Five hundred Americans accidentally die each year from carbon monoxide poisoning, and at least 10,000 people a year are treated for exposure to the colorless, odorless gas.

Thousands more never seek medical care because, with most cases occurring in winter, the early symptoms mimic those of flu and can be easily misdiagnosed. The most common early symptoms of what is known as the "silent killer" are headache, dizziness, weakness, nausea, vomiting, chest pain and confusion. Yet only 26 percent of all U.S. homes have carbon-monoxide detectors, although that is up from 18 percent four years ago.

The primary reason is financial, industry experts believe. A detector that meets Underwriters Laboratory standards costs between $40 and $70. Battery-powered smoke detectors, by comparison, retail for $5 to $20. In addition, the majority of states and municipalities require that smoke detectors be installed on every floor of a house, and some will not permit a house sale to go to settlement until that requirement is met. Just a few states and municipalities require carbon-monoxide detectors.

Carbon monoxide is most dangerous for unborn babies, infants and children. Because fetuses, infants and young children have smaller bodies and faster metabolisms, they absorb carbon monoxide more quickly and at lower levels. When a fetus is exposed, the gas destroys oxygen cells and interferes with brain development.

When carbon monoxide is inhaled, it combines with the hemoglobin in oxygen to form carboxyhemoglobin. When that happens, the hemoglobin is no longer able to carry oxygen to the brain and vital organs.

For example, if you are a healthy adult in your 40s, and the concentration of carboxyhemoglobin in your blood is 10 percent, there are no symptoms. At 15 percent, you have a slight headache. At 45 percent, you are unconscious. At 50 percent, you are dead.

For infants, toddlers and the elderly, lower percentages are as toxic as the higher levels in a healthy adult.

There are many potential sources of carbon monoxide in the house.

Fuel-burning appliances, chimneys, and flues are not designed to last forever. Even new homes have carbon-monoxide problems because of installation errors or design mistakes. The Consumer Products Safety Commission recommends installing at least one carbon-monoxide detector in the house, but favors installation of a detector on every floor, especially outside the bedroom area.

There are both battery-operated and plug-in varieties; the latter often include a battery backup in case of power failure. There are also dual-purpose models - carbon-monoxide and smoke detectors.

A detector should meet Underwriters Laboratories requirements and sound an alarm signal before most people experience the adverse effects of carbon-monoxide poisoning.

Alarms are required to sound an 85-decibel signal within 189 minutes if a level of 70 parts per million is present, within 50 minutes for 150 ppm, and within 15 minutes if 400 ppm is present, according to UL.

An alarm will not eliminate the source of the problem, though. If your house is too insulated, you'll need to find a way to bring fresh air inside - an air-exchange system.

Keep gas and charcoal grills and gas-powered equipment outside and away from windows and doors. And make sure gas stoves, dryers, furnaces, hot-water heaters, and chimneys are inspected regularly. You might not notice a cracked heat exchanger in a furnace, but you can see whether a bird has built a nest in your chimney or whether the dryer vent is clogged.

Here are some quick facts: