Carbon dioxide (\(\text{CO}_2\)) is an odorless, colorless gas present in the air inside every home. While outside air typically contains \(\text{CO}_2\) at concentrations around 400–420 parts per million (ppm), indoor levels often rise significantly higher, especially in modern, tightly sealed buildings. When concentrations exceed 1,000 ppm, this buildup can affect indoor air quality, potentially leading to symptoms like fatigue and reduced concentration. Understanding the specific sources of \(\text{CO}_2\) within a residence is the first step toward maintaining a healthy indoor environment.
The Constant Source: Human and Pet Respiration
The most consistent source of carbon dioxide in a home is the occupants themselves, including humans and household pets. This gas is the natural byproduct of metabolism, the biological process where the body uses oxygen to convert nutrients into energy. The rate of \(\text{CO}_2\) production is directly related to the metabolic rate, which changes based on activity level.
An adult resting or seated typically emits \(\text{CO}_2\) at a rate of around 19.6 liters per hour, increasing substantially during physical activity, such as jumping to over 115 liters per hour during strenuous exercise. Because this process is continuous, \(\text{CO}_2\) quickly accumulates in poorly ventilated spaces. For example, concentrations can easily exceed 1,500 ppm in bedrooms overnight.
The density of occupancy directly influences the concentration of \(\text{CO}_2\) in a room. More people or pets in a small, enclosed area means a higher generation rate without a corresponding increase in fresh air exchange. Even large pets contribute a measurable amount, with production estimated based on their body weight relative to a human. \(\text{CO}_2\) concentration is also used as an indicator of overall ventilation effectiveness and the buildup of other bioeffluents.
Emissions from Fuel-Burning Appliances
A second significant source of \(\text{CO}_2\) emissions comes from the combustion of fossil fuels, such as natural gas, propane, wood, or oil, used in household appliances. Furnaces, water heaters, gas fireplaces, and kitchen stoves all rely on this chemical reaction, which produces \(\text{CO}_2\), heat, and water vapor. The design of these appliances dictates where the \(\text{CO}_2\) is released.
Properly vented appliances, like most furnaces and water heaters, are sealed systems. They are designed to draw combustion air from outside and exhaust all byproducts, including \(\text{CO}_2\), through a chimney or flue. This design prevents the gas from entering the living space, ensuring that \(\text{CO}_2\) from these sources does not contribute to indoor levels.
However, unvented or partially vented appliances release their exhaust directly into the home. Gas cooktops and ovens are technically unvented, and their use can quickly raise indoor \(\text{CO}_2\) concentrations. Studies show that unvented appliances, such as certain gas fireplaces, can add approximately 2,000 ppm of \(\text{CO}_2\) to the existing indoor air level while operating.
When the venting system of a typically vented appliance becomes damaged, blocked, or leaky, combustion gases can backdraft or seep into the home. A faulty connection or a cracked heat exchanger in a furnace means \(\text{CO}_2\) that should be safely exhausted outdoors is instead circulating throughout the house. The use of unvented space heaters or decorative gas logs also directly contributes to this indoor buildup.
Minor Chemical and Stored Sources
Beyond the major sources of breathing and combustion, several minor or intermittent sources also contribute to the overall indoor \(\text{CO}_2\) concentration. Ambient air outside a home already contains \(\text{CO}_2\), and this air infiltrates the structure through small cracks, windows, and doors. While often a minor contributor, the outside concentration sets the baseline for the indoor environment.
Infiltration from specific localized sources can be more pronounced. This includes air drawn in from an attached garage where a car or gas-powered equipment has recently been running. The exhaust from these combustion engines can introduce concentrated \(\text{CO}_2\) into the home’s air supply.
Stored organic materials or soil beneath the home can also be a source of the gas. As organic matter decomposes, it naturally releases \(\text{CO}_2\), which can seep into basements or crawlspaces through foundation cracks. Activities like home brewing or storing large quantities of organic waste can similarly generate small amounts of \(\text{CO}_2\) through fermentation.
Mitigating Indoor \(\text{CO}_2\) Levels
The most effective strategy for reducing high indoor \(\text{CO}_2\) levels is to increase the rate of air exchange with the outdoors. This process, known as ventilation, flushes out the \(\text{CO}_2\)-rich indoor air and replaces it with fresher outdoor air. Simply opening windows, especially on opposite sides of a room to create cross-ventilation, is a straightforward way to achieve this.
Mechanical ventilation systems, such as exhaust fans in kitchens and bathrooms, should be used regularly to remove stale air directly to the outside. The range hood over a gas stove should always be operated when cooking to capture combustion byproducts and vent them outdoors. Energy recovery ventilation (ERV) or heat recovery ventilation (HRV) systems continuously exchange indoor and outdoor air while minimizing energy loss.
Controlling the sources of combustion involves ensuring all fuel-burning appliances are well-maintained. Regular inspection of furnaces, water heaters, and fireplaces helps confirm that venting systems are intact and functioning properly to exhaust \(\text{CO}_2\) outside the home. Using a dedicated \(\text{CO}_2\) monitor allows occupants to track levels in real-time and provides an indication of when ventilation is needed, ideally maintaining concentrations below 1,000 ppm.