Carbon Monoxide Poisoning: Symptoms, First Aid and What to Do When the Alarm Goes Off

Quick answer: carbon monoxide (CO) poisoning often feels like the flu without a fever: headache, dizziness, weakness, nausea or vomiting, chest pain and confusion. If a CO alarm sounds or several people or pets in the same home feel sick at once, get everyone into fresh air right away, then call 911 from outside. Do not go back in until firefighters say the air is safe. Anyone with symptoms needs medical care, because the only real treatment is oxygen given by professionals.

Carbon monoxide is a gas you cannot see, smell or taste. It comes from burning fuel, so any furnace, water heater, gas stove, fireplace, car, generator or charcoal grill can produce it. In an apartment building, a problem in one unit or in a shared garage can affect neighbors too. This guide is part of our emergency first aid guide for city households and covers what CO poisoning looks like, what to do in the first few minutes and how to keep it from happening. It is general safety information, not a substitute for emergency services or medical advice.

Sources: Centers for Disease Control and Prevention (CDC) carbon monoxide poisoning prevention pages; U.S. Consumer Product Safety Commission (CPSC) generator and CO alarm guidance; National Fire Protection Association (NFPA) CO alarm recommendations; America’s Poison Centers. When in doubt, call 911 or Poison Control at 1-800-222-1222.

Why Carbon Monoxide Is So Dangerous

Your red blood cells pick up carbon monoxide far more readily than oxygen. As CO builds up in the blood, less oxygen reaches the brain, the heart and other organs. Damage can happen quickly at high levels, and slowly at lower levels over hours.

According to the CDC, more than 400 people in the United States die each year from unintentional carbon monoxide poisoning not linked to fires, more than 100,000 visit an emergency department and more than 14,000 are hospitalized. Many of those cases happen in cold months, when furnaces run hard and people use generators or other fuel-burning devices during power outages.

People who are asleep or drunk can die from CO poisoning before they ever notice symptoms. That single fact is the main reason a working CO alarm matters more than knowing the warning signs.

Symptoms of Carbon Monoxide Poisoning

CDC describes the most common symptoms as:

• Headache, often dull and constant
• Dizziness or lightheadedness
• Weakness and unusual tiredness
• Upset stomach, nausea or vomiting
• Chest pain
• Confusion

At higher exposures, people can become clumsy, short of breath, drowsy or lose consciousness. Severe poisoning can cause seizures, abnormal heart rhythms, coma and death.

Clues That Point to CO Instead of the Flu

Because the symptoms overlap with common illnesses, look for patterns:

• Several people in the same home feel sick at the same time.
• Pets are lethargic, unsteady or vomiting too.
• Symptoms ease when you leave home and return when you come back.
• Symptoms started after the heat came on, after a generator or grill was used nearby, or after a car was warmed up in a garage.
• There is no fever, which is usual with flu.

Who Is at Higher Risk

Everyone is at risk, but CDC notes that infants, older adults, people with chronic heart disease, people with anemia and people with breathing problems are more likely to get sick from CO. Pregnant people are also a concern, because carbon monoxide can harm a developing baby even when the parent’s symptoms seem mild.

What to Do When a CO Alarm Goes Off

Treat every CO alarm as real until trained responders tell you otherwise. A short checklist helps under stress:

1. Get everyone out into fresh air, including pets. Leave the door open behind you if it is quick and safe.
2. Call 911 from outside the building, or from a neighbor’s apartment on a different floor.
3. Count heads at your meeting spot so no one goes back looking for a missing person.
4. Tell the dispatcher that a carbon monoxide alarm sounded and whether anyone has symptoms.
5. Stay out until the fire department has tested the air and says it is safe to return.
6. Before using the appliance again, have a qualified technician find and fix the source.

Do not try to find the leak yourself, and do not open windows and stay inside “to air it out.” Opening windows does not remove the source, and you would keep breathing contaminated air while you work.

If the Alarm Chirps Instead of Sounding

A single chirp every minute or so usually means a low battery or an alarm at the end of its life. A full alarm is a loud repeating pattern, often four beeps followed by a pause, though patterns vary by model. Check the label or manual on your alarm so everyone in the home knows the difference before an emergency.

First Aid for Someone With Carbon Monoxide Poisoning

The most important first aid is fresh air and oxygen. Everything else supports that.

• Move the person to fresh air if you can do it without putting yourself in danger. If the source is unknown or the air feels thick with fumes, wait for firefighters, who have breathing equipment.
• Call 911 and say you suspect carbon monoxide poisoning.
• Check breathing. If the person is unresponsive and not breathing normally, start CPR if you are trained and keep going until help arrives.
• If the person is breathing but drowsy or unresponsive, place them in the recovery position to keep the airway open, and stay with them.
• Keep the person calm and still. Activity uses more oxygen.
• Note the details: what was running, how long people were exposed and when symptoms began. Emergency crews and doctors will ask.

Do not let anyone with symptoms drive themselves to the hospital. Confusion and drowsiness can come and go, and a person can lose consciousness behind the wheel.

Why Your Pulse Oximeter Can Fool You

A standard fingertip pulse oximeter cannot tell the difference between oxygen and carbon monoxide attached to red blood cells. Someone with serious CO poisoning can show a normal-looking reading. Hospitals use a blood test or a special CO-oximeter to measure carbon monoxide levels, so ignore a reassuring home reading when symptoms are present.

How Doctors Treat Carbon Monoxide Poisoning

Emergency crews give high-flow oxygen through a tight-fitting mask, which helps clear carbon monoxide from the blood faster than breathing room air. At the hospital, staff check CO levels in the blood, watch the heart and look for signs of nerve or brain effects.

For some patients, such as people who passed out, pregnant patients or people with very high levels, doctors may recommend hyperbaric oxygen therapy, which delivers oxygen in a pressurized chamber. Not every hospital has one, so transfer is sometimes needed.

Some people develop memory problems, personality changes or trouble concentrating days to weeks after a serious exposure. Follow-up appointments matter, even if the person seems fully recovered the next day.

Common Sources of CO in Apartments and City Homes

Most poisonings trace back to a handful of sources:

Source	Typical problem	Prevention
Gas furnace or boiler	Cracked heat exchanger, blocked flue	Yearly inspection by a qualified technician; report problems to your landlord in writing
Gas water heater	Poor venting, blocked flue	Keep the area clear; schedule service
Gas stove or oven	Using it to heat the home	Never use a range or oven for heat
Portable generator	Run indoors, on a balcony or near windows	Outdoors only, far from windows, doors and vents
Car in a garage	Engine running with the door open or closed	Never idle a car in an attached or underground garage; clear snow from the tailpipe
Charcoal grill or camp stove	Used inside or on an enclosed balcony	Outdoor use only
Fireplace or wood stove	Closed or blocked chimney	Open the damper and have the chimney inspected

Power Outages and Winter Storms

Outages are the classic setup for CO tragedies. The CDC and CPSC warn never to run a portable generator inside a home, garage, basement, crawl space or shed, even with doors and windows open, and to keep it at least 20 feet from windows, doors and vents. Our guide to generator placement and carbon monoxide safety covers the details, including fuel storage.

Apartment balconies are a special trap. A generator or grill running on a balcony can push exhaust straight into your unit or the one above you. Many buildings ban them outright, and fire codes in many cities restrict open-flame grills on balconies.

Warming up a car in a garage is another winter risk, and so is sitting in a running car stuck in snow. Clear snow from the tailpipe before you start the engine and crack a window if you must run the engine for heat.

Prevention: CO Alarms Done Right

A working alarm is the single most useful CO safety tool. NFPA recommends CO alarms outside each sleeping area, on every level of the home and in other locations required by laws or codes. Many states and cities require CO alarms in rental housing, and some require landlords to install them. Ask your landlord or building manager what your building has and where.

• Choose alarms listed to a recognized standard, such as UL 2034.
• Test them monthly with the test button.
• Replace batteries according to the manufacturer, or choose a sealed long-life model.
• Replace the alarm itself when it reaches the end of the life listed on the label, often 5 to 10 years.
• Consider interconnected alarms so one alarm triggers all of them.

Combination smoke and CO alarms work well in small apartments. Place them according to the instructions, since some locations near cooking appliances or bathrooms can cause nuisance alarms. For placement mistakes and testing routines, see our CO alarm placement and testing guide.

Habits That Lower Your Risk

• Schedule a yearly check of fuel-burning heat and water heating equipment. In a rental, put repair requests in writing.
• Never cover or block vents, flues or the area around a water heater.
• Never use a gas oven or stove to heat the home.
• Never use charcoal, a camp stove or a propane heater meant for outdoor use inside.
• Keep the fireplace damper open whenever a fire is burning or smoldering.
• Learn the sound of your alarm and teach everyone in the home what to do when it goes off.

After a CO Incident

Once everyone is safe and checked by medical staff, deal with the source before anyone goes back to sleep in the home.

• Get a written report from the fire department if one is available.
• Have a licensed technician inspect and repair the appliance.
• If you rent, notify your landlord in writing and keep copies of messages. Ask when the repair will be done and where you can stay in the meantime.
• Replace any alarm that did not sound when it should have.
• Watch for delayed symptoms over the following weeks and see a doctor if memory, mood or concentration changes.

If smoke was also involved, for example from a fire that produced CO, read our page on smoke inhalation first aid, since irritated airways can worsen after the first hours.

Teaching the Household

A plan only works if everyone knows it. Practice once or twice a year.

• Show children what the CO alarm sounds like and how it differs from the smoke alarm, if your models use different patterns.
• Pick a meeting spot outside the building, such as a lobby across the street or a specific corner.
• Make sure older children know how to call 911 and what to say.
• Include pets in the plan, with leashes or carriers near the door.
• Post Poison Control’s number, 1-800-222-1222, on the fridge.

Frequently Asked Questions

What are the first signs of carbon monoxide poisoning?

A dull headache, dizziness, weakness, nausea and confusion are common early signs. When several people or pets in the same home feel unwell at the same time, suspect CO and get to fresh air.

How long does it take to get carbon monoxide poisoning?

It depends on the concentration. High levels can cause symptoms within minutes and death within a short time, while low levels can cause symptoms that build over hours or days.

Can you smell carbon monoxide?

No. CO has no color, smell or taste. A gas smell comes from an additive in natural gas, not from carbon monoxide, so rely on a CO alarm.

Should I open windows if my CO alarm goes off?

Get out first and call 911 from outside. Opening windows while you stay inside keeps you breathing contaminated air and does not stop the source.

Can carbon monoxide poisoning cause lasting effects?

Yes. Some people develop memory, concentration or mood problems days to weeks after a serious exposure. Follow-up care is important even if you feel better quickly.

Is it safe to sleep in a car with the engine running?

It is risky, especially if snow or a wall blocks the tailpipe. If you are stranded and need heat, clear the tailpipe, run the engine in short bursts and keep a window slightly open.