Mercury emissions don't pose a serious health risk because mercury has always been present in the environment
Far more mercury is in the environment now than before industrialization
There is far more mercury in the environment now than there was before industrialization, and there is no dispute that mercury is a harmful substance for humans.
A 2006 report from the nonpartisan Congressional Research Service says, “The generally accepted estimate is that roughly three to five times as much mercury is mobilized today as was mobile before industrialization. However, the author of one recent study argues that the mercury deposited from the atmosphere today is at least 10 times the amount of mercury that was being deposited 500 years ago.”
In addition, the Centers for Disease Control and prevention says:
The nervous system is very sensitive to all forms of mercury. Methylmercury and metallic mercury vapors are more harmful than other forms, because more mercury in these forms reaches the brain. Exposure to high levels of metallic, inorganic, or organic mercury can permanently damage the brain, kidneys, and developing fetus. Effects on brain functioning may result in irritability, shyness, tremors, changes in vision or hearing, and memory problems.
Short-term exposure to high levels of metallic mercury vapors may cause effects including lung damage, nausea, vomiting, diarrhea, increases in blood pressure or heart rate, skin rashes, and eye irritation.
Similarly, the Environmental Protection Agency reports that “it is estimated that more than 300,000 newborns each year may have increased risk of learning disabilities associated with in utero exposure to methylmercury.”