Carbon monoxide is invisible, tasteless and odorless. It produces mild signs and symptoms that are often mistaken for the flu until it's too late. That helps explain why it causes 50 percent of the poison deaths in the United States.

Everyone needs to be able to recognize CO poisoning and to be aware of the need to respond quickly with appropriate first aid.Most people know that cars and gas stoves emit carbon monoxide. Less familiar, and therefore more dangerous, sources are faulty furnaces, water heaters and kerosene heaters. Recreational fires, whether open flame, charcoal, Sterno or hibachi grills also give off CO.

Workplace sources include propane-powered fork lifts, acetylene torches and ice-skating rink scrapers. Firefighters, industrial workers, mechanics and miners are all at an increased risk of CO poisoning.

Smokers expose themselves to CO each time they inhale tobacco smoke - and also expose the people around them when they exhale.

CO toxicity first affects tissues with the greatest levels of oxygen consumption - in the brain, the heart, and, if present, the fetus. Children's rapid metabolic rates make them especially susceptible to CO toxicity.

CO binds with hemoglobin 200 times more readily than oxygen does. That leaves little hemoglobin available for oxygen transport.

The signs and symptoms of CO poisoning depend on the amount present in the victim's blood:

- Ten percent - shortness of breath during mild exercise and headache.

- Twenty percent - dizziness, nausea, vomiting and throbbing headache.

- Thirty percent - impaired thought processes, irritability, severe nausea and vomiting.

- Forty to 50 percent - severe headache, confusion, fainting.

- Sixty to 70 percent - loss of consciousness, seizures and respiratory failure.

- More than 70 percent - rapid death.

Although many symptoms resemble flu, you can often note differences. For example, CO poisoning does not produce a low-grade fever, generalized aching, or lymph node involvement.

CO symptoms:

- Come and go

- Worsen or improve in certain places or at certain times of day

- Involve others around you with similar symptoms

- Pets seem ill

First aid includes removing the victim from the poisoned environment immediately. As soon as possible, the victim needs 100 percent oxygen, which means calling the emergency medical services system or transporting the victim yourself to a hospital emergency department.

Victims with high CO levels should be treated in a hyperbaric oxygen chamber if possible. Hyperbaric chambers - which hold one person or as many as 16 - deliver oxygen in an atmosphere at sea level. The hyperbaric chamber pushes oxygen into the blood and tissues. About 230 hyperbaric chambers exist in the United States.

The CO poisoning environment should be checked to find the CO's source and then corrected before allowing people into the area.

- Alton Thygerson is a professor of health sciences at Brigham Young University.