Carbon monoxide poisoning is almost entirely preventable. And yet, every year, more than 400 people in the U.S. die and tens of thousands more are sickened.

Often, the culprit is a common household appliance that malfunctions or is used improperly. But carbon monoxide poisoning can be especially dangerous during power outages, when people use alternative sources of fuel or electricity such as generators.

The winter storm in Texas in mid-February was the worst carbon monoxide poisoning incident in recent history, according to three experts interviewed by ProPublica, The Texas Tribune and NBC.

At least 11 people died and more than 1,400 residents sought emergency care for carbon monoxide poisonings during the storm, data shows.

The gas is particularly dangerous because it is odorless, colorless and tasteless, and it can kill within minutes at high levels. Those who survive may suffer brain damage and other long-term health problems.

The best way to detect if you have unsafe levels of the poisonous gas in your home is to have a working carbon monoxide monitor, which will sound an alarm if you’re in danger. Here’s what you should know about carbon monoxide and how to keep your family and home safe, especially during power outages.

Where does carbon monoxide come from?

Carbon monoxide is produced when not enough oxygen is reaching a fuel-burning source. Furnaces, car engines, stoves, generators, grills, water heaters and clothes dryers are some of the sources that can release carbon monoxide because of inadequate ventilation, mechanical issues and other problems.

What are the most common symptoms of carbon monoxide poisoning?

Early symptoms can include headache, dizziness and nausea, similar to the flu. At higher levels of exposure, it can produce vomiting, weakness, shortness of breath, chest pain and confusion. Without immediate treatment, people can lose consciousness and die.

What should I do if I’m feeling any of these symptoms?

If you suspect carbon monoxide poisoning, leave the house and seek prompt medical help.

How do carbon monoxide alarms work, and should I have them in my home?

The alarms detect carbon monoxide that builds up in the air and go off when the gas reaches dangerous levels. The Centers for Disease Control and Prevention recommends that every home have at least one working carbon monoxide alarm.

What does a carbon monoxide alarm look like?

Some are white and round, similar to smoke detectors, while others are rectangular. Any approved device should clearly be labeled “carbon monoxide alarm.” You still need a carbon monoxide alarm if you have a smoke detector, but combination units are available. Carbon monoxide alarms can either be battery powered or hard-wired with a battery backup.

Where should carbon monoxide alarms be installed, and how often should I check them?

Fire safety experts recommend installing one in the hallway outside every sleeping area and on every level of the house. Because carbon monoxide is light, alarms should be placed on a wall 5 feet above the floor, according to the U.S. Environmental Protection Agency. They can also be placed on the ceiling. Alarms should be tested at least once a month and batteries should be replaced twice a year.

Is my landlord required to install a carbon monoxide alarm?

Most states require carbon monoxide alarms in newly constructed or remodeled homes, and nearly 30 states require them in some or all existing homes. Cities and counties can also pass their own requirements. Check with your local fire marshal or building code office. Landlords are required to provide them in some states and cities.

If you are a renter in a place that requires carbon monoxide alarms but your home doesn’t have one, contact the local or state office that handles tenant complaints.

What should I do if my carbon monoxide alarm goes off?

Leave your home immediately and call 911.

What else can I do to prevent carbon monoxide poisoning?

Generators, grills, camp stoves and other fuel-burning devices should never be used inside a home, basement, garage or camper. Generators should be kept away from the windows, doors and vent openings of a home. Never use an oven or gas range to heat your home.

Furnaces, chimneys, hot water heaters and other fuel-burning appliances should be properly installed, regularly maintained and well ventilated. Cars should not be left to idle in garages attached to homes, even if the garage door is left open.

Who can I call if I have questions about how to stay safe?

You can call the national Poison Control Center at 800-222-1222.