The alarm sounds. Now what?
The alarm goes off and your first thought is: it's probably nothing. The candles, maybe. Someone left the oven on. You feel fine. Your head is a little heavy, but you were up late. You are already moving toward the detector to check the display when you should already be at the door.
This is how carbon monoxide works. Not with a smell you can identify or a color you can see. With a lag. By the time your body tells you something is wrong, your brain is already running slower than it should. The heaviness you feel is the gas. The confusion is the gas. The reason the alarm sounds wrong, distant, almost annoying, is the gas.
Carbon monoxide binds to hemoglobin about 200 times more readily than oxygen. Your blood is carrying it instead of carrying air, and it is doing so quietly, without protest, without pain. Your body does not register the swap.
The gas does not announce itself. The alarm does. You only get one of those signals twice.
What your body tells you to do, and why it's wrong
Your body wants to sit down. The headache is real. The nausea is disorienting. The fatigue feels like the honest end of a long day. And because the symptoms of CO poisoning closely mirror the flu, minus the fever, the first instinct is to lie down and let it pass.
Lying down in a contaminated space does not let it pass. It accelerates.
The second wrong instinct is to start opening windows. You are buying time, not solving it. If there is an active source, an idling vehicle in the attached garage, a gas appliance with a blocked flue, a generator running near a window, open air inside the building does not stop the source. It just dilutes the symptom long enough for you to feel okay about staying.
The third wrong instinct is to go back inside to get something. A phone. A wallet. A dog. People have lost consciousness within minutes of re-entering a heavily contaminated space. The object is not worth the arithmetic.
Every second you spend deciding whether to take the alarm seriously is a second you are breathing something you cannot see and cannot smell, inside a body that cannot tell you it is losing.
What you actually do
There is one sequence. It does not branch based on how bad you feel. It does not wait for confirmation.
- 1Get everyone out of the building immediately. Everyone. Do not stop to investigate. Do not check which room the alarm is in.
- 2Leave the door open on your way out. This helps clear the space but does not change the rest of the sequence.
- 3Call 911 from outside. Not from inside while gathering things. From outside, after you are out.
- 4Do not go back in for any reason until emergency responders have cleared the space and confirmed it is safe.
- 5If anyone is showing symptoms, especially confusion, loss of consciousness, or difficulty breathing, tell the dispatcher immediately. They will send emergency medical services, not just fire.
Each year, more than 400 Americans die from unintentional CO poisoning not linked to fires, more than 20,000 visit the emergency room, and more than 4,000 are hospitalized.
Treatment for CO poisoning is 100 percent oxygen, delivered through a tight-fitting mask or, in severe cases, through a hyperbaric chamber. Neither is available inside the building. Both require you to be outside and in contact with emergency services.
Test Your Response
It is a cold January night. You wake up with a pounding headache and notice your spouse also feels nauseous. Your CO detector is chirping with a steady alarm pattern, not the single low-battery chirp. What do you do first?
Be honest. No one's watching.
Where it gets complicated
The alarm scenario is the obvious one. But carbon monoxide does not always announce itself with an alarm. Detectors have a threshold. A slow, low-level leak can accumulate over hours without triggering one, while the people inside develop headaches they attribute to stress, nausea they attribute to something they ate, and fatigue they attribute to poor sleep.
The pattern that should concern you: more than one person in the same space feeling the same symptoms at the same time, with those symptoms improving when they leave the building and returning when they go back in. That is not a coincidence. That is a pattern worth treating as a leak until proven otherwise.
Generators are a specific and serious context. The Consumer Product Safety Commission has documented that generators account for more than 900 CO deaths over a five-year period in their tracking data. Generators should run at a minimum of 20 feet from any door, window, or vent, and never inside an attached garage, even with the door open.
The CPSC notes that CO from generators can reach dangerous levels within minutes when operated indoors or in partially enclosed spaces [1]. The CDC reinforces that many of these deaths occur during and after storms, when power is out and the instinct to run a generator inside or in a garage feels reasonable [2].
Cars are the other common source. An idling vehicle in an attached garage, even with the garage door fully open, can push CO into the living space within minutes. The same principle applies to any gas-burning appliance, water heater, furnace, dryer, or range, that is improperly vented or operating with a blocked flue. Annual professional inspection is not a formality. It is the interval at which these things get caught before they become the other kind of problem.
The people who took it seriously before it happened are the ones who had nothing to prove when it did.
After you're outside
Stay out until the fire department has measured CO levels inside and cleared the building. Not until you feel better. Not until you can no longer smell anything, because you never could. Until a trained responder with a meter tells you the number is safe.
If you went to the emergency room, do not assume you are finished when the oxygen therapy ends. CO can cause delayed neurological effects. Confusion, memory problems, and mood changes have been documented in survivors days and weeks after exposure. If any of that surfaces, it belongs in a conversation with a doctor, not in a search engine at midnight.
Before anyone returns to the building, the source needs to be found and fixed. Not suspected. Not assumed. Confirmed. That means a licensed technician looking at every appliance, every flue, the garage, the HVAC system. The detector that alarmed did its job. Your job now is to find out what it found.
Detectors go on every level of the home and outside sleeping areas. They are replaced every five to seven years, because the sensors degrade. The date is usually on the back. If you do not know when yours was installed, that is the number you should be looking up right now.
400+
Americans die from unintentional CO poisoning every year. Most were inside a building with a source they did not know about.
The alarm you almost silenced is the only reason you had time to read this.