Model Dayle Haddon died of suspected carbon monoxide poisoning. Here's what to know about the 'invisible killer' and its symptoms.

The “invisible killer” gas can leak from car engines and everyday appliances.

Dayle Haddon in navy evening gown and jacket, with a bank of photographers taking pictures behind her.
Actress and model Dayle Haddon was found dead on Dec. 27 from suspected carbon monoxide poisoning. (Marc Piasecki/FilmMagic via Getty Images)

Cases of apparent carbon monoxide poisoning are raising concern about the "invisible killer" this holiday travel season. On Christmas Day, a family of four from Massachusetts was found dead inside their New Hampshire vacation home; at least one of the deaths has been attributed to carbon monoxide poisoning. And on Friday, model and actress Dayle Haddon, 76, was found dead inside a Pennsylvania home owned by her daughter, Ryan Haddon, a former journalist, and her son-in-law, actor Marc Blucas. Police said on Saturday that they believe "a faulty flue and exhaust pipe on a gas heating system" led to a carbon monoxide leak, according to CBS News. A 76-year-old man — later identified as Walter J. Blucas — was also hospitalized after being discovered unconscious in the home.

Each year in the United States, more than 100,000 people visit the emergency department due to accidental carbon monoxide, or CO, poisoning, and at least 400 people die from unintentional CO poisoning that isn't fire-related, according to the Centers for Disease Control and Prevention. Here’s what you need to know about this dangerous gas.

Carbon monoxide, also known as the “invisible killer,” is a colorless, odorless, tasteless gas. A byproduct of combustion, or burning, CO can leak from everyday appliances like stoves, dryers, water heaters or gas fireplaces, or from car or truck engine exhaust. Gas appliances are designed to vent CO outdoors, but if there’s a malfunction, they can leak it indoors and cause carbon monoxide poisoning.

Breathing in a lot of CO can make you pass out, or kill you.

Common symptoms are often flulike and include headache, dizziness, upset stomach, chest pain or confusion, but highly concentrated levels of CO can cause a person to pass out without feeling symptoms.

Because CO has no smell, color or taste, it’s impossible to detect without the right equipment, but there are some signs you can look for. It’s sometimes accompanied by other exhaust gases that you can smell, so be aware of any unusual odors. If you have a fireplace, look out for fallen soot or an upward draft in the chimney flue. And take notice of any “sooty or brownish-yellow stains” near appliances.

Since CO is lighter than air, it tends to gather near the ceiling. So CO detectors should be installed in each floor of your home at least 5 feet above ground — ideally near sleeping areas and in the same room as gas- or wood-burning appliances — and tested once a month, according to the U.S. Fire Administration. Also make sure that home heating equipment is inspected annually, including gas appliances and chimneys.

To avoid carbon monoxide poisoning in your vehicle, the CDC recommends having your exhaust system checked every year, as even a small exhaust leak results in CO buildup inside a car or truck. If your vehicle has a tailgate, open the vents or windows after opening the tailgate to make sure air is moving; otherwise CO from the exhaust can be pulled into the vehicle.