I don't know the percentages for cement plants but the US in total produces about 3% of the worlds mercury emissions, US coal plants account for 1%. Taking a rough guess cement plants probably figure into that .25% to .50%.
Mercury Emissions: A Global Problem
* Mercury emitted from coal-fired power plants comes from mercury in coal, which is released when the coal is burned. While coal-fired power plants are the largest remaining source of human-generated mercury emissions in the United States, they contribute very little to the global mercury pool. Recent estimates of annual total global mercury emissions from all sources -- both natural and human-generated -- range from roughly 4,400 to 7,500 tons per year. Human-caused U.S. mercury emissions are estimated to account for roughly 3 percent of the global total, and U.S. coal-fired power plants are estimated to account for only about 1 percent
* EPA has conducted extensive analyses on mercury emissions from coal-fired power plants and subsequent regional patterns of deposition to U.S. waters. Those analyses conclude that regional transport of mercury emission from coal-fired power plants in the U.S. is responsible for very little of the mercury in U.S. waters.
That small contribution will be significantly reduced after EPA’s Clean Air Interstate Rule and Clean Air Mercury Rule are implemented.
o U.S. coal-fired power plants emit mercury in three different forms: oxidized mercury (likely to deposit within the U.S.); elemental mercury, which travels hundreds and thousands of miles before depositing to land and water; and mercury that is in particulate form.
o Because mercury can be transported thousands of miles in the atmosphere, and because many types of fish are caught and sold globally, effective exposure reduction will require reductions in global emissions
The EPA estimates about 1/3 of mercury emissions in the US are deposited here, the rest enters the global cycle hence the reason the new have mercury emissions standards that will cost tens or hundreds of billions will only reduce deposition rates in the US 1 to 10% and will raise IQ's an estimated 2/1000 point:
Economic Valuation of Human Health Benefits of Controlling Mercury Emissions from U.S. Coal-Fired Power Plants
Changes in mercury deposition rates associated with reductions in power plant
mercury emissions are based on regional deposition modeling results from the EPA's
analysis of the Clear Skies Initiative. In its analysis, the EPA simulated current mercury
deposition rates and the changes in these rates that would result if power plants
reduced their mercury emissions from the current rate of 49 tons per year to either 26 or
15 tons per year. We used these predictions to estimate changes in deposition rates for
the freshwater regions, the Atlantic Coastal Region, and the Gulf of Mexico. Estimated
decreases range from approximately 1% to 10%.
The change in deposition rates to the
All Other Waters region is assumed to be proportional to the change in total global
emissions that would result from U.S. power plant emissions reductions, which is less