Nine people were treated for severe carbon monoxide exposure after a Sunday morning scare in Northwest DC.

RELATED: 8 people hospitalized after carbon monoxide incident in DC

Fire official said it happened at around 7:09 a.m. in a three-story boarding house along the 500 block at U St. NW. Eight people were transported. A 9th person took themselves to the hospital, said a DCFD Spokesperson.

Carbon Monoxide or “CO” (its abbreviation) is an odorless, colorless, tasteless gas that can make you sick or even kill you.

Firefighters are sounding the alarm now of CO dangers before head deeper into the cold-weather season. Sunday’s incident marked the second major CO incident in the District in just a few days.

On November 9th, DCFD said a six-story apartment building had to be evacuated on 14th St. NW for carbon monoxide.

With the Sunday incident, DCFD Captain Mark St. Laurent told WUSA9, “Illnesses were consistent with flu-like symptoms.”

He described “dizziness, fatigue” and clumsiness. However, this was not the flu.

St. Laurent said one person thankfully in the building recognized this and called 911.

We know it’s cold out and people are shutting their windows while still using their gas appliances and heat. St. Laurent said the problem arises when you don’t get your appliances checked-out.

“What will happen is they’ll start to get cumulative build-up if it does not completely exhaust the way they’re supposed to exhaust,” said the DCFD EMS Supervisor.

That’s why the Fire Department wants everyone to have carbon monoxide detectors. The Captain showed us a pulse oximetry device, what he used to check the Sunday morning patients.

St. Laurent said less than a six reading is considered “good.” He said the U St. patients registered somewhere between 28 and 33.

DC’s Department of Consumer and Regulatory Affairs said each year, more than 20,000 people visit the emergency room for CO poisoning – that’s about half of a packed Nats Park. More than 400 Americans each year from the same thing, according to the DCRA.

“That’s dangerous. I didn’t know how far it had spread,” said Jim Coleman, a neighbor next door to the U St. address. He said his home was also checked.

St. Laurent told WUSA9 they found a faulty furnace in the basement of the boarding house. A woman, cleaning the home on Sunday night, told WUSA9 everyone treated was released from the hospital. Many are staying in a hotel for the night.