Three residents of the Cove Village townhouse complex in Essex died in 2005. That's the same neighborhood where the recently hospitalized people live.

So what should you watch out for? Here's what the Environmental Protection Agency says:

At moderate levels, you or your family can get severe headaches, become dizzy, mentally confused, nauseated, or faint. You can even die if these levels persist for a long time. Low levels can cause shortness of breath, mild nausea, and mild headaches, and may have longer term effects on your health. Since many of these symptoms are similar to those of the flu, food poisoning, or other illnesses, you may not think that CO poisoning could be the cause.

Carbon monoxide is odorless, so don't wait around for a funny smell.

If you suspect carbon monoxide poisoning, go outside right away -- fresh air helps -- and get to an emergency room.

The EPA says the quality of carbon monoxide detectors varies quite a bit, so you'll want to do some research if you're planning to buy one. Even if you're satisfied with the one you get, the EPA says, "don’t let buying a CO detector lull you into a false sense of security."

The agency offers a checklist of ways to avoid problems in the first place, such as having your fuel-burning appliances checked at the start of each heating season and never idling your car in the garage.