During the winter, humid air is highly charged with positive ions which are real pollution traps. As a reminder, positive ions are negative for our health, while negative ions, contrary to what their name indicates, are beneficial. Gusts of rain, gales of wind stir up these polluted particles and bring them to the level of our respiratory tract.
We therefore happily absorb molecules of carbon dioxide, nitrogen oxide or sulfur dioxide, and other pollutants like mercury. Mercury pollution was already known from fish and vaccines. On the other hand, its presence in the air we breathe is less often mentioned while its concentration is constantly increasing. Thus the Earth is constantly sprinkled with this poisonous metal which seeps in everywhere and penetrates in particular the food chain.
Originally, it was mainly coal-fired power plants, incineration plants, the cement industry and the gold mining industry that released these toxic molecules into the atmosphere. According to the newspaper Le Monde, nearly 385,000 tons of mercury have been released into the atmosphere over the last 5,000 years (see Environmental Science and Technology magazine of December 15). The record of emissions would have been reached between 1860 and 1910, at the time of the gold rush in the United States, releasing nearly 2,600 tons of mercury each year. Mercury was then used to amalgamate the precious metal. In addition, mercury is emitted by natural phenomena such as volcanic eruptions or soil erosion.
Today, the situation has not improved much. After a slight lull at the beginning of the 20th century, we are currently witnessing new emission peaks of around 2,000 tonnes per year, i.e. 4 times the quantity emitted by the natural phenomena mentioned above.
The primary cause of this increase in pollution would be the massive combustion of coal by Asian countries. Among these, China is the largest contributor of emissions from its power plants.
It is important to know that once dispersed in the air, these toxic particles are very volatile and carried by the winds and air currents, they can sometimes make several trips around the world. In the December 19, 2011 issue of Nature, a new study based on measurements from a scientific research aircraft explains that "the upper troposphere and lower stratosphere cause oxidation of gaseous elemental mercury. This oxidized mercury is much more reactive, its molecules contained in rain or snow flood the surface of the Earth much more quickly and abundantly. Thus, "The upper atmosphere acts as a chemical reactor allowing mercury molecules to be deposited in ecosystems," explains Professor Seth Lyman, who is responsible for this study.
Once in the water, mercury oxide is then transformed by bacteria into methylmercury. This toxic substance easily penetrates the food chain, contaminates marine fauna and crops, and consequently, at the end of the chain, humans. Professor Seth Lyman points out that certain regions of the world, because of their climatic conditions (southwestern United States for example), seem more easily contaminated by this oxidized mercury.