Carbon Monoxide Indoors: Why Outdoor AQI Misses the Risk

Last updated May 20, 2026 · 7 min read

Outdoor ambient carbon monoxide has dropped by more than 80% in the U.S. since 1980 thanks to catalytic converters, fuel reformulation, and tighter emissions standards. Almost nobody dies from outdoor CO in the modern U.S. But indoor CO — from generators, malfunctioning furnaces, fireplaces, charcoal grills, and gas appliances — still kills more than 400 Americans every year and sends 100,000+ to emergency rooms, per CDC data. AQI apps don't see indoor air. This guide is about what to watch for and how to prevent the avoidable.

What CO does to the body

CO binds to hemoglobin — the oxygen-carrying protein in your red blood cells — with about 200 times the affinity that oxygen does. A small amount of CO in your bloodstream displaces a much larger amount of oxygen, and the body starts running short on oxygen delivery to tissues. The brain and heart, with the highest oxygen demand, fail first.

Symptoms by exposure level (% carboxyhemoglobin in blood):

The pattern that catches families: a slow leak from a furnace or water heater raises COHb gradually overnight. The household wakes with headaches, nausea, "flu-like" symptoms, doesn't connect it to the source, and goes back to sleep. By the second night the COHb is high enough to cause unconsciousness during sleep. Mass-casualty CO events in homes follow this pattern almost every winter.

The sources that produce real risk

Portable generators

The dominant cause of preventable CO death in the U.S. Generators are designed to be used outdoors, far from inhabited structures, in well-ventilated areas. They produce more CO per hour than a running car. After hurricanes, ice storms, and other prolonged power outages, generator-related CO deaths cluster. Standard rules:

• At least 20 feet from any door, window, or vent.
• Never inside any enclosed or partially enclosed space — including garages, basements, carports, sheds, or against an open window.
• Even an open garage door is not enough. Generator exhaust accumulates in the garage and drifts into the house.
• Tilt the exhaust away from the house, never toward it.
• Modern CO-shutoff generators (sold since the 2020 CPSC standards push) shut down automatically when CO concentrations rise. Look for "CO-Minder," "CO-Sense," or similar marketing on the unit.

Charcoal grills and chimineas indoors

Charcoal produces CO continuously even after the visible flame is out. Bringing a charcoal grill into the garage to "finish cooking out of the rain" or to provide emergency heat after a power outage has killed entire families. Never bring a charcoal grill, hibachi, or chiminea indoors for any reason.

Malfunctioning furnaces and water heaters

Cracked heat exchangers, blocked flues, downdraft from chimney issues — all can route combustion byproducts back into the home instead of out the vent. Annual inspection by an HVAC professional is the prevention. Yellow flames (instead of clean blue) on gas burners are a visual warning sign of poor combustion.

Cars running in garages

Modern emission controls make car-generated CO much lower than 1970s-era cars, but it's still meaningful, especially for older vehicles, cold starts, and idling. Don't warm up your car in a closed garage. Don't run it in an open garage with a connected living space.

Fireplaces with closed dampers

A wood fireplace with a partially or fully closed damper produces measurable indoor CO. Open damper fully before lighting; ensure good draft.

Gas appliances during power outages

Many gas furnaces, water heaters, and ovens require electricity for their venting blowers. During power outages, lighting these manually (or running an unvented gas oven for heat) accumulates CO indoors with no exhaust path. Don't use a gas oven to heat the house; ever.

CO alarms: the one item every home needs

The CO alarm is the most important indoor-air-quality device most people will ever buy, and one of the cheapest. The general guidance:

• At least one CO alarm per floor, including the basement.
• Near sleeping areas — the bedroom hallway is the standard placement.
• Battery-backed if hardwired; battery-only is fine for retrofit.
• UL 2034 listed. Random no-brand units sometimes don't meet the standard.
• Replace per the manufacturer's date. Most CO sensors expire after 5–10 years; the alarm has a date stamped on the back.
• Combination smoke + CO alarms are fine and convenient. Just verify both detection technologies are present (photoelectric or ionization for smoke; electrochemical for CO).
• Test monthly; replace batteries when the unit chirps.

The CDC's research on CO-poisoning deaths shows that the homes with the highest death rates are the ones that don't have working alarms. The cost is roughly $25–50 per alarm; the math is not close.

What to do if the alarm goes off

1. Leave the building immediately. Get everyone out — humans and pets.
2. Call 911 from outside. Don't go back in to grab anything.
3. Do not re-enter until first responders have ventilated the space and verified safe CO levels.
4. Seek medical attention for anyone with symptoms (headache, nausea, dizziness, confusion) — even after leaving the space. COHb takes hours to clear.

If everyone is symptom-free and the alarm is intermittent, the alarm may be malfunctioning or at end-of-life. Replace it. Don't ignore it.

Primary sources: CDC — Carbon Monoxide Poisoning · EPA — Carbon Monoxide · CPSC — CO Information Center