Mercury

In the news today it was announced that the EPA has new lower limits on mercury emissions from coal plants. I have often wondered how the government determines the legal level for various contaminants. For example the maximum allowed for mercury in water is 2 parts per billion (ppb). For those of you who have forgotten your chemistry I remind you of Avogadro’s number. He discovered that every gram molecular weight of any substance contains 10 the 23rd power (1 followed by 23 zeros) molecules. This means that a little over a table spoon of water weighs one gram molecular weight which means it contains 10 to the 23rd power of water molecules. Now if the allowable level of mercury is 10 to the 9th power (1ppb) then each tablespoon of water contains 10 to the 14th power of mercury. This means that in every tablespoon of water considered safe by the EPA there are 100,000,000,000,000 or 100 quadrillion atoms of mercury. If we increase the concentration to 1,000,000,000,000,000 or 1 quintillion we will be poisoned.
I believe we should have clean water but I am wondering if these people really know what the limits should be. If I told you that we now have safe drinking water because there are only 100 quadrillion atoms of mercury in a tablespoon of water instead of 1 quintillion would you feel better
This is the kind of nonsense I think about. I think I need a hobby.