Indoor air should contain as close to zero parts per million (ppm) of carbon monoxide as possible, and any sustained reading above 9 ppm warrants attention. That 9 ppm figure comes from both the U.S. Environmental Protection Agency’s national ambient air quality standard and the World Health Organization’s eight-hour exposure guideline, and it serves as a practical ceiling for what counts as safe in a home over the course of a day. But the full picture of “acceptable” is more layered than a single number, because the danger CO poses depends on concentration, duration of exposure, and who is breathing it in.
The Numbers That Define Safe Exposure
Several organizations have set guidelines, and they do not all use the same timeframe or threshold. The WHO provides tiered limits based on how long you are exposed: roughly 87 ppm for 15 minutes, about 35 ppm for one hour, 9 ppm for eight hours, and 7 ppm for 24 hours. The EPA’s outdoor standard, which is often used as a benchmark for indoor readings, allows up to 35 ppm averaged over one hour and 9 ppm averaged over eight hours. The Occupational Safety and Health Administration (OSHA) permits up to 50 ppm as a workplace ceiling over an eight-hour shift, but that is a limit for healthy working adults, not for homes where infants, elderly people, and those with heart disease may spend all day and night.
Home CO detectors sold in the United States must meet UL 2034 standards, and these alarm thresholds are deliberately set higher than health guidelines. A detector will not sound until it registers 70 ppm for somewhere between one and four hours, 150 ppm within 10 to 50 minutes, or 400 ppm within 4 to 15 minutes. The reason for the gap between what health agencies call safe and what triggers an alarm is partly practical: detectors need to avoid false alarms from brief, harmless spikes like opening an oven. But the gap also means that a home can have chronically elevated CO at, say, 15 to 30 ppm without ever setting off an alarm. That range will not kill you overnight, but it is not harmless either.
How Carbon Monoxide Does Its Damage
CO is dangerous because it competes with oxygen for a seat on hemoglobin, the protein in red blood cells that ferries oxygen from your lungs to your tissues. CO binds to the iron center of hemoglobin far more tightly than oxygen does, forming a compound called carboxyhemoglobin (COHb) that essentially locks out oxygen molecules.
1PubMed Central. Binding of Carbon Monoxide to Hemoglobin in an Oxygen Environment: Force Field Development for Molecular Dynamics The more CO you breathe, the higher your COHb percentage climbs and the less oxygen your blood can deliver. At a COHb of around 2%, the effect is subtle. By 25%, the hemoglobin’s ability to release whatever oxygen it does carry is already dramatically compromised.2PubMed. Influence of carbon monoxide on hemoglobin-oxygen binding
The harm does not stop at oxygen deprivation. CO also disrupts the cellular machinery that uses oxygen inside tissues, triggers inflammation, and can injure the heart muscle and brain even after COHb levels have been brought back down.3PubMed Central. The Diagnosis and Treatment of Carbon Monoxide Poisoning This is why carbon monoxide poisoning can leave lasting damage that seems out of proportion to what the initial blood test showed.
Symptoms and Why They Are Easy to Dismiss
Mild CO poisoning produces symptoms that could describe a dozen other things: headache, fatigue, nausea, dizziness, and general weakness.4Düzce Tıp Fakültesi Dergisi. The Relationship of Lactate Levels with Carboxyhemoglobin Levels and Clinical Findings in Patients Admitted with Acute Carbon Monoxide Poisoning At higher exposures, confusion, impaired coordination, and visual disturbances set in. Severe poisoning leads to loss of consciousness, seizures, coma, and death. One complicating factor is that the severity of symptoms does not neatly track with blood COHb levels measured at the hospital. Someone with a relatively low reading can be sicker than expected, and someone with a higher reading may appear deceptively well.5Toxicology. Carbon monoxide poisoning — a public health perspective This poor correlation makes CO poisoning especially tricky to diagnose based on blood tests alone.
Even after what looks like a full recovery, some patients develop delayed neurological problems days or weeks later. These delayed neurological sequelae can include memory difficulties, personality changes, difficulty concentrating, and movement disorders.6PubMed Central. Delayed neurologic sequelae of carbon monoxide intoxication Brain imaging in these patients shows progressive damage to deep white matter, consistent with a slow-burning inflammatory process triggered by the original exposure.7Scientific Reports. Early neuroimaging and delayed neurological sequelae in carbon monoxide poisoning: a systematic review and meta-analysis The existence of these delayed effects is one reason researchers care so much about preventing even moderate exposures: the consequences do not always announce themselves right away.
Who Faces the Greatest Risk
People with coronary artery disease are the group most clearly harmed by low-level CO exposure. In a study of 63 men with documented heart disease, raising COHb to just 2% shortened the time to exercise-induced chest pain and ischemic changes on an electrocardiogram. At a COHb of about 4%, the time to ischemic changes dropped by roughly 12%.8PubMed. Short-term effects of carbon monoxide exposure on the exercise performance of subjects with coronary artery disease A separate study found a similar pattern: raising COHb from a baseline of about 1.5% to just 3% reduced time to angina onset by 6% and showed a 20% reduction in time to measurable ST-segment depression in a subgroup of participants.9PubMed. Effects of short-term exposure to carbon monoxide in subjects with coronary artery disease These are COHb levels that a non-smoker could reach from spending hours in a home with a malfunctioning furnace or an unvented gas heater.
Pregnant women and their fetuses are another high-risk group. Fetal hemoglobin has an even higher affinity for CO than adult hemoglobin, which means fetal COHb can climb higher and clear more slowly. Fetal complications from CO exposure include anoxic brain injury and, in rare severe cases, organ damage, and the severity of fetal injury does not reliably correspond to the mother’s COHb levels.10PubMed Central. Carbon Monoxide Poisoning during Pregnancy: Presentation of a Rare Severe Case with Fetal Bladder Complications A pregnant woman who feels only mild symptoms may be carrying a fetus experiencing something much worse. Young children, older adults, and anyone with anemia or chronic lung disease are also at elevated risk, but the evidence is strongest and most specific for cardiac patients and fetuses.
Where the CO in Your Home Comes From
Any device that burns fuel can produce carbon monoxide if combustion is incomplete. The usual suspects in a home include gas furnaces, gas water heaters, gas or propane stoves, wood-burning fireplaces and stoves, pellet stoves, coal stoves, kerosene heaters, and attached garages where cars idle. A study measuring CO in homes using different heating fuels found elevated levels across all fuel types, but the highest hourly concentrations showed up in homes burning wood and coal, with some readings exceeding WHO guidelines on both an hourly and eight-hour basis.11Science of The Total Environment. Low-cost measurement techniques to characterize the influence of home heating fuel on carbon monoxide in Navajo homes
Unvented appliances are particularly concerning because they dump all their combustion byproducts into the living space rather than routing them outside through a flue. An unvented gas space heater or a gas oven used for heating can steadily raise indoor CO. Even properly vented appliances produce CO when a flue is blocked, a heat exchanger cracks, or backdrafting pulls exhaust back into the house. Generators are an extreme case: running a portable generator inside a garage, basement, or near an open window is one of the leading causes of fatal CO poisoning after storms and power outages.
The Flu Season Misdiagnosis Problem
One of the most unsettling facts about carbon monoxide is how often chronic, low-level poisoning gets diagnosed as something else entirely. Because CO poisoning symptoms overlap so heavily with influenza and other viral illnesses, and because CO exposure tends to peak in winter months when heaters are running, emergency physicians regularly miss it. In a study conducted during February at a university hospital, every patient who presented with flu-like symptoms was tested for COHb. About 24% of the self-selected group had COHb levels at or above 10%, a clear sign of CO exposure. Not a single one of those patients was diagnosed with CO poisoning by the emergency physicians who treated them. Patients using wood heat were significantly more likely to have elevated levels.12Annals of Emergency Medicine. Carboxyhemoglobin levels in patients with flu-like symptoms
The pattern is consistent enough that CO poisoning has been called the “great mimicker.” Because symptoms are nonspecific, the true incidence of acute CO poisoning is almost certainly underestimated.13Chest. Management of Carbon Monoxide Poisoning A useful rule of thumb: if everyone in a household develops headaches and nausea around the same time, especially during heating season, and the symptoms improve when people leave the house, CO should be suspected before a virus. Pets may also show symptoms or behave strangely, since they are affected by CO just as humans are.
The Murky Territory of Chronic Low-Level Exposure
Most research on CO poisoning focuses on acute, high-dose events. The health effects of breathing low concentrations over weeks or months are much less well understood. Anecdotal evidence and case reports suggest that chronic exposure to levels below the alarm threshold can produce mild neurological symptoms: difficulty concentrating, memory problems, mood changes, persistent headaches. But nailing down the connection scientifically has been difficult because people who experience chronic low-level exposure are often also exposed to occasional higher spikes, making it hard to untangle which type of exposure is responsible for their symptoms.14PubMed. Effects on health of prolonged exposure to low concentrations of carbon monoxide
This uncertainty is actually part of the practical answer to the title question. “Acceptable” levels are defined conservatively because the long-term risks are not fully mapped. If your home routinely reads 5 to 8 ppm, you are technically under every guideline, but that does not mean you should ignore it, especially if anyone in the household has heart disease, is pregnant, or is very young or old. The source of the CO should be identified and addressed. A properly functioning furnace in a well-ventilated home should not produce detectable CO indoors at all.
Detector Placement, Maintenance, and the Law
A common question is whether CO detectors should go high on the wall (because warm combustion gases rise) or low (because CO is slightly heavier than air). The answer: it does not matter much. When researchers infused CO into a sealed chamber, the gas distributed evenly throughout the space regardless of where it was introduced, as predicted by basic gas diffusion principles. In a real home, with drafts, air currents from HVAC systems, and people moving around, mixing would be even faster.15The Journal of Emergency Medicine. Should the Placement of Carbon Monoxide (CO) Detectors be Influenced by CO’s Weight Relative to Air? Place your detector where you can hear it and where it is easy to maintain, ideally near sleeping areas so it can wake you.
Many U.S. states and localities now require CO detectors in homes, and the evidence suggests these laws help. After New York City’s mandatory CO alarm law took effect, investigations that found indoor CO levels above 9 ppm increased nearly sevenfold, meaning the alarms were catching problems that had previously gone undetected.16PubMed Central. Impact of Mandatory Carbon Monoxide Alarms: An Investigation of the Effects on Detection and Poisoning Rates in New York City A Wisconsin study found that the odds of CO poisoning were about three times higher among people who had no detector compared to those whose detector alarmed.17PubMed. Carbon monoxide detector effectiveness in reducing poisoning, Wisconsin 2014-2016 And a cost-effectiveness analysis concluded that state legislation requiring residential CO alarms is probably cost-effective, even though the full extent of their benefit is still being measured.18Undersea and Hyperbaric Medicine. Cost effectiveness of residential carbon monoxide alarms
The weak link is maintenance. In New York City’s experience, while 83% of residents reported having CO alarms by 2009, only 54% had recently tested or replaced the batteries.16PubMed Central. Impact of Mandatory Carbon Monoxide Alarms: An Investigation of the Effects on Detection and Poisoning Rates in New York City A detector with dead batteries is decoration. Most CO detectors also have a limited lifespan of five to seven years, after which the sensor degrades and should be replaced entirely.
What to Do When a Detector Sounds
Surveys reveal a surprising amount of confusion about what a CO alarm actually means. Some people mix up carbon monoxide and natural gas, and many are unsure how to respond when the alarm goes off.19PubMed. Risk and protective behaviours for residential carbon monoxide poisoning Unlike natural gas, CO has no smell. You cannot detect it without an instrument. If your CO alarm sounds, the response is straightforward: get everyone out of the house, including pets, and call 911 or your local fire department from outside. Do not re-enter to search for the source. Do not assume the detector is malfunctioning just because no one feels sick yet: remember, mild symptoms are vague and easy to rationalize, and the alarm may be catching a problem before symptoms become obvious.
If anyone in the household is experiencing headache, dizziness, or nausea, mention that to the emergency responders, as it can change how they prioritize the call. The fire department will use professional-grade CO monitors to check the home and identify the source. Your furnace, water heater, or other appliance should be inspected and repaired before you run it again.
Fingertip CO Monitors and Their Limits
Pulse CO-oximeters, devices that clip onto a finger and attempt to read COHb levels through the skin, have been available since 2005 and are sometimes used by first responders for quick screening. Their accuracy, however, is limited. A meta-analysis of diagnostic studies found that these devices had a pooled sensitivity of about 77% and specificity of about 83%, with wide limits of agreement that make them unreliable for either confirming or ruling out CO poisoning with confidence.20PubMed Central. Accuracy of pulse CO-oximetry to evaluate blood carboxyhemoglobin level: a systematic review and meta-analysis of diagnostic test accuracy studies A separate meta-analysis of studies specifically in adults with suspected acute poisoning found even lower sensitivity, around 65%, though specificity was higher at 93%.21PubMed Central. The diagnostic accuracy of carbon monoxide pulse oximetry in adults with suspected acute carbon monoxide poisoning: a systematic review and meta-analysis
In practical terms, a positive reading on one of these devices is fairly trustworthy, but a negative reading does not mean you are in the clear. The gold standard remains a venous or arterial blood draw analyzed by a laboratory CO-oximeter. If you have been exposed to CO and a finger-clip device says you are fine, but you feel terrible, insist on a blood test. Standard pulse oximeters, the kind used routinely in hospitals and doctor’s offices to measure oxygen saturation, cannot distinguish between oxyhemoglobin and carboxyhemoglobin at all. They will often read normal even during serious CO poisoning, which is one reason the condition gets missed.
Reducing CO in a Home That Tests Above Zero
If a professional inspection or a low-level CO monitor reveals persistent readings above zero in your home, the priority is identifying and fixing the source rather than improving ventilation as a workaround. A cracked heat exchanger in a furnace, a blocked flue, a malfunctioning water heater, or degraded seals on a gas appliance are all problems that require repair or replacement. Annual inspection of all fuel-burning appliances by a qualified technician is the single most effective preventive step.
Ventilation does help as a secondary measure. Homes that are very tightly sealed for energy efficiency can trap combustion byproducts more effectively than drafty older homes, so if you have recently air-sealed or weatherized your home, make sure your combustion appliances still have adequate combustion air supply and that flues are drafting properly. Running a kitchen exhaust fan vented to the outdoors while using a gas stove reduces short-term CO spikes. Never use a gas oven or stovetop burners for supplemental heating. Never operate a charcoal grill, camp stove, or generator indoors or in an enclosed space like a garage, even with the door open.
For people who heat with wood or coal, keeping the stove or fireplace well-maintained and burning seasoned, dry fuel reduces incomplete combustion. Wet or green wood produces substantially more CO. Ensuring the chimney is clean and the damper is functioning properly allows combustion gases to exit rather than seep into the living space. Even so, homes heated primarily with wood or coal tend to have higher indoor CO on average, which is worth knowing if anyone in the household falls into a high-risk category.11Science of The Total Environment. Low-cost measurement techniques to characterize the influence of home heating fuel on carbon monoxide in Navajo homes