The result is a five to twenty percent drop in the amount of oxygen in the blood. This reduces the amount of oxygen that reaches the organs of the body.

　　One effect of this lack of oxygen to the brain is a headache. When this happens, the heart attempts to fix the situation by beating harder and faster. This can make the traveler feel tired.

　　These signs of hypoxia are not dangerous in a healthy person. But a drop in oxygen level can cause a health emergency in people with heart or lung problems. They might lose consciousness or even suffer a heart attack.

　　Experts say that smoking cigarettes and drinking alcoholic liquids also reduce the body’s ability to use oxygen. So they suggest that people not drink alcohol or smoke cigarettes either before or during a flight. They also say persons with heart or lung problems should seek advice from their doctors before flying.

　　36. The author writes the passage mainly to ___.

　　A. warn people in airplanes of possible dangers

　　B. offer some advice on avoiding health problems

　　C. explain the causes and effects of hypoxia

　　D. advise people not to smoke or drink alcohol

　　37. According to the passage, what leads to hypoxia?

　　A. The brain lacks enough oxygen.

　　B. The air pressure in a plane is too high

　　C. Our body needs less oxygen during takeoff.

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