Rule #6: Clean the duct system. If you have a duct system, get it cleaned properly if someone in your family seems to be suffering from the effects of bad air or if you see large dust or mold deposits. (Only a lab can tell you if what looks like mold is really mold; some microbiology firms can do an analysis off a piece of sticky tape with some of your sample on it for about $50.) Just make sure that whoever does the cleaning follows EPA recommendations; otherwise, you may be doing more harm than good.
Finally, make sure they clean the entire system, use HEPA vacuum equipment, and protect your furnishings during the work. Better yet, if you really want to know what should be done and when, contact your local EPA office for the fine pamphlet "Should You Have the Air Ducts in Your Home Cleaned?"
Don't hire any firm to clean your ducts that says it's EPA certified -- there's no such thing -- or that wants you to clean ducts regularly or touts the health benefits (as yet unclear). Call the National Air Duct Cleaners Association and check references for at least three candidates as well as state licenses if they're relevant (as they are in California, Texas, and Florida, among others). If the servicer wants to use chemical sealants or biocides, think about it very carefully; the EPA has very mixed feelings on their application.
Rule #7: Give space heaters some space. Those of you who are in old buildings with less-than-adequate heat may be tempted to use unvented space heaters to notch up the temperature a degree or two. Do so only with appropriate precautions. Old or malfunctioning models can release all sorts of noxious elements into the air while sucking out the oxygen (in fact, in many states, unvented heaters are illegal and/or may vitiate your insurance coverage if used).
If you need the heat, get a UL-approved newer model with an automatic sensor that shuts the heater off when the oxygen gets too low. And crack a window, please.