Carbon dioxide (CO₂) is an invisible gas with no smell perceptible by human senses.

Composed of a carbon atom and two oxygen atoms, indoor CO₂ is very common and mostly harmless in small amounts. But at high concentrations, CO₂ can displace oxygen and cause harm or even death.

Ventilation with fresh air is the primary method for reducing indoor CO₂ concentrations. Indoor CO2 build-up can be especially problematic in many spaces because of the lack of ventilation when windows and doors are closed on hot or polluted days.

Rising CO₂ emissions worldwide also pose a threat to indoor CO₂ levels, as outdoor atmospheric CO₂ (a greenhouse gas) can seep indoors. Since 1990, CO₂ emissions have risen nearly 61 percent, from around 20 gigatonnes to nearly 35 gigatonnes in 2021 (see Figure 1).¹

Figure 1: Global CO2 emissions 1990-2021 (in gigatonnes), with an overall increase of nearly 61% since 1990. Source: International Energy Agency (IEA)

Although CO₂ emissions dropped by nearly 6 percent in 2020 from reduced human activities due to the COVID-19 pandemic, CO₂ levels have largely returned to their pre-pandemic levels and are expected to rise by at least another 5 percent during 2021.

Opening windows and doors can help temporarily reduce indoor CO₂.

Additionally, mechanical ventilation can help reduce indoor CO₂. This can also help reduce other common indoor pollutants, such as volatile organic compounds (VOCs) particulate matter, viruses, and bacteria through dilution.

Monitoring indoor CO₂ is also critical to understanding the scale of CO₂ air pollution in a space, its relationship to other air pollutants in that space, and reducing its health effects.

Read on to learn more about CO₂ and its impact on the indoor environment, including:

• the most common sources of indoor CO₂
• acceptable vs. unhealthy levels of CO₂
• the relationship between CO₂ and air pollution
• how to effectively monitor CO2 indoors and help reduce indoor CO₂

Sources of indoor CO₂

By far, humans breathing—specifically, exhaling—is the most common source of indoor CO₂.²

Inhaling brings oxygen (O2) into the lungs and bloodstream, where red blood cells carry it throughout the body to support bodily cells. Carbon dioxide is created as a waste byproduct of oxygen being used by cells to generate energy for metabolism. Red blood cells then carry carbon dioxide back to the lungs, where it’s exhaled back into the air.

When exhalation is the main natural source, indoor CO₂ buildup is primarily based on two factors: room size and number of inhabitants.

The smaller the space and the more people in the space, the quicker CO₂ can build up in the space. This is one of the reasons that the air in crowded conference rooms or classrooms can begin to feel stale and make someone feel drowsy or disoriented after even a short time.³

Other common sources of indoor CO₂ include:

• fumes from stovetop flames or ovens
• smoke from fireplaces or tobacco use
• vehicle exhaust from garages or nearby roads and highways
• heating devices powered by gas or kerosene
• organic matter breaking down in soil beneath buildings
• outdoor CO₂ seeping indoors, especially from nearby fossil fuel-burning sources like factories

Understanding indoor CO₂ levels

Indoor CO₂ is measured in parts per million (ppm). The higher the ppm, the more concentrated CO₂ build-up is.

Typical indoor CO₂ ranges from around 400-1,000 ppm, but can rise as high as 40,000 ppm in extreme cases.⁴

Small, temporary increases in indoor CO₂ are not typically a major threat to human health. These short spikes can often be resolved simply by ventilating the space or using a high-efficiency HVAC mechanical ventilation and purification system.

At higher levels from 2,000 to 5,000 ppm and above, CO₂ can cause short-term symptoms that interfere with attention and cognition as well as health effects from long-term exposure.

Normal: 400-1,000 ppm
Normal indoor CO₂ concentrations hover around 400-1,000 ppm. This means that the space is properly ventilated and has consistent air exchange.

A well-ventilated space not exposed to any nearby sources of CO₂ emissions, such as a factory or busy highway, will generally experience CO₂ on the lower end of this scale. A space lacking ventilation or located close to a major CO₂ emission source can begin to creep up this scale.

Newer homes, schools, and office buildings designed with tight building envelopes for greater energy efficiency are more likely to experience high CO₂ due to a lack of air exchange with outdoor fresh air. This is especially likely when doors and windows are closed or mechanical ventilation and filtration technology is insufficient.⁵

Mild symptoms: 1,000-2,000 ppm
Above 1,000 ppm, CO₂ begins to cause noticeable symptoms as oxygen in the air is displaced by CO2 molecules.⁶

Common but mild symptoms often resulting from CO₂ in this range include:

• drowsiness
• feeling of stuffiness
• mild confusion
• disorientation

Proper CO₂ ventilation can help reduce these symptoms as well as levels of other harmful indoor air pollutants. As a result, some legislatures have mandated average daily indoor CO₂ targets in the lower end of this range to encourage consistent ventilation.

In this vein, the California state legislature passed AB-841 in late 2020. Among other requirements for school ventilation and filtration, this bill set an upper limit of indoor CO₂ at 1,100 ppm in California classrooms and required schools to set up indoor CO₂ monitors to ensure compliance with this limit.⁷

Moderate symptoms: 2,000-5,000 ppm
Beyond 2,000 ppm, CO₂ can cause disruptive health and cognitive symptoms, including:

• headaches
• feeling sleepy
• tightness in chest
• increase in heart rate
• reduced attention
• lack of concentration
• nausea

Figure 2 illustrates a CO₂ reading in this range, along with readings of indoor particle pollution (in green) and outdoor particle pollution (in yellow).

Figure 2: CO₂ levels measuring above 2,000, indicating moderately high levels of indoor CO₂. Source: IQAir AirVisual Pro

High CO₂ in this range is also associated with sick building syndrome (SBS).⁸ SBS refers to a variety of symptoms that accompany poor air quality in a building that’s not properly ventilated. Lack of ventilation can lead to a build-up of indoor air pollutants like CO₂ and other contaminants like bacteria, viruses, and volatile organic compounds (VOCs).⁹

Severe or life-threatening symptoms: 5,000-40,000 PPM
Beyond 5,000 ppm, oxygen displacement caused by high indoor CO₂ results in noticeable and potentially life-threatening symptoms, increasing the risk of:

• losing consciousness
• blurred vision
• sweating
• shaking
• high heart rate
• asphyxiation
• death

At this high level of exposure, a respirator or emergency medical treatment may be required to help an individual get enough oxygen to breathe normally again, especially after long periods of exposure.¹⁰

Many regulatory bodies, such as the U.S. Occupational Safety and Health Administration (OSHA), have set strict limits to help prevent CO₂ in the workplace from surpassing 5,000 ppm.¹¹ Specific sampling methods for accurate monitoring are also often enforced.¹²

Most regulations treat CO₂ as an asphyxiant gas and do not permit 8-hour CO2 exposure in the workplace to exceed 5,000 ppm. Lack of compliance can result in violations punishable by fines and even prison if individuals are severely injured or die due to CO₂ exposure.

CO₂ and air pollution

There is no direct correlation between indoor CO₂ and other common indoor air pollutants, such as particulate matter (PM) or VOCs.

In some cases, indoor CO₂ may exhibit behavior opposite that of other indoor air pollutants. For example, opening a window on a polluted day may reduce indoor CO₂ but increase PM10, PM2.5, and other outdoor air pollutants that penetrate the indoor space.

However, conditions that lead to high levels of CO₂ can also increase indoor concentrations of PM or VOCs. In a poorly ventilated or unfiltered space, both CO₂ and other indoor air pollutants can build up to dangerous levels and result in a wide variety of health effects.¹³

In a shared office space or classroom, for example, exhalation can quickly cause CO₂ and infected respiratory aerosols to build up to high levels. The use of common appliances like printers and copiers can also produce PM2.5 and ultrafine particles (UFPs) that remain airborne for long periods of time in the absence of ventilation or filtration.

Airborne infections linked to viruses, bacteria, and mold are also more likely in unfiltered, unventilated spaces. Biocontaminant aerosols from coughing, sneezing, breathing, or talking can be as small as 0.003 microns and linger in the air for hours, exposing building inhabitants to infection long after aerosols are produced.¹⁴

How to monitor indoor CO₂

CO₂ is a gas and cannot be monitored with typical light-scattering laser sensors used to measure PM.

Instead, CO₂ is best measured with sensors that use infrared (IR) light to estimate the number of CO₂ molecules in ambient air.

Here’s how this works:

1. Ambient air passes through a CO₂ sensor assembly made up of an IR light source, reflective gas cell, and IR light detectors.
2. IR light shines onto CO₂ molecules that pass through the assembly. The CO₂ molecules absorb much of this light.
3. The remaining light that isn’t absorbed by CO₂ molecules passes through to detectors.
4. IR light detectors calculate the change in IR wavelengths from what was produced by the IR light source to what remains after CO₂ absorbs the IR light.
5. The change in wavelength indicates the concentration of CO₂, which is converted to a ppm reading.

A standalone CO₂ sensor may indicate the presence of elevated indoor CO₂ and meet basic CO₂ monitoring requirements for workplaces and schools. A 2021 study published by the American Chemical Society suggests that indoor CO₂ levels may be one tool to help indicate the relative risk of exposure to infectious aerosols in the same space.¹⁵

However, basic CO₂ sensors do not provide critical data about other air pollutants that threaten the health of building inhabitants.

An air quality monitor that measures both PM and CO₂ provides the most useful picture of indoor air quality, including how ventilation and filtration affects these pollutants. Measuring temperature and humidity can also help improve understanding of how atmospheric conditions affect indoor concentrations of PM and CO₂.

The takeaway

Below 1,000 ppm, indoor CO₂ is not a major air quality issue.

However, indoor CO₂ beyond 1,000 ppm can reduce concentration and cognitive performance and cause harm at increasingly high levels. This can have high costs to productivity, academic performance, and health in workplaces and classrooms where air pollutants like PM2.5 and airborne infections are already critical concerns.

Ventilation with fresh outdoor air is the primary solution for reducing indoor CO₂. When outdoor air is polluted or weather is extreme, using mechanical ventilation and filtration can help reduce CO₂ and other indoor air pollutants that affect the health and performance of building occupants.