The most common symptoms of carbon monoxide poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion, according to the CDC. If you think anyone at home has them because of carbon monoxide, get everyone outside to fresh air and call 911.

Carbon monoxide is easy to miss. You can't see it or smell it, and the CDC says its symptoms are often described as "flu-like." Each year, more than 400 Americans die from carbon monoxide poisoning not linked to fires, more than 100,000 visit an emergency room and more than 14,000 are hospitalized. People who are asleep or have been drinking can die before they have any symptoms at all.

Early signs and serious signs

Symptoms tend to get worse the longer someone breathes it. Cleveland Clinic lists these early warning signs:

• A mild headache
• Nausea
• Shortness of breath

Signs that it's getting serious, from Cleveland Clinic and Mayo Clinic:

• A severe headache
• Dizziness, weakness or fainting
• Vomiting
• Chest pain
• Confusion or blurred vision
• Loss of muscle control or coordination
• Sleepiness or loss of consciousness

In babies and small children the signs can be harder to spot. MedlinePlus says to watch for fussiness, trouble feeding or behavior that seems off. Pets may show symptoms before people do.

Why it gets mistaken for the flu

Low-level carbon monoxide poisoning feels a lot like the flu. The U.S. Consumer Product Safety Commission points out one difference: there's no fever. Watch for these clues:

• Headache, nausea or dizziness with no fever.
• Feeling better after you leave the house, then worse when you come back. The EPA says symptoms may clear up after you leave home.
• Symptoms that start or get worse when the furnace, a gas stove, a fireplace or a space heater is running. The Minnesota Department of Health lists this as a warning sign.
• A pet that seems sick or sluggish at the same time.

What to do right now

1. Get everyone outside. Don't stay indoors to make the call. If you can't get out safely, open a window or door and stay right next to it while you call.
2. Call 911. MedlinePlus says carbon monoxide poisoning needs medical care right away. Call even if you don't know much about what happened.
3. Tell the dispatcher each person's age, whether they're awake and alert, and how long they may have been exposed.
4. Stay out until emergency responders say the house is safe.
5. Call Poison Help with questions. The national line is 1-800-222-1222. It's free, confidential and open 24 hours a day, 7 days a week, for any question about a poisoning.

If anyone is unconscious or struggling to breathe, 911 comes first, every time.

Get checked even if you feel better

Fresh air is the first step. A doctor is the second. Treatment can include pure oxygen through a mask, and sometimes high-pressure oxygen in a special chamber. MedlinePlus says recovery depends on how much carbon monoxide someone breathed and for how long, and that problems with thinking can return 1 to 2 weeks after a person seemed fine. Mayo Clinic lists memory loss, personality changes and trouble with movement as problems that can show up after recovery.

Who is most at risk

Anyone can be poisoned. The CDC says infants, older adults and people with chronic heart disease, anemia or breathing problems are more likely to get sick from it. Mayo Clinic adds unborn babies, because their blood takes up carbon monoxide more easily. If your parents live on their own with a gas furnace or stove, put their house on your list too.

The warning you can count on: an alarm

Carbon monoxide has no smell, color or taste (more on that in what carbon monoxide smells like). Your senses can't pick it up, so an alarm is the only reliable way to know it's there.

The CPSC recommends carbon monoxide alarms on every level of your home and outside sleeping areas, either battery-operated or with a battery backup. Our guides on how many detectors you need and whether carbon monoxide rises or falls walk you through placement.

Then add a Hero Dual Threat Detector in each room with a stove, a furnace or an engine, and one where you sleep. It plugs into any outlet and checks the air for carbon monoxide and gas leaks, day and night. A live readout shows both levels, carbon monoxide from 0 to 1,000 parts per million, and an alarm sounds if either one rises, then a faster, urgent one at higher levels. It runs on wall power with no battery, so keep your battery CO alarms too.

Know what's in the air tonight. Get the Hero Dual Threat Detector.