Diffusion of Carbon Monoxide Through Gypsum Wallboard

Carbon monoxide (CO) poisoning is a significant US health problem, responsible for approximately 500 accidental deaths annually,¹ and a risk of 18% to 35% for cognitive brain injury 1 year after poisoning.² Most morbidity and mortality from CO poisoning is believed to be preventable through public education and CO alarm use.

States have been enacting legislation mandating residential CO alarm installation.³ However, as of December 2012, 10 of the 25 states with statutes mandating CO alarms exempted homes without fuel-burning appliances or attached garages, believing that without an internal CO source, risk is eliminated. This may not be true if CO diffuses directly through wallboard material.

A Plexiglas chamber divided by various configurations of gypsum wallboard was used to determine whether CO diffuses across drywall. Single-layer 0.25- and 0.5-inch thick wallboard, double-layer 0.5-inch thick wallboard, and double-layer 0.5-inch thick wallboard painted on 1 side were tested.

Carbon monoxide test gas (3000 ppm) was infused into 1 chamber at 15 L per minute to a concentration of 500 to 600 ppm and then CO concentrations were measured once per minute in each chamber for 24 hours with monitors having a resolution of 1 ppm and range of 0 to 999 ppm (Biosystems Toxipro Single-Sensor Gas Monitor, Honeywell Inc). Six trials for each of the plain wallboard configurations were performed (3 in each direction) and 3 trials with the painted double-layer wallboard, infusing CO on the unpainted side. Experiments were performed from March 22, 2012, through November 28, 2012.

We sought to determine how rapidly a concentration of CO toxic to humans (100 ppm) would be reached in the noninfused chamber and whether diffusion would then continue, measuring the time until the chambers reached less than 5% of their initial concentration difference. Mean and standard deviation results were calculated using GraphPad statistical software (GraphPad Software Inc).

Carbon monoxide diffused across single-layer gypsum wallboard of 2 thicknesses, double-layer wallboard, and painted double-layer wallboard (Table). When CO was infused into 1 chamber, the concentration reached 500 ppm within 7 minutes. The increase in CO concentration in the noninfused chamber followed rapidly, reaching 100 ppm 17 to 96 minutes after the infusion stopped, depending on the configuration. Concentrations of CO were less than 5% different between the 2 chambers by 12 hours in all configurations.

The Figure illustrates the kinetics of gas movement across 0.5-inch wallboard. In all experiments, the CO concentration in the infused side increased rapidly to 500 ppm, then decreased precipitously when the infusion was discontinued. Concentrations of CO in the noninfused side simultaneously began to increase as CO rapidly diffused across the wallboard.

A recent report⁴ described 3 instances of CO poisoning attributed to CO diffusion through floorboards among residents living above restaurants that cooked with charcoal. To our knowledge, our study is the first examining CO diffusion across wallboard.

There are numerous media reports describing simultaneous CO poisonings in different units of multifamily dwellings. Even though CO might have traveled through ventilation ducts, hallways, or stairways, the building configurations in many such cases are inconsistent with this explanation, raising the possibility that CO passes through walls.

This study showed that CO can pass through gypsum wallboard. Gypsum’s permeability to CO is due to its porosity. Pores in 0.5-inch thick wallboard averaged 466 μm in diameter in 1 report.⁵ Because a CO molecule is about 0.387 nm in diameter, it is not surprising that it could easily travel through a pore 1 million times its size. Diffusion across painted drywall was slower, likely due to pore obstruction by paint. Even though this was a laboratory-based experiment that should be replicated in a real-world setting, a recent study⁶ found similar results using generators in a model house garage.

The ability of CO to diffuse across gypsum wallboard may explain at least some instances of CO poisoning in contiguous residences. Exempting residences without internal CO sources from the legislation mandating CO alarms may put people in multifamily dwellings at risk for unintentional CO poisoning.

Corresponding Author: Neil B. Hampson, MD, Virginia Mason Medical Center, 1100 Ninth Ave, H4-CHM, Seattle, WA 98101 (neil.hampson@vmmc.org).

Author Contributions: Dr Hampson had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Hampson, Courtney, Holm.

Acquisition of data: Courtney, Holm.

Analysis and interpretation of data: Hampson, Courtney, Holm.

Drafting of the manuscript: Hampson, Courtney, Holm.

Critical revision of the manuscript for important intellectual content: Courtney, Holm.

Statistical analysis: Hampson, Courtney, Holm.

Administrative, technical, or material support: Courtney, Holm.

Study supervision: Hampson, Courtney, Holm.

Conflict of Interest Disclosures: The authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported.