117. Read the following passage, then answer the question below.

Plastic does not disappear; it breaks into smaller pieces. Fragments smaller than five millimeters are called microplastics, and scientists have found them almost everywhere they have looked, from Arctic ice to deep ocean trenches and the air above cities. Researchers distinguish two types. Primary microplastics are manufactured small, such as the beads once added to facial scrubs or the pellets used as raw material by factories. Secondary microplastics form when larger objects such as bottles and bags break apart under sunlight and wear.

The surprising part is where much of the material comes from. Surveys suggest that laundering synthetic clothing releases thousands of tiny fibers with each wash, and that the wear of vehicle tires on roads may be a still larger source. Wastewater plants capture a large share of these particles, but they end up in the leftover sludge, which is often spread on farmland as fertilizer. In this way, a pollutant that was removed from water may return to the environment on land.

What these particles do to living things is much less certain. Laboratory studies show that animals that swallow microplastics can suffer inflammation and reduced feeding, yet the doses used in experiments are often far higher than those found in nature. Researchers have also detected particles in human blood and tissue, but they have not shown that these cause disease. Some scientists worry that the particles carry harmful chemicals on their surfaces, whereas others point out that natural particles such as dust have always been present in the body. Because the evidence is incomplete, policymakers have focused on what can be done without waiting for proof: banning microbeads in cosmetics and encouraging filters on washing machines. Whether such measures will be enough remains an open question.

Why does the author mention that experimental doses are higher than those in nature?