The Importance of CO2 in Indoor Air Quality

Urbanisation has made the global population spend most of their time indoors, making indoor air quality (IAQ) a public health issue. Poor IAQ causes respiratory and multiple cognitive impairments. One of the critical pollutants affecting IAQ is carbon dioxide (CO2). While CO2 is often discussed in the context of outdoor air pollution and climate change, its presence and levels indoors are equally significant. This article explores the importance of CO2 in IAQ, its impact on health and productivity, and strategies to manage and monitor CO2 levels effectively.

Carbon dioxide is a colourless, odourless gas present in the Earth’s atmosphere. It is produced by respiration in humans and animals, combustion processes (stoves and heaters), and industrial activities. Indoors, respiration is the primary source of CO2. The global baseline for CO2 in atmospheric air is assumed to be 400-420ppm. Sixty years ago, this baseline was estimated to be 300 ppm. This rise in the baseline is attributed to climate change and global warming.

An average adult performing sedentary activity breathes about 12-20 times per minute. When we breathe in our indoor spaces that are mechanically ventilated, every exhalation causes a hundredfold increase in the CO2 content. If we inhale air with a CO2 content of 400 ppm, the air exhaled will contain 40,000 ppm of CO2. This phenomenon causes a build-up of CO2 levels when the ventilation is insufficient.

Sustained exposure to higher levels of CO2 causes headaches, drowsiness, dizziness, and nausea. The evidence points to a 21% drop in the cognitive function scores of occupants for every 400 rise in CO2 levels. These health concerns affect our productivity and health.

We inhale rebreathed air whenever we breathe in a poorly ventilated space. Since air is invisible, it escapes our attention. Inhaling is a continuous act, and the rebreathed air makes CO2 the real concern.

David Elfstrom, an Independent Energy Engineer based in Canada, has simplified the calculations of rebreathed air for our understanding.

Fig. 1 indicates that when the CO2 levels in our indoor space are at around 2400ppm, 5% of the air that we inhale or 1 in every 20 breaths, is the air that has come out from someone else’s lungs. Large offices, auditoriums, banquet halls, restaurants, or other public spaces have dynamic indoor spaces. We assume the given space will be used as designed, but that is not the case usually. The monitoring of air quality is not stringent. Also, research shows that viruses tend to remain active longer in spaces where CO2 levels are elevated, making it necessary to mitigate CO2 levels and improve ventilation.

With deliberate observation, the conclusion is that CO2 build-ups in indoor spaces and even cars/cabs as we commute through slow and congested traffic. Here are a few case studies that will explain:

A passenger boarded a cab at Mumbai airport for a 10-minute ride. The CO2 levels in the cab just before alighting was 3504ppm, indicating that the passenger and the driver were inhaling about 8% of the air from their lungs. Cars have a feature that allows outside air intake, which helps contain the CO2 levels to some extent, but this feature is not automated. The concurrent way to determine if this air leads to an increase in PM levels, another pollutant that harms our health, is missing. Wearing a good-quality mask and rolling down the window for a few minutes are some solutions for this issue.

Most office spaces have poorly ventilated meeting rooms. This is because they are mostly conditioned using a split AC or a system not connected to the rest of the office since such rooms are not designed for continuous occupancy. A case study conducted in a meeting room in Mumbai with 20 occupants revealed that the CO2 levels had crossed the 4500ppm mark within an hour. Each occupant was inhaling about 11% of the air that had come out of their lungs.

Hotel guest rooms nowadays do not have any operable windows and rely on bare mechanical ventilation. After monitoring the air quality in a guest room at a hotel in Ahmedabad for 72 hours, an analysis revealed that while the recent push for decarbonisation and rising energy costs incentivises energy efficiency, it comes at the expense of occupant health. Businesses try to keep their operating cost lower by prioritising energy over health. The hotel where the study was conducted has an SOP to switch OFF the treated fresh air (TFA) at 10 pm and switch ON the TFA at 6 am every day. A classic case that “might” save energy but will provide discomfort to the occupants

Air conditioners are highly used in bedrooms, especially during the summer. This is the space where we spend about 33% of our time. With climate change driving ambient temperatures to new highs, the aim is to reduce electricity costs while maintaining thermal comfort. As a result, we tightly seal doors and windows to prevent any air leakage. However, we often wake up feeling fatigued, attributing this to various factors while overlooking the primary issue: the rising CO2 levels. In a bedroom occupied by two people, CO2 levels can typically rise to around 3000 ppm within just a few hours of bedtime.

Monitoring CO2

Regulations mandating every public building/space to monitor and display IAQ pollutants should be practised. We need an app that tells us the IAQ rating of a public building so visitors/occupants can make an informed choice, similar to food review applications.

Buildings must deploy effective ventilation measures. A good demand control ventilation system based on indoor CO2 levels will not increase energy costs and optimise the HVAC system. Increasing awareness and incorporating behavioural changes amongst the building occupants will also bring a change.

Incorporating CO2 management into sustainable building practices is beneficial for the occupant health and environmental goals. When sustainable buildings prioritise energy efficiency, it can sometimes lead to reduced ventilation.

Managing CO2

Energy efficiency is a major part of managing CO2. Heat Recovery Ventilation (HRV) systems exchange stale indoor air with fresh outdoor air while retaining heat energy, ensuring efficient energy use without compromising IAQ. Energy Recovery Ventilation (ERV) systems also recover moisture, maintaining both temperature and humidity levels, which can be crucial in varying climates.

Certifications like LEED (Leadership in Energy and Environmental Design) and WELL Building Standard emphasise IAQ as a critical component. These certifications encourage CO2 monitoring and advanced ventilation systems to ensure buildings meet high air quality standards.

Smart buildings use Integrated Building Management Systems (BMS) to monitor and control various aspects, including CO2 levels. These systems automatically adjust ventilation based on real-time data, optimising air quality and energy use. Internet of Things (IoT) devices provide granular data on IAQ, including CO2 levels, enabling more precise control and management of air quality.

Practical Tips for Occupants

Avoid using indoor combustion appliances without proper ventilation. This includes gas stoves, fireplaces, and kerosene heaters, which can significantly increase CO2 levels. Instead, use exhaust fans in kitchens and bathrooms to remove stale air and bring in fresh air.

Managing indoor CO2 levels goes beyond immediate health and comfort benefits. It has broader implications for public health, productivity, and educational outcomes. For instance, there is a high rise in workplace productivity. Employees and students operating in environments with good air quality tend to perform better, with fewer sick days and higher work satisfaction.

Good IAQ can reduce the prevalence of respiratory illnesses and other health issues linked to poor indoor environments. This, in turn, can reduce healthcare costs and improve overall community well-being.