例子：加氟自来水（adding fluoride in public water）
3 对骨骼有好处（decrease bone diseases）
The speaker refutes the reading‘s argument about the benefits of adding fluoride to pipe water by providing contradictory evidence about pipe water‘s fluoride‘s effectiveness in preventing cavities in teeth, killing bacteria, and decreasing bone diseases.
First, the speaker points out that the short contact of fluoride with teeth will not enable the fluoride to prevent cavities, as the reading argues. Instead, the water will pass the mouth and teeth quickly and go directly to the stomach. Another argument raised in the lecture is that fluoride could undergo chemical reactions with heavy metals in the pipe, such as lead. This will create toxic substances which will pose serious health threat. In this way fluoride could be even more deadly than bacteria. Although the fluoride, as the reading says, will kill some bacteria, the cure here is worse than the disease.
In the end, the speaker maintains that overdose of fluoride will backfire to human bones. Although the reading illustrates that fluoride will improve bone strength, the lecture says overdose will cause serious damage to human bones.
Water fluoridation is the controlled addition of fluoride to a public water supply in order to reduce tooth decay. Its use in the U.S. began in the 1940s, following studies of children in a region where water is naturally fluoridated; too much fluoridation causes dental fluorosis, which mottles or stains teeth, but U.S. researchers discovered that moderate fluoridation prevents cavities. Water fluoridation has been contentious for ethical, safety, and efficacy reasons, and only about 5.7% of people worldwide drink fluoridated water.
The history of water fluoridation can be divided into three periods. The first (c. 1901–33) was research into the cause of a form of mottled enamel called "Colorado brown stain", which later became known as fluorosis. The second (c. 1933–45) focused on the relationship between fluoride concentrations, fluorosis, and tooth decay. The third period, from 1945 on, focused on adding fluoride to community water supplies.