Carbon Dioxide (CO2): What It Is, Why It Matters, and How It Affects Health and Air Quality

Carbon dioxide (CO2) is a naturally occurring gas that plays a central role in Earth’s atmosphere, human biology, and indoor air quality (IAQ). As the most widely discussed greenhouse gas, CO2 heavily influences global temperatures, climate change, and environmental stability.

Beyond its outdoor global impact, CO2 also affects you every day in the built environment. Elevated concentrations of indoor CO2, driven by poor ventilation, directly impact your health, cognitive performance, and productivity.

Understanding what carbon dioxide is, where it comes from, and how to maintain safe CO2 levels is essential for both environmental awareness and personal health and well-being. This article will look at the science of carbon dioxide, its sources and uses, the health effects of high CO2 exposure, and the practical steps you can take to ensure safe indoor air quality in homes, offices, and schools.

What Is CO2 (Carbon Dioxide)?

What Is the Difference Between Carbon Dioxide and Carbon Monoxide?

Many confuse carbon dioxide (CO2) with carbon monoxide (CO). Despite similar names, their toxicity profiles are very different from each other:

Why Is CO2 Essential for Life and the Environment?

Carbon dioxide is a fundamental ingredient for photosynthesis, the process by which plants produce food from sunlight. Without CO2, plant life and, by extension, animal life, would not survive.

It is also regulated by the carbon cycle, a natural system that moves carbon through the environment:

• Release: Animals exhale CO2 during respiration; organic matter decomposes.
• Absorption: Plants, oceans, and soil act as "carbon sinks," absorbing CO2 from the atmosphere.

How Does CO2 Affect the Global Climate?

Atmospheric CO2 acts as a thermal blanket, trapping heat radiating from Earth’s surface to maintain a habitable climate (the greenhouse effect). However, since the Industrial Revolution, anthropogenic activities have disrupted the natural balance.

• Natural baseline: Pre-industrial CO2 levels were approximately 280 ppm.

• Current levels: Global atmospheric CO2 is now approximately 420 ppm.

• Causes: The combustion of fossil fuels (coal, oil, natural gas) and deforestation release carbon stored underground faster than natural sinks (oceans and forests) can absorb it.

Why this matters for buildings: Indoor CO2 levels naturally sit several hundred ppm above outdoor concentrations. As the global outdoor baseline rises, the "floor" for indoor CO2 also rises, requiring more aggressive ventilation strategies to maintain healthy IAQ.

What Are the Commercial Uses of CO2?

Beyond its role in respiration and the climate, CO2 has several commercial and industrial applications:

• Food and beverage: Carbonation for soft drinks and beer; dry ice (solid CO2) for freezing and non-liquid cooling

• Safety: Fire extinguishing systems, to displace oxygen and suffocate the flame

• Medical: Minimally invasive surgeries and respiratory stimulation

• Energy: Enhanced oil recovery (EOR) and carbon capture and storage (CCS) projects

What Are the Main Sources of Indoor CO2?

While outdoor air provides a baseline, indoor activities are the primary drivers of high concentrations in the built environment.

• Human respiration: This is the primary indoor source. Exhaled air contains CO2 concentrations hundreds of times higher than ambient air, causing rapid spikes in spaces that are enclosed or have high occupancy.

• Poor ventilation: Modern offices and homes are often built to be airtight and energy efficient. While this saves on utility costs, it traps exhaled CO2 inside and prevents the exchange of stale indoor air with fresh outdoor air.

• Recycled air: Many commercial HVAC systems recycle indoor air to conserve energy rather than continuously cycling in new air. This moves the contaminated, CO2-rich air around, resulting in high CO2 in the building and a deterioration in overall IAQ.
• Combustion sources: Any form of fire will produce CO2. Unvented gas stoves, fireplaces, candles, and smoking are the most common sources of combustion in the built environment.

What Are the Health Effects of High CO2 Levels?

High CO2 levels primarily impact health by causing cognitive impairment, fatigue, and headaches, starting at concentrations as low as 1,000 ppm. In the workplace, this translates to low productivity, higher rates of sick leave, and poor academic performance.

How Does CO2 Impact Cognitive Performance?

Research indicates a direct correlation between elevated CO2 levels and reduced workforce productivity. The COGfx Study from Harvard’s T.H. Chan School of Public Health published the latest major findings on this link:

• Employees’ cognitive function scores decreased by 50% when the CO2 level in their office building increased from 550 ppm to 1,400 ppm.
• A 100 ppm increase in CO2 (a minor fluctuation that is common in a standard building) was associated with significant reductions in four creative thinking scores, including an 11% decrease in both originality and elaboration scores.
• Increasing the ventilation rate from 20 cubic feet per minute (CFM) of outdoor air per person to 40 CFM of outdoor air per person (thereby reducing the indoor CO2 level) led to a 2x increase in cognitive function test scores, compared to those working in spaces with average levels of indoor pollutants.
• The cognitive benefits of doubling the ventilation rate created an 8% increase in employee decision-making performance.
• This generates as much as $6,500 in increased productivity per person per year. This number is more than 150 times greater than the energy costs of doubling the ventilation rate.

Considering the significant negative impact of CO2 on cognitive performance, as well as the major return from creating a healthier indoor environment, the business case for optimizing CO2 alone (not to mention all of the other indoor pollutants that impact productivity) couldn’t be stronger. In short, when you lower CO2, it’s a win-win for both the employees and the business!

What Are Safe Levels of Indoor CO2?

Safe indoor CO2 levels are generally defined as staying below 1,000 ppm, with optimal levels for health and cognition falling below 750 ppm.

• ASHRAE Standard 62.1: Recommends indoor CO2 be no more than 700 ppm above outdoor levels (approximately 1,000 - 1,200 ppm total).
• WELL v2 Certification: Recommends maintaining levels below 900 ppm, ideally under 750 ppm.

Based on widely accepted IAQ and health standards, here are the CO2 concentration guidelines to know when it comes to occupant health and well-being:

How Can You Reduce Indoor CO2 Levels?

Effective CO2 reduction requires increasing the exchange rate of fresh air. Follow these four strategies to reduce CO2 in indoor spaces:

1. Increase Outdoor Airflow

• Natural ventilation: Open windows and doors regularly, even for a few minutes, to create a cross-breeze and flush out the stale air. But first, make sure that the outdoor CO2 concentration is at a healthy level.

• Mechanical systems: Ensure your HVAC system is regularly maintained and set to draw in the maximum amount of fresh outdoor air, rather than just recirculating indoor air.

2. Mitigate Combustion Sources

• Gas stoves and fireplaces: Always use a functioning exhaust fan that vents outside when operating gas cooking appliances or using a fireplace. 

• Smoking: Prohibit smoking indoors, as well as near any outdoor entrances, as it releases a large amount of CO2 and other harmful pollutants.

3. Manage Occupancy 

• Limit crowding: Limit the number of occupants in smaller rooms, particularly offices or classrooms, to match the size and ventilation capacity of the space. CO2 concentrations in a crowded room can spike to an unhealthy level in a matter of minutes.

• Manage time: Be mindful of how much time people spend in high-density spaces like meeting rooms. Scheduling regular breaks allows the room to “air out” and keep a healthy level of CO2 throughout the day.

• Utilize data: Measure occupancy to understand how to adjust ventilation rates (e.g., higher ventilation when occupancy is high). This process can be automated by syncing this data with the building’s automation system.

4. Deploy Continuous CO2 Monitors

• Confirm HVAC performance: Use historical data to verify that your system is actively pulling in fresh air during peak occupancy hours, rather than relying on assumed settings.
• Automate ventilation: Integrate monitors with your building management system (BMS) to trigger demand-controlled ventilation (DCV). This automatically increases fresh air intake only when CO2 levels rise, balancing health with energy efficiency.

Why Is Continuous CO2 Monitoring Important?

Because CO2 is colorless and odorless, it’s difficult to detect when levels are high until it’s too late and you or your occupants are already experiencing physical discomfort and cognitive impairment. Monitoring CO2 continuously and in real time is critical to preventing these harmful CO2 spikes and the health impacts that follow.

Using the data from continuous CO2 monitors can also help you:

• Verify ventilation performance: CO2 data can help you understand whether or not your HVAC system is effectively removing CO2 and introducing fresh air into your space. If the HVAC system is not performing optimally, use CO2 data to determine how the HVAC settings need to change or if there is a larger issue with the system as a whole.
• Identify problem areas: Continuous data helps pinpoint which rooms, HVAC zones, or times of day are most vulnerable to a buildup of CO2. Proactively adjust ventilation rates or building policies (e.g., opening doors in meeting rooms) to keep CO2 levels steady.
• Surface root causes: In some cases, high CO2 is the result of a more complex issue than low ventilation rates or high occupancy. For example, if you see a CO2 spike in a specific room at the same time every day, it might indicate that a ventilation damper in your HVAC system is stuck. Continuous data makes it simple and quick to identify these underlying causes and determine the right course of action to fix the problem.
• Improve your overall IAQ: CO2 can serve as a general proxy for human-generated pollutants in the built environment. By increasing ventilation to manage CO2, you are simultaneously reducing the concentration of other contaminants like volatile organic compounds, particulate matter, etc., creating an overall healthier indoor environment.

While monitoring CO2 alone provides valuable data to guide building operations, the only way to maximize the insights and outcomes you get from this data is by also monitoring several other indoor environmental quality parameters (PM2.5, PM10, TVOC, NO2, ozone, temperature, humidity, etc.) at the same time.

By monitoring a comprehensive list of parameters that affect your indoor environment, you gain a much deeper understanding of how your space works, how the different parameters affect each other, and what optimizations can be made to enhance the health and well-being of everyone inside.

If you’re interested in monitoring CO2 and want to explore your options, reach out today to connect with an air quality monitoring expert to see which solution is best for your space.