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Indoor CO2 Sensors for COVID-19 Risk Mitigation: Current Guidance and Limitations

Last updated: 05-29-2021

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Indoor CO2 Sensors for COVID-19 Risk Mitigation: Current Guidance and Limitations

» Indoor CO2 Sensors for COVID-19 Risk Mitigation: Current Guidance and Limitations
Indoor CO2 Sensors for COVID-19 Risk Mitigation: Current Guidance and Limitations
Eykelbosh, A. Indoor CO2 Sensors for COVID-19 Risk Mitigation: Current Guidance and Limitations. Vancouver, BC: National Collaborating Centre for Environmental Health. 2021 May.
Publication Date
Angela Eykelbosh
Primary inquiry: Information was requested regarding whether CO2 monitoring could be implemented as a COVID-19 risk mitigation tool in restaurants. Building on a previous NCCEH document in which we explored some of the concerns around CO2 monitoring as a public health tool, the scope was expanded to include all public health guidance regarding CO2 monitoring by occupants to address ventilation adequacy during pandemic conditions.  
Summary statement: CO2 monitoring is an established tool to assess occupancy and ventilation requirements for indoor spaces, typically for the purposes of increasing energy efficiency. During the pandemic, it is critical that spaces are adequately ventilated for the number of occupants and type of activities, as one means to reduce risk of transmission. CO2 monitoring can help to address ventilation inadequacy, provided that users are able to install, monitor, interpret, and react to these devices. However, indoor CO2 levels should not be interpreted as a proxy for COVID-19 risk; SARS-CoV-2 transmission depends on multiple factors, of which ventilation is only one.  There are numerous scenarios in which reliance on indoor CO2 levels may cause occupants to under- or over-estimate the risk of transmission. In general, increasing CO2 concentrations may indicate a lack of acceptable ventilation and an increase in COVID-19 risk if an infected person is nearby.
Disclaimer: The information provided here is for the purpose of addressing a specific inquiry related to an environmental health issue. This is not a comprehensive evidence review and has not been subjected to peer review. The information offered here does not supersede federal, provincial, or local guidance, regulations, or occupational health and safety requirements and/or the advice of a medical professional (where applicable).
SARS-CoV-2 transmission via aerosols – tiny droplets that do not settle due to gravity – is known to play some role in the pandemic.1 It is unclear how often aerosol transmission occurs, but we know that we must act to mitigate it, through a combination of actions such as increasing fresh air supply (with natural or mechanical ventilation), cleaning indoor air with filters and germ-killing devices,2 limiting the occupancy of spaces, and limiting activities that increase the emission of respiratory droplets. However, even when measures to improve ventilation are taken,3 poor air mixing and low air flows can make it difficult to know whether ventilation is truly adequate for the number of people in a space and their activity levels, which change over time.
Monitoring CO2 levels may useful in this regard because indoor CO2 concentrations scale fairly reliably with the number of occupants and their activity level.4 As the number of people in the room increases, or as their rates of respiration increase (for example, through exercise), CO2 concentrations in the room will gradually climb. If CO2 levels are increasing over time, it means that there is not enough fresh outdoor air getting into the room for the number of people present. It is necessary to manage CO2 levels either by reducing occupancy or increasing the amount of outdoor air.
Despite the widespread use of CO2 monitoring in building systems to save energy and reduce costs, the use of CO2 monitoring as a public health tool has only recently gained attention.5 However, indoor CO2 monitoring carried out by non-expert occupants can be difficult to operationalize. Despite its seeming simplicity, some expertise is needed to interpret indoor CO2 concentrations and what they mean for occupant health,6 which may lead to unintended adverse effects if non-expert occupants are asked to monitor and react to these devices. CO2 monitoring and enhanced ventilation are also limited because they only address the aerosol route of SARS-CoV-2 transmission, and are unlikely to decrease the risk of transmission during close interactions between one or more unmasked people. What role then could CO2 monitoring play in reducing COVID-19 transmission in indoor spaces?
The aim of this field inquiry is to better understand the reasoning behind CO2 monitoring and to communicate clearly about the potential benefits and limitations for COVID-19 risk mitigation. We used the following multi-pronged approach to address this inquiry:
Expert consultation with specialists in environmental health, indoor air quality, and occupational health to clarify the challenges associated with CO2 monitoring and suggest potential approaches;
A rapid review of any pertinent academic or NCCEH resources;
A grey literature search for existing guidelines or resources.
Expert consultation
Based on dialogue with environmental health experts within the NCCEH and partner agencies, several potential challenges related to CO2 monitoring for COVID-19 risk mitigation were identified:
The lack of any direct linkage between indoor CO2 concentrations and COVID-19 transmission risk, and the high likelihood that CO2 levels would be misinterpreted in this way;
The variety of CO2 monitoring products in the marketplace, their validity, and their averaging times;
Poor general understanding of how CO2 fluctuates over time as people enter and leave an enclosed area;
The need to identify health-based thresholds for action and practical measures to reduce increasing CO2 concentrations when detected;
The likelihood that CO2 monitoring would exacerbate risk perception and anxiety rather than alleviate it, whether due to differences in guideline values across jurisdictions, differences in CO2 values in the same building, or differences in instrumentation;
The need to devise more nuanced tools to understand the interrelationship between occupancy and fresh air requirements.
These concerns were used to guide a series of questions related to how CO2 monitoring might be operationalized as a risk mitigation measure, supported by selected resources from the academic and grey literature.
Rapid review of academic and grey literature
A rapid literature review was conducted to identify resources related to indoor CO2 monitoring and COVID-19 risk mitigation. Specifically, we sought public health guidance documents or intervention studies that included occupants actively using CO2 monitoring to address indoor air quality (IAQ). EBSCOhost databases (including MEDLINE, CINAHL, Academic Search Complete, etc), Google, and Google Scholar were used to search for articles in English from 2011-2021. The search was conducted using variants and Boolean operator combinations related to indoor environments (room, office, restaurant, condo, etc), exposure to or monitoring of CO2, the use of sensors and outcomes related to risk mitigation (transmission, infection, etc). Full search terms are available upon request.
Who is currently recommending CO2 monitoring?
The literature search revealed a great deal of pre-pandemic information regarding the use of indoor CO2 monitoring by experts to assess occupancy and ventilation. These studies were primarily related to energy savings and efficiency, although a growing body of literature has focused on CO2 in schools with a comfort or health lens. However, there were very few guidance documents or studies that addressed CO2 monitoring as a public health tool during the pandemic. Of the public health-oriented documents, several studies used CO2 monitoring to assess or model infection risk (discussed previously7). Only a few documents (Table 1) advised on how to operationalize CO2 monitoring for use by occupants as a risk reduction tool. No information was available regarding experiences or evaluation of occupants using CO2 monitoring in this way.
Table 1. Public health guidance or other documents on the use of CO2 monitoring to assess ventilation adequacy and reduce SARS-CoV-2 transmission risk.
A portable air cleaner should be considered for spaces that cannot be maintained below 800 ppm.
Washington State15
Public health guidance document
CO2 monitoring required to ensure that “open air” eating places (i.e., patios or restaurants with large open windows) are truly open to the outdoors.
If seated occupants are exposed to > 450 ppm for 15 min, they must be moved to a better ventilated table.
What is an appropriate action limit for indoor CO2?
Health Canada has recently adopted a long-term indoor exposure guideline of 1000 ppm CO2 for residential and other settings,4 which is consistent with the comfort-based threshold of 1000 ppm promoted by professional organizations such as ASHRAE and REHVA. The Health Canada guideline was based on a review of the existing epidemiological studies and controlled exposures, in which indoor CO2 levels were associated with various non-specific respiratory and neurophysiological symptoms (e.g., headache, fatigue). These studies did not demonstrate a causal link between CO2 and symptoms, but rather reflect that one or more factors related to poorly ventilated environments may be having adverse impacts on occupants. 
However, the 1000 ppm guideline value and others noted in Table 1 may not be suitable for pandemic conditions. During pandemic conditions, we should seek to keep indoor air as close to “fresh” outdoor conditions as possible, where outdoor air generally has a CO2 concentration < 450 ppm. When CO2 levels are consistently increasing over time, this is a strong signal that ventilation is inadequate for the number of occupants and/or their activities. However, because of the need to heat or cool air to keep the indoors comfortable, 100% fresh air is not always possible and some amount of CO2 buildup is unavoidable. Notably, the US CDC and REHVA have reduced their recommended indoor CO2 levels to 800 ppm during pandemic conditions.12,14
How should occupants be using CO2 monitors?
The objective of installing a CO2 monitor is for occupants to observe changes in CO2 levels and then take action. There are two approaches to doing this (Table 2):
Table 2. Threshold vs. trend-based approaches to occupant-centre CO2 monitoring.
Select an appropriate action limit and instruct occupants to take action if they approach or surpass that limit.
Action limits are relatively easy to understand and some CO2 monitors can be programmed to alarm when the action limit is passed, reducing the attention required.
Occupants will only take action when CO2 levels are high, rather than acting to keep levels low.
Use the data logging feature to display a CO2 curve. Occupants take action when the curve is trending upward, indicating not enough fresh air for current use.
Reacting to increasing levels keeps CO2 as low as possible; this is more consistent with public health messaging than the use of an arbitrary threshold.
Must be monitored closely. Trends over time may be not be immediately apparent. Occupants entering/exiting will lead to large oscillations, which may lead to user frustration/distraction.
For both approaches, the actions necessary to reduce CO2 levels are the same:
Open windows and doors;
Increase fresh air supply via the HVAC system;
Keep ventilation system fans running during occupied periods, after hours, and on weekends;
Install local exhaust, although increasing exhaust is not as effective as increasing fresh air supply.
There may be some cases in which it is not possible to increase ventilation or reduce occupancy to maintain levels as desired. In such cases, it may be worthwhile to consider whether the activity can be moved to a different space or outdoors.
If it is not possible to move to a different space, a portable air cleaner may be useful.2 Portable air cleaners do not add fresh air to a space or reduce the concentration of CO2. However, if CO2 levels indicate that ventilation is inadequate, then the people within that space may be at greater risk of infection if a sick person enters the space. Air cleaners can reduce the concentration of aerosols, but their effectiveness depends on positioning and other factors.16,17 Occupants may not be aware if their air cleaner is not functioning as expected, so moving to a better ventilated space is preferable.
What types of CO2 sensors are available?
There are various types of CO2 monitors in the marketplace, but the simplist and most common technology is the non-dispersive infrared (NDIR) sensor. These sensors estimate the amount of CO2 present by measuring how much light is absorbed across specific wavelengths. Because CO2 absorbs light at specific wavelengths, there is minimal interference from other gases present, although humidity and temperature can affect the readout.
Numerous NDIR-CO2 sensors are available. Accuracy ranges widely and price is not always an indicator of quality. Although there are many options, the following are some useful functions to look for:
The monitor has an appropriate range. In most indoor spaces, CO2 levels will range between outdoor concentrations (< 450 ppm) and about 3000 ppm (a stuffy indoor environment). In contrast, some CO2 sensors are intended to function as personal safety devices in occupational settings with potentially very high exposures (up to 50,000 ppm); these devices should not be used for typical indoor environments.
CO2 data are averaged over time. Some CO2 sensors provide a real-time point estimate of CO2 concentration that can fluctuate wildly as air moves around. To smooth out some of the “noise,” it can be useful to have a CO2 monitor that displays CO2 concentrations averaged over a set period of time. Averaging periods of 5-15 minutes are ideal for providing useful information without random fluctuations.
The monitor has a datalogging function. Many typical indoor air quality monitors will continuously datalog temperature, relative humidity and CO2 over adjustable time periods. The data can be downloaded to a computer or mobile device and graphed to show how CO2 concentrations change during the course of the day or with varying occupancy.
The monitor shows trends over time. Because many sensors show point estimates, and can fluctuate wildly, it is preferable for the CO2 monitor to have a function that displays trends over time, either as a CO2 curve or as an arrow indicator. If ventilation in the space is adequate, CO2 concentrations should stay more or less steady throughout the day. However, if CO2 levels are trending upward, this means that CO2 is accumulating and the fresh air exchange is not adequate.
The monitor comes with adequate instruction. Depending on the type of monitor purchased, the device may require maintenance and calibration to ensure accurate results over time.
Where do I put the monitor in the occupied room?
Properly locating the monitor is critical to accurately reflecting CO2 concentrations. The sensor should be placed in a location with unobstructed air flow, but not in a location where it will be directly affected by windows, a ventilation system, or concentrated plumes of exhaled air. Ideally, sensors should be installed at a height relatively level with the height of occupants, between 0.5 m and 2 m above the floor. The following locations should be avoided:
At or near a window;
Near an air supply for a mechanical ventilation system;
Within 2 m of any human occupant;
Within 2 m of any open flame, such as a fireplace.
What are the limitations of CO2 monitoring and increasing ventilation?
The resources reviewed here identified numerous limitations to the use of CO2 monitoring to assess ventilation when used by non-expert occupants:
It can be difficult to collect accurate data in poorly mixed spaces, especially when occupancy is low, or the interior air volume is large.13 Low CO2 levels measured in such spaces may not reflect conditions throughout the space. It may be necessary to have more than one monitor or relocate a single monitor multiple times, to cover the entire space.
Although ventilation can help to reduce exposure to the smallest respiratory particles present throughout the room, ventilation does not mitigate risk of transmission due to close contact with an infected person. Ventilation does not act fast enough to interrupt the rapid exchange of respiratory particles during a face-to-face interaction.
Ventilation also does not prevent larger particles from settling on surfaces, where they may contribute to potential fomite transmission.18
As such, CO2 monitoring and ventilation do not replace any of the other necessary public health measures, such as distancing, masking, surface cleaning, hand hygiene, etc.
CO2 monitoring does not replace the expertise of an HVAC professional; HVAC systems must be maintained and assessed periodically to ensure that they are working as designed.
What are the risks of using CO2 sensors?
The two key risks of CO2 monitoring as a COVID-19 risk mitigation tool are technological dependency (discussed previously7) and misinterpretation. CO2 levels are easily misinterpreted as COVID-19 risk and may generate confusion and IAQ complaints if their uses and limitations are not clearly communicated. To facilitate communication, the following FAQs were devised:
Does a high CO2 level mean that COVID risk is high?
No. Being exposed to high CO2 levels does not mean you will contract COVID-19, and having low CO2 levels does not mean that you cannot contract COVID-19.
Although CO2 has been proposed a proxy for COVID-19 risk,19 the readout on a CO2 sensor is not a direct indicator of COVID-19 risk.13 The risk of COVID-19 transmission is based on numerous individual, environmental, and epidemiological factors20 that vary independently of CO2 fluctuations. For example, transmission risk varies with factors such as community transmission rates, which affect the probability that one or more infected people may be in the room. In addition, strategies such as air filtration and masking reduce COVID-19 risk, but do not affect CO2 levels. The most important factors in transmission are the proximity and duration of contact,20 as these factors increase an individual's exposure to respiratory droplets of all sizes. For this reason, close interactions with others will remain the most important determinant of COVID-19 risk.
If CO2 levels in the space are trending upward, take action to reduce them – but high CO2 does not indicate that you will contract COVID-19. Similarly, indoor CO2 levels may be low, but risk of transmission is high if distancing and masking are not observed.
Are there other long-term health consequences of indoor CO2 exposure?
No. There are no long-term health consequences of being exposed to CO2 at levels that are typically encountered in a non-industrial workplace or residence.
Although some research has found that CO2 levels greater than 1000 ppm may be linked to poor cognitive performance, headaches, or other symptoms,4  many other similar studies have not demonstrated these effects.21 If any such effects do occur, they are reversible with removal to fresh air. Prolonged exposure to CO2 levels greater than approximately 6000 ppm, which are rarely encountered in non-industrial settings, may elicit more serious effects, such as fatigue, visual impairment, and cardiovascular effects,4 but these are again reversible with removal to fresh air.
Can we reduce ventilation if CO2 levels can be maintained below 1000 ppm?
In some facilities, CO2 monitoring is used to estimate occupancy, which in turn is used to reduce outdoor air intake if/when occupancy is low. This is known as demand control ventilation and in non-pandemic conditions can be a useful energy-saving feature. However, under pandemic conditions, facilities should strive to keep the air as fresh as possible. Demand control ventilation should be disabled, and the aim should be to maintain CO2 concentrations as close to outdoor levels as possible (< 450 ppm) while maintaining human comfort.
Throughout the pandemic, building managers and the public have been asked to ventilate their spaces adequately through HVAC adjustments, opening windows, installing local exhaust, etc. Without the assistance of an HVAC professional, it can be very difficult for building occupants to know whether ventilation objectives have been met. CO2 monitoring is attractive in this sense: monitors are inexpensive and widely available, and they makes indoor air quality visible, which can help to identify poorly ventilated spaces for remediation.
However, the variety in quality and functionality of sensors, the need to install and operate them correctly, and the tendency to misinterpret CO2 levels as COVID-19 risk means that this strategy must be considered carefully before implementation. Care must be taken to communicate very clearly around the proper use and limitations of this tool, and to emphasize that CO2 monitoring and ventilation are part of a larger suite of public health tools to reduce transmission risk.
The author would like to acknowledge the contributions of Dr. Sarah Henderson (BCCDC), Mr. Geoff Clark (WorkSafeBC), Dr. Steve Rogak (UBC), Dr. Adam Rysanek (UBC), Mr. Gary Mallach (Health Canada), and Dr. Lydia Ma and Michele Wiens (NCCEH).
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