Carbon monoxide (CO) is often referred to as the silent killer because it is a colorless, odorless, and tasteless gas produced by the incomplete combustion of fuels. The danger of this invisible poison is magnified by questions about its movement, particularly whether it can penetrate the barriers in a home. Understanding how this gas interacts with building materials is paramount to protecting occupants from accidental poisoning. The movement of CO through and around structural walls is a significant concern for residents of multi-unit dwellings.
The Physical Properties of Carbon Monoxide
Carbon monoxide is a simple molecule composed of one carbon atom and one oxygen atom, giving it a molecular weight of approximately 28.01 grams per mole. This is very close to the average molecular weight of air, which is around 29 grams per mole, making CO slightly less dense than air. Because of this minimal difference in density, carbon monoxide does not simply collect near the floor or rise rapidly to the ceiling like smoke. Instead, it readily mixes with the surrounding air and distributes itself relatively evenly throughout a space. Its lack of detectable color, odor, or taste means humans cannot sense its presence, making specialized detection equipment the only reliable safeguard against exposure.
How Gases Move: Diffusion and Airflow
Gases move through two primary mechanisms: diffusion and bulk transport, also known as airflow or convection. Diffusion is the slow, molecular movement of gas from an area of high concentration to an area of lower concentration. This process is driven purely by the random motion of individual molecules and can allow gases to pass through microscopic pores in materials.
Airflow is the rapid movement of a gas mass driven by external forces like pressure differences or temperature gradients. This occurs when air is moved by mechanical systems, such as fans or heating, ventilation, and air conditioning (HVAC) ducts. Airflow can also be caused by natural phenomena like the stack effect, where warm air rises, or by negative pressure created by exhaust fans. While diffusion is a slow process, airflow provides a highly effective pathway for quickly moving large volumes of gas.
The Reality of CO Intrusion in Buildings
The question of whether carbon monoxide can travel through walls has a two-part answer that depends on the wall material and the mechanism of movement. CO molecules can diffuse directly through porous construction materials like standard gypsum wallboard, commonly known as drywall. The microscopic pores within the compressed gypsum are many times larger than the small CO molecule, allowing the gas to slowly pass through the barrier. While painted drywall slows this process, it does not completely prevent the infiltration of the gas at potentially dangerous levels from a highly concentrated source.
The primary way carbon monoxide travels between spaces in a real-world structure is not through diffusion, but through bulk airflow. CO often finds passage through structural flaws and unsealed openings in the surrounding walls, floors, and ceilings. These air pathways include gaps around utility penetrations, such as pipes and electrical wiring, as well as unsealed joints and cracks in the building envelope.
Pressure differentials are particularly concerning, especially in homes with attached garages or shared walls in apartment buildings. If the living space is under negative pressure, perhaps due to an operating clothes dryer or exhaust fan, it can actively draw air from the adjacent area. This suction can rapidly transport concentrated carbon monoxide from a source, such as a running car in a garage or a faulty furnace in a utility room, through any available breach in the shared wall. This bulk transport mechanism is significantly faster and moves far greater quantities of the gas than molecular diffusion alone.
Essential Carbon Monoxide Safety Measures
Because carbon monoxide moves primarily with air and can diffuse through porous walls, effective safety measures must address both the source and the means of travel. A fundamental step is the installation of reliable carbon monoxide detectors on every level of the home, especially near sleeping areas. Placing additional detectors in rooms adjacent to potential CO sources, such as attached garages or furnace rooms, provides an early warning system.
Prevention and Sealing
- Regular maintenance of all fuel-burning appliances is necessary to prevent the gas from being produced in the first place.
- This includes annual inspections of furnaces, water heaters, fireplaces, and gas stoves by qualified professionals.
- Homeowners should also proactively seal obvious air gaps and structural breaches in walls that separate the living space from high-risk areas.
- Using fire-rated caulk to seal openings around utility lines.
- Ensure the garage-to-house door has tight-fitting weatherstripping to significantly reduce the risk of intrusion via airflow.