Nine months of pregnancy -- the journey of an ecologist becoming a mother

Author: (American) Stan, translated by Hu Zhongle
Publisher:
Publishing Date: 2004-04-01
Features: Consequences that arise unintentionally are not always consequences that are unpredictable. Although the environment has its own ways of dealing with metals like mercury, certain ecological rules still apply. At the core of this is the biomagnification principle, which refers to the concentration of persistent toxins as they move up the food chain. The organisms at the top of the food chain have the highest concentration. In water, toxic substances can often become highly concentrated because the food chain there is longer than on land. (Due to the buoyancy of water, aquatic organisms require relatively less heat to survive compared to land-based organisms constrained by gravity. Because they spend less energy supporting themselves, the efficiency of energy transfer from one link in the chain to the next is higher. Additionally, because less energy is lost between links, the entire chain can have more links.) Taking Minamata as an example, fish caught on a fishing boat's deck have mercury levels in their flesh that are over a million times higher than the concentration in the seawater they inhabit. In other words, when Chisso Corporation discharged mercury into the bay, in addition to the bacteria living there converting it into a more toxic substance in terms of quality, the concentration had increased by a million times by the time it returned to land. Generally speaking, when persistent pollutants are released into the overall environment, people who consume large amounts of fish and other aquatic animals are more likely to come into contact with these substances. Among all human beings, fetuses are the most vulnerable to toxins. We now know that the placenta can even further amplify the effects of toxic chemicals. Furthermore, developing organs are more sensitive to damage than adult organs. The period most vulnerable to methylmercury exposure is from the fourth to the sixth month of pregnancy—much later than the periods for rubella and thalidomide. This is the time when brain cells migrate. Just as a spider can descend from the ceiling by releasing a fine thread, fetal brain cells can move from the center of the brain to the surface using their own axon fibers. Eventually, the brain is filled with cells resembling spiders and the neural networks they subsequently weave. Methylmercury interferes with the movement of nerve cells from the center to the surface. Once the critical period for brain cell migration is missed, it will never return. The threshold for toxic chemicals may not exist for fetuses. The concept of a threshold, like the image of the placental barrier, is deeply ingrained in our collective consciousness, and discarding it is equally difficult. Similar to the belief in the placental barrier, the threshold belief provides a sense of safety from harm. People often assume that as long as exposure to a certain toxin is below a certain limit, the risk can be ignored. This assumption is tempting, and the idea has been around for a long time in terms of human perception. The concept of a threshold is derived from the basic principle of the Middle Ages: "the dose determines the toxicity." New research on Minamata disease challenges the question of whether there is a threshold for mercury's harm to fetuses. When the initial survey of congenital Minamata disease patients was conducted, the question of "what is the lowest amount of mercury that causes harm?" was not anticipated. Unfortunately, during the last count, only 47 victims were still alive; at this point, it would be difficult to distinguish between their exposure to mercury before and after birth. However, many families in Minamata