fatigued, even though you just had a 10-hour sleep and woke up at 10 am. You felt dizzy, got a headache and even had a feeling of nausea. It seems like you never got enough rest and never had high-quality sleep. One thing that you may ignore but is essential to your health is the air quality in your room, or “Indoor Air Quality (IAQ)”.
IAQ plays a critical role in human beings’ physical condition: research shows that people spend at least 90% of their time indoors , including 69% in residence and 5% in office factories . However, it is estimated that 96% of buildings have at least one type of IAQ issue . Long-term exposure to poor IAQ may lead to severe acute and chronic health problems, such as respiratory diseases, lung cancer and heart diseases . Besides health issues, poor IAQ also makes an impactful ramification to economics: indoor air pollutants may reduce human productivity, work efficiency and the ability to process information. It is estimated that 33% to 50% of commercial buildings in the US are affected by poor IAQ, contributing to over 10 million lost workdays per year . Obviously, low-quality indoor air causes not only adverse health effects to human beings but also an economic loss to society. Thus, feasible solutions to detecting, quantifying indoor pollutants, and improving air quality are essential from the perspective of personal health, economics and society.
IAQ measurement: what to identify and how to measure
Pollutants affecting IAQ usually come from sources inside a building, though some of them may originate from outsides and are drawn in. The most common pollutants include volatile organic compounds (VOCs), carbon monoxide (CO) and particulate matter (PM) (See Figure 2): VOCs are the main contributors to poor IAQ. These organic chemicals are emitted as gases from indoor products and processes, such as cleansers and disinfectants, wood preservatives, paints, pesticides, dehumidifiers and building materials . Over-exposure to VOCs may lead to eye, nose and throat irritation, headaches, nausea and damaged central nervous system. CO molecules are generated from clothes dryers, water heaters, boilers and stoves . A small amount of CO may cause fatigue and chest pain, and a large amount of CO with long-term exposure may lead to faintness and death*. As CO is colorless, odorless and tasteless, people may not detect the problem until they have already become ill and sick. Thus, it poses a health risk for human beings as an invisible killer. PM is a mixture of solid particles and liquid droplets suspended in the air, such as dust, pollen, smoke and soot. The particles vary in size, and those with a diameter of 10 micrometers or less (PM10) are concerned since they are inhalable and may lead to adverse health effects*. PM2.5, particles with a diameter of 2.5 micrometers or less, have higher respiratory deposition fraction and are more likely to travel into and deposit on the deeper parts of the lung, which may induce tissue damage and lung inflammation  *. Besides pollutants, indoor temperature and humidity levels also influence IAQ: high building temperatures and a humid environment may induce chemical reactions, leading to the emission of harmful chemicals.
To measure IAQ, different types of measurements are conducted. Conventionally, VOCs are detected by a photo-ionization detector (PID), CO by a carbon monoxide detector and PMs by an optical particle counter. Over the past few years, low-cost IAQ consumer sensors have become available and are mostly used to detect and quantify the levels of various indoor air pollutants. They should be placed in the breathing zone, a height range between 0.9 to 1.8 meters above the floor. Also, they should be placed at least 5 meters away from windows, doors and mechanical ventilation systems.
*According to air quality standards by US Environmental Protection Agency (US EPA), the concentrations of CO and PM10 should not be exceeded more than once per year to the level of 35 ppm during one hour and 150 μg/m3 during 24 hours, respectively, and the mean concentration of PM2.5 should be less than 12.0 μg/m3 during one year.
Challenges in IAQ monitoring
Why is it so important to choose where to place your IAQ sensors? This is because it is not easy to achieve accurate IAQ monitoring! The main reason is that we assume that indoor air pollutants are quickly well-mixed up, which, in reality, is not true. There are various static (e.g., carpets, furniture, wood products) and dynamic pollutant-emission sources (e.g., human beings, pets) in the building. As the concentration of a pollutant is highest at the source and gradually decreases, pollutants are inhomogeneous in the buildings and take time to reach homogeneous conditions. Thus, measurement errors take place and are hard to be avoided.
As it takes time for indoor air pollutants to become homogeneous in the building, the placement of IAQ sensors significantly affects the measurement accuracy: indoor pollutants may accumulate on the floor, on the ceiling and at corners; sensors installed in these regions may receive inaccurate results. Direct sunlight in the region nearby windows, personal heater and humidity in the room may induce chemical reactions; polluted chemicals may be released from these regions and influence the accuracy of IAQ measurement, since the result may overestimate the average of pollutants indoors. Also, regions nearby windows and floor fans may have a stronger ventilation rate than any other region in the room; the measurement may underestimate the concentration of indoor contaminants.
Other than sensors themselves, the location of buildings may also influence IAQ measurement: while buildings in the outskirts and mountain regions may have clean outdoor air, houses in/nearby the manufacturing zones and downtown may have terrible outdoor pollution. If the measurements are undergone in these heavily-polluted regions with ventilation systems on, outdoor pollutants, rather than indoor pollutants, may be dominated in the building; the sensors may detect the concentration of outdoor pollutants instead of indoor pollutants, which may get unexpected results.
Besides the aforementioned factors, the difficulty of airflow rate measurement, the reliability and precision of low-cost consumer sensors themselves, the lack of sufficient data on IAQ studies and robust methods of data analysis over IAQ make accurate IAQ monitoring still difficult.
From here, we understand that IAQ matters a lot not only to your health and your working productivity but also to society and the economy. A small number of indoor pollutants, such as VOCs, CO and PMs, may interfere with your sleeping quality and working efficiency and even harm your health. Also, IAQ measurements could help monitor IAQ, but many factors may disrupt the accuracy of monitoring.
Some feasible approaches can be applied to improve your IAQ: first, reduce the emission sources in your building, such as using low-VOC carpets on the floor. Second, keep buildings dry. This can prevent you from harmful sources to your respiratory system. Third, protect against outdoor pollutants, so that contaminants will not come in. Finally, ventilate well, so that you will obtain fresh air in the building all the time.
Next time before sleeping, remember to open the window or the door a bit in your room. Improve the ventilation. Let the fresh air in and let the polluted air out. Maybe this small change could make a big improvement in your sleeping quality!
Reference Indoor Air Quality Monitoring for Enhanced Healthy Buildings, by Gonçalo Marques and Rui Pitarma, IntechOpen, Sep 2018. https://www.intechopen.com/chapters/63898  Indoor Air Quality in Buildings: A Comprehensive Review on the Factors Influencing Air Pollution in Residential and Commercial Structure. DOI: 10.3390/ijerph18063276 https://www.researchgate.net/publication/350285067_Indoor_Air_Quality_in_Buildings_A_Comprehensive_Review_on_the_Factors_Influencing_Air_Pollution_in_Residential_and_Commercial_Structure  The best indoor air quality monitors for identifying indoor pollutants, by Michael Ansaldo, TechHive, Feb 2022 https://www.techhive.com/article/583710/best-indoor-air-quality-monitor.html  CHALLENGES, CONSIDERATIONS, AND CONCERNS OF INDOOR AIR QUALITY, by Bryan Heizmann, Pennsylvania Housing Research Center, June 2015. https://www.phrc.psu.edu/assets/docs/Publications/PHRC%20IAQ%20BB%20FINAL.pdf  Indoor Air Quality Monitoring: How To Test, Measure & Improve, by IotaComm, May 2019 https://www.iotacommunications.com/blog/how-can-you-improve-indoor-air-quality/  Volatile Organic Compounds’ Impact on Indoor Air Quality, United States Environmental Protection Agency. https://www.epa.gov/indoor-air-quality-iaq/volatile-organic-compounds-impact-indoor-air-quality  Carbon Monoxide Poisoning: Health Effects (AEN-166) https://learn.kaiterra.com/en/resources/where-to-install-commercial-air-quality-monitor