The Dangers of Carbon Monoxide

On October 30, 2022, three American travelers were found dead at a Mexico City Airbnb after a water heater filled their unit with carbon monoxide overnight. The victims were schoolteachers Jordan Marshall, 28, and Courtez Hall, 33, and entrepreneur Kandace Florence, 28, on a Day of the Dead visit. Building security noticed a strong gas smell and called the authorities, which is the detail worth holding on to: what anyone could smell that night was the odorant added to the fuel gas, not the carbon monoxide, which has no smell of its own at any concentration. CDC puts the toll at more than 400 Americans a year from unintentional non-fire CO exposure, with more than 100,000 emergency department visits and more than 14,000 hospitalizations alongside it. The dangers of carbon monoxide follow a recognizable pattern: a gas you cannot see or smell, a malfunctioning combustion source, and a setting where exhaust accumulates faster than ventilation can clear it. This guide explains what CO does inside the body, which exposure scenarios drive most incidents, and which actions reduce the risk for homeowners, renters, and travelers.

Quick Safety Summary

• CO replaces oxygen on red blood cells, starving the heart and brain first.
• A working CO alarm is the only reliable warning, your senses give zero advance notice.
• Most fatal incidents happen during sleep, when symptoms cannot be acted on.
• Renters, travelers, and STR guests carry the same risk as homeowners with less inspection access.
• If an alarm activates: leave first, call for help second, never investigate inside.

What CO Does to the Body

Inside the lungs, carbon monoxide takes the spots on red blood cells that oxygen normally fills. The cells that need oxygen most, the brain and the heart, are the cells that feel the loss first. The deeper science is on our CO chemistry page; for danger assessment, the practical points are simple. CO holds onto red blood cells far more strongly than oxygen does, so even a low concentration in the air leads to a much bigger drop in usable oxygen than the same concentration of any common gas would cause. Removing the exposure reverses the binding, but slowly enough that fresh air alone takes hours rather than minutes, which is why emergency departments take suspected CO exposures seriously.

How Exposure Feels as It Worsens

Symptoms intensify with how much CO has built up in the blood and how long it has been there. The progression is not linear and people do not feel a percentage; they feel the pattern. Early on, the most common signs are headache, dizziness, and nausea that fit a hundred other explanations (flu, dehydration, hangover). Sustained or higher exposure adds confusion, weakness, fast pulse, and chest discomfort. Severe exposure causes loss of consciousness, seizures, and respiratory failure.

The actionable signal is rarely the number on any chart. It is the cluster of details that fit together: symptoms that get worse the longer you stay in a room, ease when you step outside, hit several people in the same space at once, or come on overnight with no other explanation. If that pattern fits or a CO alarm sounds, leave the building and call emergency services. Do not attempt to figure out the source while still inside.

Three Incidents That Show What CO Looks Like in Real Life

Statistics flatten the experience. Three documented cases show how CO kills people and where the protection layers failed. Each one ends with what its jurisdiction requires today, because the question a reader has after a 2013 hotel death is whether the room they booked last week is any different.

Mexico City, October 30, 2022 (short-term rental)

Jordan Marshall, Courtez Hall, and Kandace Florence rented a La Rosita Airbnb for Day of the Dead. Kandace called her boyfriend during the night, said she felt unwell, and the call dropped. The boyfriend asked the host for a wellness check. Building security found all three dead and detected gas inside the unit. Inspection traced the source to a water heater installed against code that vented combustion gases into the apartment. None of the three knew the heater existed. Their families later sued Airbnb and pushed for mandatory CO detectors in all platform listings.

Would that apartment be legal today? We cannot tell you, and that is the answer worth having. Our records cover 51 US jurisdictions and stop at the border, so for a listing in Mexico City there is no statutory answer on this site or on any other US safety page. Airbnb has not made alarms mandatory platform-wide in the years since. For a booking outside the United States, the alarm you can rely on is the one you packed, and that is a different conclusion from the two cases below rather than the same one repeated.

Best Western Boone, North Carolina, April-June 2013 (hotel)

Daryl and Shirley Jenkins, a Washington State couple in their 70s, died in Best Western Boone Room 225 on April 16, 2013. Two months later, on June 8, 11-year-old Jeffrey Williams died in the same room while staying with his mother. Investigators found that hotel staff had replaced a pool heater in October 2011 using an unlicensed crew with no permit, no inspection, and an exhaust path that vented combustion gases into the floor directly above. The room had no CO alarm. North Carolina later adopted new lodging CO requirements, but criminal charges against the hotel manager were dismissed.

Would that room be legal today? No. North Carolina now reaches hotels and short-term rentals, and it reaches existing buildings rather than only new ones, so Room 225 would carry an alarm today without waiting for the hotel to be rebuilt. That is the part of the story worth keeping: the room was legal in 2013 and is not legal now, and the difference is a statute rather than a change in the building.

Plymouth, Massachusetts, January 2005 (single-family home)

Nicole Garofalo, age 7, died when snowdrifts blocked a furnace exhaust vent on the exterior of her family's home, forcing CO back into the living space overnight. The home had no CO alarm. Massachusetts passed Nicole's Law in 2005, mandating CO alarms in all residences with fossil-fuel equipment or attached garages. Snow- and ice-blocked exterior vents remain a documented cause of winter CO incidents in northern climates.

Would that house be legal today? No, and not only in Massachusetts. Massachusetts reaches homes, rentals and lodging, and it reaches existing buildings, so the alarm the Garofalo house did not have is now owed by the house rather than by whoever next renovates it. The wider answer is the one this dataset exists to give: 39 of 51 jurisdictions now apply a requirement to owner-occupied homes. The 2005 statute was early rather than unique, and a reader who wants their own answer can open their state's record instead of counting on ours.

The Scenarios Most Guides Skip

CDC and Mayo cover symptoms and prevention well. What general guides usually miss is how the danger redistributes across user groups. The same gas plays out differently depending on who you are and where you sleep.

• Renters and apartment dwellers: CO may originate from a shared boiler, a neighbor's unvented heater, or an attached parking garage. Local law usually requires the landlord to install alarms, but enforcement varies. See apartment CO response and the state-by-state laws hub.
• Hotel and STR guests: Airbnb, VRBO, and most US hotel chains do not mandate CO alarms platform-wide. Both cases below occurred where the guests had no way to inspect or test the source. See hotel and Airbnb CO safety and consider a portable detector for travel.
• Generator users during outages: CPSC attributes about 100 US deaths per year to portable generators, with 79% occurring at residential addresses (2012-2022 review). Most involve garages, basements, or porches near a doorway or window. See generator distance guidance.
• Homeowners with attached garages: Vehicle exhaust from a car warmed up in the garage migrates upward into living space within minutes. Keyless ignition systems are a quiet contributor: drivers leave running, walk inside with the fob, and never hear the engine stop. See CO in your car.
• Cold-climate households: Snow- or ice-blocked exterior vents (the Garofalo scenario above) reroute combustion gases back indoors. Check intake and exhaust terminations after every major snowfall.

What to Do If You Suspect CO

The response order is fixed: leave, call, do not return. Two compressed cases drive that order: people who paused to investigate often did not make it out, and emergency responders cannot help anyone who is still inside when they arrive. Move everyone (including pets) to outdoor air, call your fire department from outside, and stay out until responders measure the air with a calibrated meter and clear the building. For the expanded checklist, see what to do if you suspect a carbon monoxide leak. For alarm-specific guidance, see what to do if a CO alarm goes off.

How to Reduce the Risk

Prevention is layered. No single step is sufficient; the combination keeps you ahead of the failure modes.

• Working CO alarms on every level, especially near sleeping areas. Test monthly. Replace at the manufacturer's end-of-life date (most are 7-10 years).
• Annual professional inspection of furnace, water heater, fireplace, and chimney before each heating season.
• Generators outdoors only, at least 20 feet from the house, exhaust pointed away from doors and windows. Models built to ANSI/PGMA G300-2018 stop the engine before CO measured an inch or two above the machine passes 400 ppm as a 10-minute rolling average, or 800 ppm in a single reading. That threshold is why the feature is weakest outdoors, where the concentration around the machine may never reach it.
• Garage discipline: never run a vehicle in an attached garage, with or without the door open. Test keyless ignition daily until you know the engine has stopped.
• Travel preparation: pack a battery-powered portable CO detector for hotels and STRs. Place it on the nightstand. The Best Western Boone deaths above all occurred in a room without an installed alarm.
• Cold-weather vent check: clear snow and ice from the exterior side of furnace, water heater, and dryer vents after every storm.

Common Mistakes That Worsen Outcomes

• Treating a chirp as "I will deal with it tomorrow." A chirp means low battery or end-of-life, both of which leave you unprotected. Replace the battery or unit the day you notice the chirp.
• Running a generator on a covered porch. Roof coverage traps exhaust at the same height as nearby doors and windows. CPSC data consistently shows porch generators among the most fatal placements.
• Using a gas oven for room heating. Gas ovens are not designed for sustained operation as space heaters and can drive indoor CO past detector thresholds within hours.
• Assuming an appliance you cannot inspect is safe. A water heater is the source in a striking share of lodging cases, including the Mexico City Airbnb deaths and, in the 2005-2018 hotel case series, 22 of 115 incidents.
• Pausing to figure out the source while inside. Source identification is a job for fire department or HVAC professionals with calibrated meters, not for occupants smelling for clues that do not exist.

Sources & References

• CDC, Carbon Monoxide Poisoning
• CDC MMWR, Unintentional Non-Fire CO Deaths 1999-2010 QuickStats
• ATSDR/CDC, Toxicological Profile for Carbon Monoxide
• CPSC, New CPSC Report Shows Upward Trend in CO Fatalities (2023)
• CPSC, Carbon Monoxide Alarm Conformance Testing to UL 2034 (Table 3 gives the response times quoted in the FAQ: 60-240 minutes at 70 ppm, 10-50 at 150 ppm, 4-15 at 400 ppm)
• NBC News, Families of the Americans who died in Mexico City (the water heater, attributed to the families' attorney)
• Gloucester MA, Nicole's Law Reference
• CNN, Mexico City Airbnb CO Deaths (November 2022)
• Watauga Democrat, Best Western Boone CO Deaths Timeline

Last updated: May 17, 2026