Carbon monoxide can pass directly through common wall materials like drywall. A study published in JAMA demonstrated that CO diffuses through standard gypsum wallboard, the material that lines most interior walls in homes and commercial buildings. Because CO is a colorless, odorless gas with extremely small molecules, it moves through porous building materials and through gaps in building assemblies with relative ease. This means a CO source on one side of a wall, floor, or ceiling can poison someone on the other side who has no idea the gas is even present.
How Carbon Monoxide Moves Through Drywall
The key study on this question tested whether CO could travel through gypsum wallboard, which is by far the most common interior wall material in North American construction. The researchers found that CO passes through gypsum readily, and the explanation is straightforward: gypsum board is porous. The pores in a standard half-inch sheet of drywall average about 466 micrometers in diameter, while a carbon monoxide molecule measures roughly 0.387 nanometers across. That makes the pore about one million times wider than the molecule trying to pass through it.1JAMA. Diffusion of Carbon Monoxide Through Gypsum Wallboard Imagine rolling a marble through a tunnel the width of a football field. The gas does not need cracks or holes to get through; the wall material itself is permeable enough.
This permeability is not unique to gypsum. Research on gas permeability across a range of building materials has found that how easily a material lets COâ‚‚ through correlates closely with how easily it lets water vapor through.2Journal of Building Engineering. Carbon dioxide permeability of building materials and their impact on bedroom ventilation need The same principle applies to CO: materials that breathe, in the building-science sense, tend to allow small gas molecules to pass. Concrete block, brick, wood framing, and plaster all have some degree of porosity. Dense, nonporous materials like metal sheeting, glass, or heavy vapor barriers are far more resistant to gas diffusion, but most residential walls are not made from those materials alone. Even when a wall assembly includes a vapor barrier, it is rarely continuous and sealed enough to stop gas migration completely, because those barriers are designed to slow moisture movement rather than to create a gas-tight seal.
It Is Not Just Diffusion Through Materials
While the gypsum study proves that CO can travel through the wall material itself, in real buildings the bigger pathway is often simpler: gaps, cracks, and penetrations in the building envelope. Walls in practice are not solid, continuous slabs. They have electrical outlets, plumbing penetrations, gaps at the top and bottom plates of framing, cracks around window and door frames, and joints between different materials. Air moves through these openings due to pressure differences caused by wind, temperature differences between floors, and mechanical systems like furnaces, exhaust fans, and dryers.
A telling case involved two adjacent retail stores where CO built up to dangerous levels in one store even though the source was in the other. Investigators determined that carbon monoxide created by liquid petroleum gas-fueled welders operating on the first story of one store migrated into the neighboring store through unsealed floor penetrations in electrical receptacles.3Journal of Occupational and Environmental Hygiene. Indoor carbon monoxide exposures in a retail occupancy The CO did not need to seep through a solid wall. It found the path of least resistance through openings that nobody had thought to seal.
This distinction matters practically. Sealing obvious penetrations between spaces, such as gaps around pipes, wiring holes, and unsealed ductwork, does far more to reduce CO migration than worrying about the permeability of the wall material itself. The diffusion through the material is real but slow; the airflow through gaps is fast and can deliver dangerous concentrations quickly.
Multi-Unit Buildings and Shared Spaces
The risk of CO traveling between separate living or working spaces is highest in multi-unit buildings: apartment complexes, row houses, mixed-use buildings with commercial spaces on the ground floor and residences above. In these settings, the people at risk often have no knowledge of, or control over, the CO source.
A particularly well-documented pattern involves restaurants with charcoal-burning ovens. In a series of incidents in the UK, residents living in flats above restaurants were poisoned after workers left charcoal smouldering overnight in traditional or specialty ovens while ventilation systems were turned off. The CO produced by the smouldering charcoal traveled through floorboards and accumulated to dangerous concentrations in the apartments above.4Journal of Public Health. Carbon monoxide from neighbouring restaurants: the need for an integrated multi-agency response The victims were asleep and had no reason to suspect a hazard. Many of these buildings had wooden floor assemblies with no fire-stopping or gas-tight barriers between the commercial and residential spaces, so CO moved upward through the floor structure freely.
These cases highlight a recurring theme: CO poisoning from a neighboring space tends to happen at night or during hours when people are sleeping, because that is when ventilation is lowest, when building occupants are least likely to notice symptoms, and when commercial operations sometimes leave fuel burning unattended. The combination of a continuous CO source, poor ventilation, and sleeping occupants is what turns a low-level leak into a life-threatening situation.
Attached Garages Are a Common Source
For single-family homes, the most common scenario for CO migrating through a wall involves an attached garage. Running a car engine, a gas-powered lawn mower, a generator, or any combustion equipment in an attached garage sends CO into the air. That gas can then move into the house through the shared wall, the door between the garage and the house, gaps around ductwork, and any other penetrations in the garage-to-house barrier.
This is such a well-recognized problem that building codes in most jurisdictions now require the shared wall between an attached garage and the living space to be built to specific air-sealing standards. In practice, though, many older homes predate these requirements, and even newer homes often have imperfect sealing. One engineering approach tested specifically for this problem involved installing a dynamic air terminal device in the garage ventilation system. The idea was to maintain steady airflow in the garage to flush CO away from the shared wall and prevent it from infiltrating into the house. Testing showed that the device did reduce CO infiltration into the living space.5PubMed Central. Improving Household Safety via a Dynamic Air Terminal Device in Order to Decrease Carbon Monoxide Migration from a Gas Furnace
For most homeowners, though, the practical lesson is simpler: never run combustion engines or equipment in an attached garage, even with the garage door open. The open door helps, but pressure dynamics in the building can still pull CO into the house through the shared wall. If you must warm up a car in winter, back it out of the garage first.
Why You Cannot Smell, See, or Feel It Coming
Part of what makes CO migration through walls so dangerous is that carbon monoxide gives you no sensory warning. It is colorless, odorless, and tasteless. Unlike natural gas, which utilities add a rotten-egg scent to so that leaks are noticeable, CO has no such additive. Unlike smoke, which is visible and irritating, CO is invisible and nonirritating at the concentrations that cause harm. You can breathe air with dangerous levels of CO in it and feel nothing unusual until the poisoning is already underway.
Early symptoms of CO exposure, headache, dizziness, nausea, and fatigue, overlap heavily with common illnesses like the flu. This means that chronic low-level exposure from a neighboring unit or an attached garage can go misdiagnosed for weeks or months. A family might visit a doctor repeatedly for headaches and fatigue without anyone considering an environmental cause, especially if the CO source is on the other side of a wall in a space they do not control or even enter. The clue that often cracks these cases is that symptoms improve when the person leaves the building and return when they come home, a pattern that points toward an indoor air quality problem rather than an infectious one.
Where to Place CO Detectors
Because CO can move through walls, floors, and ceilings, detector placement matters more than many people realize. A detector on the wrong wall or at the wrong height may not pick up a dangerous concentration quickly enough, and a home with only one detector may miss CO entering from an unexpected direction.
CO mixes fairly evenly with air because the two have similar densities. CO is slightly lighter than air but not enough to make it rise dramatically like helium. In practice, this means CO distributes throughout a room relatively quickly once it enters. Mounting a detector on the ceiling or high on a wall works fine, and most manufacturers design their units for those positions. The more important factor is location within the home:
- Near sleeping areas: Every floor with a bedroom should have a CO detector within earshot of sleeping occupants. Nighttime exposure is the most dangerous because you cannot notice symptoms while asleep.
- Near the garage wall: If your home has an attached garage, place a detector on or near the shared wall between the garage and the living space, not in the garage itself. Garage detectors will alarm constantly from brief engine starts and are not required by most codes.
- Near fuel-burning appliances: A detector near your furnace, water heater, or gas fireplace catches malfunctions at the source.
- On every level: CO from a basement furnace can reach the second floor. A single detector on the main floor may not alarm in time to protect someone sleeping upstairs.
In multi-unit buildings, a detector in your own unit is your only line of defense against CO generated by a neighbor’s appliance, a commercial tenant below, or shared mechanical systems. You cannot control what happens on the other side of the wall, but you can be warned when the result reaches your air.
Sealing Measures That Actually Help
If you live in a situation where CO migration is a concern, whether from an attached garage, a neighboring commercial space, or a shared wall with another unit, there are practical steps that reduce the risk beyond simply installing detectors.
Air sealing the shared boundary is the most effective measure. This means caulking or foam-sealing around any penetrations in the wall, floor, or ceiling between the two spaces: electrical boxes, plumbing pipes, HVAC ducts, cable conduits, and any visible gaps. Fire-rated caulk or intumescent foam is appropriate for penetrations through fire-rated assemblies. The goal is not to make the wall perfectly gas-tight, which is extremely difficult to achieve in practice, but to close the large pathways that allow bulk airflow to carry CO quickly from one space to another.
Maintaining proper ventilation on both sides of the barrier also helps. In the case of the restaurant poisoning incidents, the immediate cause was that commercial ventilation was turned off overnight while a combustion source continued to operate.4Journal of Public Health. Carbon monoxide from neighbouring restaurants: the need for an integrated multi-agency response Had the restaurant exhaust fans been left running, much of the CO would have been vented outdoors rather than accumulating and migrating upward. For homeowners with attached garages, ensuring the garage has at least passive ventilation (vents to the outdoors) and that the house-side door seals tightly when closed makes a meaningful difference.
Pressure management is another factor. Exhaust fans, dryers, and certain HVAC configurations can depressurize the house relative to the garage or a neighboring space, effectively pulling contaminated air through any available opening. Making sure that the house is not running at negative pressure relative to spaces where CO could be present reduces the driving force that pulls the gas indoors. A home energy audit, the kind that includes a blower-door test, can identify both the air leakage paths and the pressure imbalances that could allow CO migration.
When Building Materials Are the Last Line of Defense
In most real-world CO incidents, the gas reaches people through a combination of material permeability and air leakage, and it is usually the leakage doing the heavy lifting. But there are scenarios where the building material itself is genuinely the pathway, particularly in buildings where the assembly between two spaces has been carefully sealed but is made of porous material. The JAMA gypsum study exists precisely because investigators encountered cases where CO appeared to have traveled through the wall itself, not through gaps in it.1JAMA. Diffusion of Carbon Monoxide Through Gypsum Wallboard
This means that sealing alone, while helpful, is not a complete solution in high-risk scenarios. If a high-output CO source exists on one side of a gypsum wall, the gas will eventually diffuse through the material even if every gap is sealed. The rate is slower than through open pathways, so the concentrations reached on the other side are lower and take longer to build, but they can still reach harmful levels over time, especially in an unventilated room. For this reason, the most robust approach in high-risk separations (such as the wall between a commercial kitchen and a residential unit) combines air sealing with continuous mechanical ventilation on the habitable side and CO detection as a final safety layer. No single measure is sufficient on its own.
Misconceptions About Brick, Concrete, and “Solid” Walls
Many people assume that a brick or concrete wall is effectively impermeable to gas, but this is not the case. Both brick and concrete are porous materials. Concrete block, in particular, has large internal voids and a pore structure that allows gas to pass, although more slowly than through gypsum. Even poured concrete has some permeability, which is why radon, another invisible gas, can enter basements through concrete slabs and foundation walls. The permeability of building materials to small gas molecules correlates with their permeability to water vapor, so any material that can develop moisture issues is also letting gases through to some degree.2Journal of Building Engineering. Carbon dioxide permeability of building materials and their impact on bedroom ventilation need
Materials that are genuinely gas-tight include glass, sheet metal, and some types of plastic membranes. These are not typical wall-construction materials in residential buildings, although they do show up in specialized applications like laboratory containment or clean-room construction. In ordinary homes and commercial buildings, every wall has some permeability, and the practical question is whether the rate of CO diffusion through the material, combined with any airflow through gaps, can produce hazardous concentrations in the occupied space on the other side. The answer depends on the strength of the source, the thickness of the barrier, the ventilation rate in the receiving space, and how long the exposure lasts.
CO Detectors in Rental and Multi-Unit Housing
Renters in multi-unit buildings face a specific challenge: they often cannot control the CO sources on the other side of their walls, floors, and ceilings, and they may not have the authority to perform air-sealing work on shared boundaries. In many jurisdictions, landlords are required by law to provide working CO detectors in rental units, but enforcement varies and the requirements sometimes cover only units with their own fuel-burning appliances, missing the risk from neighboring spaces entirely.
If you rent in a building with commercial tenants, especially restaurants, bakeries, or any business that uses open-flame cooking, smouldering charcoal, or gas-fired ovens, a CO detector in your unit is not optional equipment. The incidents documented in the UK involved residents who had no CO detectors and no idea that the restaurant below them was producing the gas.4Journal of Public Health. Carbon monoxide from neighbouring restaurants: the need for an integrated multi-agency response A battery-operated or plug-in CO detector with a digital display costs very little relative to the protection it offers, and unlike a smoke detector, it should be placed low enough to read the display easily so you can check ambient levels even when the alarm is not sounding. Some models log peak readings, which can be useful for documenting intermittent exposure from a neighbor’s appliance that runs only at certain times of day.
If you suspect CO is entering your unit from a neighboring space, contact your local fire department. Most fire departments carry portable CO monitors and can measure concentrations in real time throughout your unit and in adjacent spaces. This is usually a free service, and fire departments take CO complaints seriously because the potential for fatalities is high. Documenting the measurements also gives you evidence to present to a landlord or local housing authority if the situation requires remediation.