Carbon dioxide, or CO2, is a colourless and odourless gas. On average, it is found at about 400 parts per million (ppm) in the atmosphere; though most people will be aware that this concentration has been rising at an increasingly rapid rate since the first Industrial Revolution.
The gas is a product of both combustion and respiration, and it is this latter characteristic that makes it a useful proxy for air quality in the indoor environment.
Elevated levels of CO2 are in most cases not immediately harmful to human health. However, they can be used as an indication of insufficient dilution of airborne pollutants such as volatile organic compounds (VOCs) and other bio-effluents, which are emitted or brought into a space by occupants. In short, high levels of CO2 usually mean that not enough fresh air is being provided.
After a slow start, most health organisations and governments have acknowledged that SARS-CoV-2, the Coronavirus that causes the disease COVID-19, is primarily spread in the form of droplets and aerosols expelled through respiration. In addition, the generation of these aerosols is increased when people breathe more forcefully, for example by singing, talking loudly, or exercising – activities that also generate more CO2.
This correlation between CO2 concentration and the risk of COVID-19 infection has led many organisations, such as the UK government's Scientific Advisory Group for Emergencies (SAGE) and Chartered Institution of Building Services Engineers, to recommend the use of CO2 monitors in some indoor spaces.
This allows occupants, who may well have no expertise in the science of building ventilation, to assess quickly whether a space is poorly ventilated, and hence presents a higher risk of COVID-19. A high-profile example of this approach is the government's recent distribution of 300,000 CO2 monitors to schools in England and Wales.
It should be noted that SAGE highlighted a number of cases where CO2 is not a good indicator of infection risk. These include large spaces where a significant reservoir of indoor air may disguise more local transmission risks.
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The use of CO2 monitors has also been included in the recent revision of the English Building Regulations Approved Document F, which takes effect on 15 June.
For new buildings, offices that have a floor area between 50m2 and 320m2 – corresponding roughly to volumes between 125m3 and 800m3 – and rooms where 'singing, loud speech and aerobic exercise' are likely to take place, should now assess indoor air quality using CO2 monitors or other sensors
While the Building Regulations have contained guidelines on common air pollutants for a number of years, this is the first time that continuous indoor air quality measurement has been legally required.
As a result, CO2 monitoring will almost certainly become a standard part of ventilation design, likely connected to a central building management system (BMS). Smart designs will use the CO2 monitors to control the amount of fresh air supplied into the space, reducing fan power and assisting the drive towards net zero.
As well as an indicator of the fresh air supply to a space and its relationship to COVID-19 risk, the impact of CO2 itself on health and productivity has been studied for decades.
At very high levels – more than 5,000ppm – there are known health and safety risks, which are covered by health and safety legislation. Even at levels exceeding 2,000ppm, which may be experienced regularly in poorly ventilated spaces, most research shows an increase in drowsiness and lower performance.
At levels less than 1,500ppm, which are more typical of modern buildings, the impact of CO2 on human health and performance is less consistent. A study by Xiaojing Zhang and colleagues found no clear relationship between CO2 and cognitive performance. Some studies even found that higher levels of CO2 on its own actually increased performance, up to a point.
This inconsistency likely results from the extreme difficulty of trying to prove causation between a change in the environment and a person's productivity. There are almost endless confounding factors that could lead to a change in performance, including other indoor pollutants.
The indoor environment is a cocktail of many chemical and biological elements, many of which are not actively harmful. If the source of these elements is indoors, as it is with many VOCs, using CO2 monitoring to increase awareness of poor ventilation may have the added benefit of reducing the health risks associated with them.
But the air outside is not always fresh, and for many city-dwellers the concentration of pollutants such as nitrogen dioxide (NO2), particulates (PM2.5 and PM10) and ozone (O3) can regularly exceed healthy levels. By opening a window to reduce CO2 levels, occupants may actively make air quality worse.
So using CO2 as the sole proxy for air quality has limitations, and will not always lead to improved air quality. If there are other internal or external sources that are not measured and managed effectively, then the health of occupants can still be at risk.
The complexity inherent in ensuring good indoor air quality is reflected in the different standards used across the UK, let alone the world, in defining the target CO2 level.
The Health and Safety Executive (HSE) has produced guidance for using CO2 monitors for COVID-19 risk assessment. This states that CO2 levels of less than 800ppm are likely to indicate that a space is well ventilated, and this is recommended for areas with high levels of aerosol generation. A reading higher than 1,500ppm is indicative of poor ventilation, and requires corrective action such as opening windows.
The standard BS EN16798-1 predates COVID-19 and gives a value of 950ppm CO2 for a high level of indoor air quality for general use. Meanwhile, Building Bulletin 101 covers school ventilation in the UK, and suggests a daily average of 1,000ppm for mechanically ventilated and 1,500ppm for naturally ventilated spaces, which reflects the variability of wind-driven ventilation.
Overall, a level below 1,000ppm is acceptable in most spaces; 800ppm is good, and required for spaces with higher levels of aerosol generation; and 600ppm is excellent, but may come with an energy penalty. To get below 600ppm in a well-occupied space means providing significant amounts of outdoor air, which requires more energy for ventilation fans, and heating and cooling. Providing visual feedback to occupants about CO2 levels – typically in the form of a traffic light system or similar – will enable them to act if air quality is poor.
This brings us on to the issue of the ongoing climate emergency, and the global push to reduce the energy consumption of buildings.
With a global COVID-19 pandemic raging, and the link made between poor air quality and infection risk, many organisations and governments pushed for ventilation to be set to maximum for as long as people were in a building. Energy considerations were side-lined in favour of saving lives and reducing immediate impacts on health.
In the long term, this approach is unsustainable, and would lead to excessive heating and cooling of buildings, just as the time these should be drastically reduced. Thankfully, the new focus on CO2 monitoring devices has provided an excellent opportunity to incorporate demand control ventilation (DCV). This helps reduce energy consumption in many building types, while maintaining high levels of indoor air quality. DCV works by taking readings from CO2 monitors in a space and increasing or decreasing the air supply accordingly.
By reducing the outdoor air supply, fan speed is significantly decreased, and less energy is required to heat or cool the air before supplying it to the occupied space. Integrating the CO2 monitors with building control systems where possible can support DCV. This will both improve health and well-being and reduce energy consumption.
It should be noted, though, that CO2 monitors are not infallible. Much of the interest in monitoring has been prompted by a reduction in the cost of these devices and the ease of connecting them to the internet. Non-dispersive infra-red (NDIR) sensors are generally regarded as the minimum standard, and are specified as such in the new Part F and other guidance.
Sensors that identify a CO2 equivalent (CO2e) are typically cheaper. These estimate air quality according to whichever bundle of chemicals they are able to detect, typically VOCs. However, this introduces significant uncertainty, and should be avoided.
As with any measurement device, CO2 monitors also require ongoing calibration. This should usually be undertaken annually and can be managed alongside other calibration processes, although some manufacturers use techniques that may allow longer periods between calibration. Alternative approaches include hot-swappable sensors in the devices, and takeback schemes where the sensors are replaced periodically.
Internet of things (IoT) devices, which are typically connected to building networks and send data to a central store or link to a building management system for control purposes, can also present network security risks, which need to be managed by IT teams. In short, you should select a suitable NDIR monitor, be aware of the calibration requirements, how to use them in different types of spaces, and how to connect them safely to building systems.
It should be clear that the issue of CO2 monitoring is not a straightforward one. But following a methodical approach to the issues it raises should lead to buildings that are energy-efficient, safe and healthy.