Carbon monoxide is a silent, deadly killer, and you need to take it seriously. The colorless, odorless gas is a by-product of combustion; it can be produced by gas and oil furnaces, charcoal and wood burners, and gas appliances, such as water heaters, space heaters, dryers, and stoves. If these are vented and operating properly, there is no danger, but malfunctions can release enough carbon monoxide to kill.

Ideally, you should place a carbon-monoxide detector on every floor of your house, near bedrooms. You want to be sure it wakes you up.

Have furnaces and other appliances checked annually, and make sure fresh air circulates in your home.

The symptoms of carbon-monoxide poisoning include headache, dizziness, nausea, and weakness, symptoms similar to those of the flu. If family members show these symptoms and you suspect a problem, get out of your house (don't forget pets) and call 911 or go to a hospital. Then call the gas company or your heating contractor.

Radon is another serious pollutant. It is a naturally occurring carcinogen that comes from soil and seeps in through cracks in your basement. If your home hasn't been tested, consult a professional home inspector. Then call a special contractor, who can install a ventilation pipe that sucks the radon from below your house up to the outside air and make other fixes.