It's getting chilly out, and that means it's carbon monoxide season. It's time to take a few minutes-and maybe spend a few bucks-to make sure you don't wake up dead some morning.

Let's start with the basics: Carbon monoxide is a colorless, odorless, tasteless gas. If you inhale too much of it, it can kill you. In non-lethal doses, the effects start with generally feeling lousy and then work up through headaches, dizziness, and nausea.

There are all kinds of ways to get a lungful of carbon monoxide. One really effective way is to roll tobacco into a paper wrapper, stick it in your mouth, set it on fire, and suck on it. A fair number of people do this on purpose. Don't get me started.

I worry a whole lot more about folks who are overdosing on carbon monoxide and don't even know they're doing anything wrong. For instance, people who have dirty-burning, neglected gas furnaces. I've seen more than a few houses with furnaces as old, evil, and deadly as the Death Furnace from The Shining, and the people who live in these houses often tell me, "The furnace man said it was awright."

Listen to me: In our part of the world, your average furnace guy will bless a furnace that's rustier than a Wisconsin Edsel and spews soot up a flue that's either as porous as a shot-gunned colander or as clogged as Elvis' arteries.

For the life of me, I can't explain this. Why would people who sell, install, and repair heating equipment never want to pronounce a system dead or defective? Do they get fired, or cussed out, or beat up for being the bearers of bad tidings? My best guess is that it's always easier to say what a person wants to hear-and nobody wants to hear that they need a new furnace.

Every day, I see furnaces that people swear to me have just been serviced, and they're so full of rust and dirt and bug carcasses that I know they haven't been cleaned in my lifetime. The dirtier the furnace, the dirtier the burn, and the dirtier the burn, the more carbon monoxide. Now, if a dirty burn is the only problem, it's not such a big deal. As long as the furnace's heat exchanger and flue are in good condition, the carbon monoxide just goes up the chimney.

But if the heat exchanger is cracked, the carbon monoxide gets pumped into the house through the ductwork. It's devilishly hard to find a crack in a heat exchanger without taking the metal jacket off the furnace. That takes all day and costs a lot of money, so it hardly ever happens.

In the real world, cracks in heat exchangers are usually discovered by the time-honored method of watching the flame pattern in the furnace when the fan kicks on. There's a catch here: The furnace needs to be clean before this "eyeball" test is reliable. So somebody tell me: How is it that all these old furnaces are getting excellent reviews from service people when some of the service people aren't even cleaning them out first?

As bad as a lot of furnaces are, the flues are worse. Even though it's been considered bad practice for years now, there are still heating contractors who will vent a furnace up an old brick chimney. This is a double whammy, as carbon monoxide can leak out the loose brick joints. Or if the flue gets clogged with leaves, berries, bird nests, raccoon carcasses, and such like, carbon monoxide just spills out wherever it can, usually at the draft hood at the top of the furnace.

Now, this is where I'd love to be able to tell you how to choose a good heating contractor. But I'm stumped. I don't think there are enough good ones to go around. But you can start this way: Get your furnace serviced every year. After the service guy tells you he's cleaned your furnace, take off the user-friendly front cover and look inside the thing. If it's still full of crud, you've been lied to. If your contractor tells you it's OK to vent your furnace up an old masonry flue, look for a new contractor.

An excellent precaution: Buy some carbon monoxide detectors and put them in all your bedrooms and near any sources of combustion, including fireplaces, gas logs, and freestanding kerosene heaters.