Carbon monoxide (CO) is a colorless and odorless gas. It forms as a byproduct of incomplete combustion when carbon-containing fuels do not burn completely. Since CO is lethal, people often ask about its physical behavior: does it rise toward the ceiling or pool near the floor? This question stems from a misconception that CO acts like smoke or propane, but the reality requires understanding the gas’s density compared to air.
Understanding Carbon Monoxide Density
The movement of any gas in an enclosed space is governed by its molecular weight relative to the average molecular weight of the surrounding air. Carbon monoxide (28.01 g/mol) is only marginally lighter than air (28.97 g/mol), which is a mixture of nitrogen and oxygen. This difference is too slight to cause significant stratification.
Due to this near-identical density, CO does not behave like a heavier gas that sinks, such as propane, nor like a much lighter gas that rapidly rises, such as helium. Instead, the gas mixes relatively evenly with the air in a confined space.
The primary factor determining the initial movement of CO is often the temperature of its source. Since CO is usually produced by a combustion process, it is released hot, causing it to initially rise with the warm air currents. As the gas cools, it diffuses throughout the room, eventually creating a uniform concentration throughout the entire volume of air.
Air currents, ventilation, and the shape of the room exert a far greater influence on CO distribution than its minimal density difference. Therefore, relying on the gas to collect in one specific area, either high or low, is a dangerous assumption. Hazardous levels can exist at any height.
Common Sources of Carbon Monoxide Exposure
Carbon monoxide is generated any time a carbon-based fuel is burned without sufficient oxygen. Inside the home, the most frequent culprits are heating systems and water heaters that are improperly maintained or vented. Faulty furnaces, especially those that use natural gas or oil, can release CO directly into the living space through cracks in the heat exchanger or vent system.
Other common household appliances, such as gas stoves, ovens, and clothes dryers, can produce the gas if they are not operating correctly or if their ventilation is blocked. Wood-burning or gas fireplaces also pose a risk if the chimney flue is obstructed or closed during use. These sources typically produce low levels of CO, but a malfunction can quickly escalate the danger.
The highest concentrations of CO poisoning often originate from equipment intended for outdoor use that is mistakenly operated indoors. This includes portable generators, charcoal grills, and power tools that use gasoline, which should never be used inside an attached garage or near an open window. Vehicle exhaust from a running car idling in an attached garage can also quickly seep into the home.
How Carbon Monoxide Harms the Body
The danger of carbon monoxide lies in its ability to hijack the body’s oxygen transport system. When inhaled, CO passes through the lungs and enters the bloodstream, where it immediately competes with oxygen to bind with hemoglobin, the protein in red blood cells that carries oxygen. This process creates a compound known as carboxyhemoglobin (COHb).
Carbon monoxide binds 200 to 250 times more readily to hemoglobin than oxygen. Even a small concentration of CO in the air can rapidly displace oxygen molecules, severely limiting the blood’s capacity to deliver oxygen to vital organs and tissues. The resulting condition is effectively a form of cellular asphyxiation.
The formation of carboxyhemoglobin also causes a shift in the oxygen-hemoglobin dissociation curve, making the remaining oxygen molecules cling more tightly to the hemoglobin. This prevents the release of oxygen in the tissues that need it most, such as the brain and heart. Initial symptoms of poisoning, such as headaches, dizziness, and nausea, are often mistaken for the flu, delaying appropriate medical response.
Placement of CO Detectors
Because carbon monoxide mixes uniformly with air, the placement of a CO detector is less dependent on height and more focused on location and audibility. Safety experts recommend installing detectors near or inside every separate sleeping area to ensure the alarm wakes occupants. It is also advised to place a detector on every level of the home, including the basement, to monitor for leaks from heating systems located there.
Detectors should not be placed directly adjacent to fuel-burning appliances. A small amount of CO is often emitted upon start-up, which can cause false alarms. A distance of at least 15 feet away from cooking or heating appliances is recommended.
When mounting the device, follow the manufacturer’s instructions, as detector designs can vary. Some manufacturers recommend a wall-mounted height of about five feet from the floor, while others are designed for ceiling placement, often when combined with a smoke detector. The most effective strategy is ensuring devices are installed in the path of air moving from any potential source toward the living spaces.