Since 1939, numerous studies have been conducted to determine whether smoking is a health hazard. The trend of the evidence has been consistent and indicates that there is a serious health risk. Research teams have conducted studies that show beyond all reasonable doubt that tobacco smoking is associated with a shortened life expectancy.

Cigarette smoking is believed by most research workers in this field to be an important factor in the development of cancer of the lungs and cancer of the throat and is believed to be related to cancer of some other organs of the body. Male cigarette smokers have a higher death rate from heart disease than non-smoking male s. Female smokers are thought to be less affected because they do not breathe in the smoke so deeply.

Apart from statistics, it might be helpful to look at what smoking tobacco does to the human body. Smoke is a mixture of gases, vaporized chemicals, minute particles of ash and other solids. There is also nicotine, which is powerful poison, and black tar. As smoke is breathed in, all those components form deposits on the membranes of the lungs. One point of concentration is where the air tube and bronchus divides. Most lung cancer begins at this point.

Filters and low tar tobacco are claimed to make smoking to some extent safer, but they can only slightly reduce, not eliminate the hazards.

练习：

1). It is easy to determine whether smoking is hazardous. -W

2). Smoking reduces one’s life expectancy. -R

3). Smoking may induce lung cancer. -R

4). There is evidence that smoking is responsible for breast cancer. -N

5). Male smokers have a lower death rate from heart disease than female smokers. -W

6). Nicotine is poisonous. -R

7). Filters and low tar tobacco make smoking safe. -W