People develop severe head pain as the amount of gas continues to enter their blood. They will begin to feel very tired and sleepy. They may have terrible stomach pains.

FAITH LAPIDUS: Carbon monoxide is measured in parts per million in a normal atmosphere. Breathing in two hundred parts per million of carbon monoxide will cause the first signs of poisoning. It will result in head pain, stomach problems and a feeling of tiredness after two to three hours.

A level of eight hundred parts per million will cause a person to lose consciousness. Victims will not know what is taking place around them. This will happen within two hours of breathing in this amount of carbon monoxide. Twelve thousand parts per million of the gas will cause death in one to three minutes.

A patient poisoned by carbon monoxide receives treatment at a hospital in China's Jilin province in 2006

CHRISTOPHER CRUISE: Medical experts say carbon monoxide affects people differently. For example, a small child will experience health problems or die much quicker than an adult will. The general health of the person or his or her age can also be important.

An older adult with health problems may suffer the effects of carbon monoxide more quickly than a younger person with no health problems. People with heart disease may suffer chest pains. They may begin to have trouble breathing.

Carbon monoxide does not always cause death. But it can cause many medical problems. Breathing low amounts of the gas for long periods of time can lead to permanent damage in the heart, lungs or brain. Experts say small amounts of carbon monoxide over a long period of time can greatly harm an unborn baby.