NEW ULM, Minn. (AP) -

Two of the three people who were sickened by carbon monoxide at their New Ulm home remain hospitalized.

Authorities say this emergency serves as a reminder to check your home's outdoor furnace vents.

Ninety-three-year-old Arnold Schweiss and his 90-year-old wife, Velma, are in satisfactory condition at Hennepin County Medical Center. Their son was treated and released from New Ulm Medical Center Monday.

Onalaska Fire Chief Don Dominick said you should make sure snowdrifts around your home aren't blocking these vents, which can cause dangerous levels of carbon monoxide to build up inside your home.

CO detectors are your best line of defense. And there are some warning signs that can alert you to a serious problem.

"Because it's not going to be a smell, it's not going to be an odor," Chief Dominick said. "It's odorless, it's tasteless, it's colorless. One of the early warning signs, people start to get cherry red skin. Everyone in the family starts to come down with flu like symptoms simultaneously. Because your early warning signs of carbon monoxide poisoning are quite closely related to the flu."

Chief Dominick also pointed out this isn't just a winter problem either. Animals nesting in those vents other times of the year can cause the same problem if they aren't cleared out.