The conventional design of buildings as airtight spaces with minimal outdoor air exchange was primarily done to improve energy efficiency. Reduced ventilation can result in energy savings because outdoor and indoor air exchange is controlled thus having a lesser need to heat or cool buildings inside. Having less ventilation, though, can also lead to an accumulation of carbon dioxide (CO2) in the air which studies have shown to cause adverse effects on the health and comfort of building occupants. CO2 is a good measurement not only of ventilation rates in built environments but also the condition of indoor air quality. When there are higher levels of CO2 in the building, there is a lower amount of fresh air exchange which could lead to potentially critical effects on the performance and health of occupants. The landmark study by Harvard in 2015 has been cited in numerous articles because it highlighted how even a small increase in ambient CO2 levels can significantly impair the cognitive performance of working professionals.

What are safe levels of CO and CO2 in rooms?


250-400 ppm – Normal background concentration in outdoor ambient air

400-1,000 ppm – Concentrations typical of occupied indoor spaces with good air exchange

1,000-2,000 ppm – Complaints of drowsiness and poor air

2,000-5,000 ppm – Headaches, sleepiness and stagnant, stale, stuffy air. Poor concentration, loss of attention, increased heart rate and slight nausea may also be present

5,000 ppm – Workplace exposure limit (as 8-hour TWA) in most jurisdictions

40,000 ppm – Exposure may lead to serious oxygen deprivation resulting in permanent brain damage, coma, even death

– Source: Occupational Health & Safety, April 2016 


Carbon dioxide is a natural, colorless, non-flammable gas produced when people breathe. Each exhaled breath by an average adult contains 35,000 to 50,000 parts per million (ppm) of CO2 – about 100 times higher than is typically found in the outside air (,2013). CO2 is also produced by burning fossil fuels like coal, gas, and natural oil. When you consider the CO2 concentration levels in indoor environments, you need to take into account several factors such as the number of people in the building and how long they have been staying inside, the air exchange rate, and even outdoor CO2 levels. Higher indoor CO2 levels means ventilation is insufficient to dilute and remove CO2 that is continuously being produced and accumulated in the air.

Although the pandemic has indeed made people more conscious about indoor air quality in relation to the virus, there is a need to bring more awareness about the dynamics of the different air quality factors and how they may affect the risk of virus survival and transmission indoors. CO2 levels for instance, is shown by numerous studies to be a critical factor in the spread of viruses in poorly ventilated areas. In 2019, researchers in Taiwan reported on the effect of ventilation on a tuberculosis outbreak at Taipei University where many of the rooms in the school were insufficiently ventilated and CO2 levels were above 3,000 ppm. When air circulation was improved and CO2 levels were brought down under 600 ppm, the outbreak
completely stopped. According to the research, the increase in ventilation was responsible for 97% of the decrease in transmission. (,2019)

CO2 levels may be easy to measure but if the data is interpreted and managed incorrectly, it can also lead to poor indoor air quality strategies. For instance, a CO2 reading taken at the start of the day when the space is vacant would be different when measured at another time during the day when there are more occupants and possibly higher CO2 levels outside. This is especially the case in workplaces where CO2 levels typically rise and stay elevated during working hours Similarly, CO2 readings may vary in different areas of an indoor environment at any given time. Relying on only one or two CO2 data points to manage your ventilation levels and design your system around it will lead to erroneous assumptions that eventually creates more problems and costs.

Real-time monitoring of CO2 concentration levels, at the breathing zone, is one strategy that is highly recommended by many experts not only to give you an indication if there is adequate ventilation but more importantly provide you valuable insights for identifying trends, establishing thresholds, trouble-shooting problem areas, and validating your IAQ solutions. Combined with a measurement of other factors such as temperature, humidity, particulate matter, and nitrogen dioxide (NO2), it can present a better understanding of the complex and variable elements in your indoor environment so you can properly assess and mitigate the risk of virus survival and transmission. It also leads to more energy savings because it can immediately alert your facility manager when there are ventilation issues that need to be promptly addressed. Moreover, smart indoor air quality monitors deployed at the breathing zone that can connect with and automate your HVAC systems can help you incorporate good IAQ tools like using demand control ventilation systems where air exchange rate changes based on actual occupancy.

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