The current concern with COVID-19 transmission in public buildings has put the focus on CO₂ as the most critical parameter of indoor air quality (IAQ) for a good reason. Experts have noted that CO₂ concentrations give an indication of how much of the air that occupants are breathing in is coming out of other people’s respiratory systems – a salient factor in gauging the likelihood of viral transmissibility. Worried parents in some areas have even started sending pocket-sized CO₂ monitors to school with their kids to covertly collect classroom data.

While the pandemic has made it an IAQ focal point, CO₂ concentration is not an acceptable proxy for overall indoor air quality. To be clear, checking CO₂ levels is an essential part of any IAQ assessment, but it gives a deceptively narrow view, when taken by itself, of what constitutes healthy indoor air.

Even before the pandemic, U.S. schools and office buildings were facing an indoor air quality crisis. A 2020 U.S. General Accountability Office survey found that a third of American school districts – roughly 36,000 – have facilities with HVAC systems that need to be updated or replaced. In the early 1990s, the U.S. Occupational Safety and Health Administration (OSHA) started investigating ‘sick building syndrome’ (SBS), a malaise typified by headache, nausea, cough, fatigue, eye irritation and muscle pain that office workers in older buildings had complained of for decades. 

Shifting the Focus from CO₂ to Contaminants of Concern

Improving IAQ, arguably the most crucial element of a healthier building, starts with a comprehensive assessment of a building’s design and furnishings, its HVAC infrastructure, its airborne pollutants and contaminants and their possible sources. According to the International WELL Building Institute, common indoor air pollutants are often traced to building materials, paints, fabrics, cleaning products, personal care products and even air fresheners – all of which can emit volatile organic compounds (VOCs) and other contaminants of concern.

The identification of contaminants of concern as a common source of indoor pollution has driven changes in air quality standards and, as a result, in how IAQ assessments are conducted.

CO₂ Levels Are Not a Proxy for IAQ

According to ASHRAE Standard 62.1-2019, “…CO₂ concentration is not a good indicator of the concentration and occupant acceptance of other indoor contaminants, such as volatile organic compounds off-gassing from furnishings and building materials. Thus, CO₂ concentration is not a reliable indicator of overall building air quality.”

ASHRAE defines 11 common VOCs as ‘contaminants of concern’ (CoCs) in public buildings: acetaldehyde, acetone, benzene, dichloromethane, formaldehyde, naphthalene, phenol, tetrachloroethylene, toluene, 1.1.1-trichloroethane and xylene. Nearly all have been identified as human carcinogens or been found to cause cancer in laboratory animals. ASHRAE also includes CO₂, carbon monoxide, particulate matter of 2.5 microns or smaller (PM2.5), ozone, radon and ammonia as CoCs.

IAQ sensors can detect most of these CoCs, so facility managers need no longer rely solely on CO₂ concentration as a proxy for IAQ.

The Balancing Act of IAQ Optimization

Increasing ventilation can reduce exposure to VOCs and contaminants of concern as well as transmissibility of airborne pathogens in classrooms and offices. A June 2020 report from Harvard’s T.H. Chan School of Public Health outlined IAQ strategies for reopening schools that include replacing the air in each room five times every hour, or once every 12 minutes. While ventilation (fresh air intake) can be increased without significant capital investment, conditioning the fresh air can increase operational costs and energy use.

An integrated building management solution can enable the use of multi-modal optimization that employs indoor sensors for VOCs and other contaminants of concern as well as exterior sensors that gauge outdoor air quality and weather conditions. For example, if a particular classroom has recently been repainted, sensors will detect elevated VOC levels and direct the HVAC system to pull more fresh air into that space rather than into the entire facility. To minimize energy consumption, that fresh air will be conditioned only to the extent required, based on readings from outdoor sensors.

The same holds for a conference room in an office building that’s designed for 30 occupants but often gets used by only two or three. By monitoring either the number of occupants or the CO₂ they generate, sensors determine on a real-condition basis how much fresh air the HVAC needs to pull into that space – rather than supplying a static, wide-open level of ventilation that would not only expend needless energy but likely freeze those two or three occupants out of the room.

Buildings require a balancing act of IAQ optimization. It keeps occupants’ well-being in mind by actively monitoring conditions and occupancy in each space and responding dynamically. At the same time, it can use energy more efficiently and sustainably – not only to reduce operational costs but also to be more mindful of a building’s impact to its environment.

Learn more about Indoor Air Quality Assessment

• Washington Post, “The coronavirus is airborne. Here’s how to know if you’re breathing other people’s breath,” Chris Mooney, February 10, 2021. [Accessed October 14, 2021]
• New York Times, The hot new back-to-school accessory? An air quality monitor, Emily Anthes, October 10, 2021. [Accessed October 15, 2021]
• ASHRAE Standard 62.1-2019, Ventilation for acceptable indoor air quality. Appendix D. [Accessed October 19, 2021]
• U.S. General Accountability Office, “K-12 Education: School Districts Frequently Identified Multiple Building Systems Needing Updates or Replacement,” June 4, 2020. [Accessed October 15, 2021]
• U.S. Occupational Safety and Health Administration, Technical manual section III, chapter 2: indoor air quality investigation. [Accessed October 19, 2021]
• WELL Certified Institute, Features: Air / Background, Q3 2021. [Accessed October 15, 2021]
• Harvard University, T.H. Chan School of Public Health, Schools for Health: Risk Reduction Strategies for Reopening Schools, June 2020. [Accessed October 20, 2021]