Anna Faris
Credit: Matt Winkelmeyer, Getty Images

Anna Faris’s Thanksgiving weekend nearly ended in tragedy when she and her 12 family members were exposed to toxic amounts of carbon monoxide at their rented home in North Lake Tahoe. The actress shared the frightening ordeal on social media, thanking first responders for saving the lives of her loved ones.

The North Tahoe Fire Protection District issued a statement about the frightening incident, explaining that the situation would likely have turned tragic, if two family members hadn’t opted to have their symptoms checked out at a local hospital.

According to their report, “multiple generations” of the family all fell ill to “varying degrees” after arriving at the rental home. At first, they brushed off the symptoms as altitude sickness. However, when two individuals decided to visit the hospital, they were diagnosed with carbon monoxide poisoning, and the North Tahoe Fire Protection District was notified. Once firefighters arrived at the scene, they treated nine family members at the home and sent another two to a local hospital.

While the maximum recommended indoor carbon monoxide level is 9 parts per million, the reading at the home measured as high as 55 PPM—even with windows and doors open for ventilation. Additionally, the home was not equipped with CO alarms.

What is carbon monoxide poisoning and how does it occur?

Carbon monoxide is an odorless, colorless gas that can cause sudden illness or death, according to the Centers for Disease Control and Prevention—which is why it is often referred to as the “silent killer.” Unintentional CO poisoning not linked to fires kills more than 400 people a year and lands more than 20,000 in the emergency room, with 4,000 getting hospitalized, according to the CDC.

The U.S. National Library of Medicine explains that carbon monoxide is found in combustion fumes—such as those made by cars and trucks—but also in lanterns, stoves, gas ranges, and heating systems. When there isn’t a good flow of fresh air, CO from these fumes can build up and poison those inhaling them.

What are the symptoms of carbon monoxide poisoning?

According to the U.S. National Library of Medicine, the most common symptoms of carbon monoxide poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion.

However, one of the reasons carbon monoxide is so dangerous is because most of the symptoms are incredibly similar to those of other illnesses, such as altitude sickness—like Faris mentioned—or the flu. Also, many CO poisoning deaths occur while people are sleeping or drunk, before any symptoms arise or become apparent.

How can you protect yourself against carbon monoxide poisoning?

First and foremost, the CDC suggests every home has a battery-operated (or battery back-up) CO detector installed and replaces the batteries bi-annually (the best way to remember is to replace the battery when you change the time on your clocks each spring and fall). If you have a CO detector and you hear the alarm sound, leave your home immediately and call 911. It’s also important to replace the CO detector itself every five years.

Other ways to prevent CO poisoning in your home include having your heating system, water heater, or any other gas, oil, or coal-burning appliances serviced by a qualified technician each year; never using a generator, charcoal grill, or other similar device inside your home, basement, or garage; not running a car or truck inside a garage attached to your house; not heating your house with a gas oven; and never using a generator, pressure washer, or gas-powered engine less then 20 feet from any window, door, or vent.

And if, by some chance, you do begin feeling the classic symptoms of CO poisoning, get out of your home immediately and get medical attention ASAP.

This story originally appeared on by Leah Groth.