Carbon dioxide (CO2) is a colorless, odorless gas naturally present in the Earth’s atmosphere. This gas is a product of metabolic processes in humans and animals. While CO2 is not inherently toxic at typical concentrations, its buildup in enclosed spaces is a direct and measurable proxy for the quality of indoor air. Monitoring the concentration of this gas, measured in parts per million (ppm), provides a straightforward indication of a building’s ventilation efficiency. The levels found indoors often exceed outdoor concentrations, making CO2 a significant factor in human well-being.
Establishing the Baseline CO2 Concentration
The first step in defining a safe indoor level is to understand the atmospheric baseline, which is the concentration of CO2 in the fresh air outdoors. Current global averages for outdoor air hover around 420 ppm, and this measurement provides the lowest possible reading one can expect indoors when ventilation is maximized. Air quality experts and engineering organizations use specific thresholds to categorize indoor CO2 levels based on health and comfort.
An optimal indoor environment aims to keep CO2 concentrations below 600 ppm, which is considered highly representative of excellent air exchange. This range ensures that the air is frequently refreshed with outdoor air, effectively diluting any accumulated pollutants. Once levels rise above this point, air quality begins to transition into an acceptable range, which is generally considered to be between 600 and 1,000 ppm. This is the level at which many organizations, such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), often recommend as a maximum for adequate ventilation.
Specifically, ASHRAE guidelines suggest that indoor CO2 should not exceed the outdoor concentration by more than 700 ppm, which typically places the acceptable ceiling around 1,100 to 1,200 ppm. When CO2 concentrations climb into the 1,000 to 2,000 ppm range, the air quality is considered poor, often leading to noticeable complaints of stuffiness and drowsiness. These elevated levels signal that the ventilation is inadequate for the number of occupants present in the space.
Concentrations exceeding 2,000 ppm move into a zone of immediate concern, as the direct physiological effects of the gas itself become more pronounced. While workplace safety standards for long-term exposure, such as those set by the Occupational Safety and Health Administration (OSHA), have limits as high as 5,000 ppm over an eight-hour period, these are regulatory ceilings for healthy adults and are not indicative of comfortable indoor conditions. For residential and general office settings, sustained readings above 2,000 ppm necessitate immediate action to increase fresh air intake.
Sources of Indoor CO2 Accumulation
The primary source of CO2 accumulation in occupied indoor spaces is human respiration. As people breathe, they inhale air containing the ambient outdoor concentration of CO2 and exhale air where the concentration is significantly higher, sometimes reaching 35,000 to 50,000 ppm. In tightly sealed buildings, this constant exhalation by occupants quickly leads to a buildup of the gas, making CO2 a reliable measure of how much fresh air is being supplied per person. The more people in a room and the longer they stay, the faster the CO2 level will rise if the ventilation rate remains unchanged.
CO2 can also be introduced into the indoor environment from various combustion sources. Poorly vented or unvented appliances that burn natural gas, propane, kerosene, or wood, such as gas stoves, fireplaces, and furnaces, generate CO2 as a byproduct. If the venting for these appliances is compromised, the exhaust gases can enter the occupied space, contributing to the overall concentration.
Other Sources
Other, less common sources of indoor CO2 include certain industrial processes or the use of specific materials. For instance, the sublimation of dry ice, which is solid carbon dioxide, can rapidly elevate concentrations in a confined space. Fermentation processes, such as those occurring in home brewing or commercial food production, also release CO2 as a natural result of yeast activity. In all cases, the accumulation is exacerbated by insufficient exchange with outside air.
Health Effects of Elevated CO2
The human body responds to elevated CO2 levels in the air by increasing the depth and rate of breathing, a process that attempts to restore the blood’s acid-base balance. Even moderate increases in concentration, starting as low as 500 ppm above outdoor air, can begin to affect physiological responses, such as a slight increase in heart rate and blood pressure. These subtle changes are usually unnoticed by the occupant but indicate the body is working harder.
A significant area of concern involves the impact on cognitive function, which begins to manifest at levels well below what is considered acutely dangerous. Studies have demonstrated that CO2 concentrations of 1,000 ppm can lead to a moderate decline in performance across several metrics, including reduced decision-making capabilities and increased fatigue. Individuals in offices and classrooms may experience difficulty concentrating, slower reaction times, and an overall feeling of malaise.
As concentrations rise further, into the 2,000 to 2,500 ppm range, the cognitive effects become more substantial, with a greater reduction in strategic thinking and the ability to utilize information effectively. This level is associated with more pronounced physical symptoms like headaches, noticeable drowsiness, and a feeling of stuffiness or poor air quality.
Exposure to very high concentrations, such as those exceeding 5,000 ppm, is generally only seen in industrial or confined settings. At these extreme levels, symptoms can escalate to include rapid breathing, confusion, and a risk of long-term health issues like metabolic dysregulation and increased strain on the cardiovascular system. For the general public, the primary health concern remains the consistent degradation of comfort and mental acuity caused by concentrations between 1,000 and 2,000 ppm.
Strategies for Monitoring and Mitigation
The most effective way for the general public to manage indoor air quality is by actively monitoring CO2 levels. Consumer-grade CO2 sensors typically use Non-Dispersive Infrared (NDIR) technology, which offers high precision and reliability for measuring gas concentration. NDIR sensors work by shining an infrared light through a chamber and measuring how much of that light is absorbed by the CO2 molecules present.
For the most accurate assessment of the air people are breathing, the monitor should be placed within the “breathing zone,” generally four to six feet from the floor. Monitors should be situated away from direct drafts, heat sources, or open windows, which could skew the readings with rapidly changing air samples.
When monitoring reveals that CO2 levels are approaching or exceeding the 1,000 ppm threshold, the primary mitigation strategy is to increase the rate of fresh air exchange. The simplest immediate action is to open windows and doors to allow for natural cross-ventilation, which can rapidly lower concentrations. For a more controlled approach, mechanical ventilation systems, such as the home’s heating, ventilation, and air conditioning (HVAC) system, should be checked to ensure they are operating correctly and introducing sufficient outdoor air.
Ventilation Methods
Other methods to improve air quality include:
- Using spot ventilation, such as kitchen exhaust fans and bathroom fans, to remove stale indoor air and draw in fresh makeup air from outside.
- Ensuring that all flues and vents for unvented combustion appliances are clear and functioning to prevent CO2 and other combustion byproducts from entering the living space.
- Performing regular maintenance of HVAC filters and ductwork to ensure the mechanical system can efficiently circulate and replace the air.