The most comprehensive estimate of mercury released into the environment is putting a new spotlight on the potent neurotoxin. By accounting for mercury in consumer products, such as thermostats, and released by industrial processes, the calculations more than double previous tallies of the amount of mercury that has entered the environment since 1850. The analysis also reveals a previously unknown spike in mercury emissions during the 1970s.

The finding doesn’t indicate a greater risk to human health; scientists already know how much mercury most people are exposed to. But it does show how tighter regulations over the past 4 decades have lowered the total amount of mercury emitted to the global environment—even as some industries in the developing world continue to expand.

Mercury is a dangerous neurotoxin, and it is most harmful when microbes convert the element into a compound called methylmercury. This form of the metal accumulates in food webs, concentrating in fish, which is how most people are exposed. Researchers in developed countries typically know how much methylmercury is in fish, which they use to set consumption advisories.

But scientists and public health advocates also want to know how mercury is used and where it eventually ends up in the environment. This knowledge can help inform policy and regulations.

First, some key figures in mercury accounting. Researchers have estimated that about 720,000 metric tons of the element have been taken out of the ground since 1850, when major silver and gold rushes were under way. Mercury is particularly useful for extracting silver from pulverized ore.

Mercury has also been used in many products and devices, such as thermometers and switches, and in various industrial processes. In addition, coal contains trace amounts of mercury, so burning this fossil fuel has spewed a lot of mercury into the atmosphere. All told, analysis of sediment cores and other research show that background levels of mercury have risen threefold since the Industrial Revolution.

The hard work for experts has been to figure out how much mercury all of these products and processes have used and what happens to the waste and pollution. Previous studies focused on emissions into the atmosphere. Roughly 100,000 tons of this pollution came from the use of mercury in silver mining, which peaked in the 1890s. Since then, most atmospheric mercury has been emitted by power plants, smelters, and chlor-alkali plants, which use it to make chlorine and sodium hydroxide for industry.

Environmental modeler Hannah Horowitz, a graduate student at Harvard University, and her colleagues wanted to account for all the uses of mercury over time, quantifying for the first time how much mercury has been released worldwide since 1850. The team found the largest peak in the 1970s, instead of the late 19th century. This time frame matches what is seen in sample cores taken from lake sediments and peat marshes.

To produce their new estimates, Horowitz and colleagues compiled historical information from the scientific literature and various government reports. Along with commercial products, a major source in the 20th century was latex paint, which used mercury compounds as a preservative. After the 1970s, this use declined in the United States, until it was finally banned by the Environmental Protection Agency in 1991. Another important source in the 20th century continues to release mercury into the environment: the production of vinyl chloride, an important ingredient in plastics and vinyl.

All told, Horowitz and her co-authors say, they have accounted for 540,000 additional tons of mercury in the environment, including in soil and water. That’s two-and-a-half times more than the amount suggested by the previous estimates, they report in the 2 September issue of Environmental Science & Technology.

“This is quite a high number,” says Jozef Pacyna, a research director at the Norwegian Institute for Air Research, near Oslo, who was not involved in the research. “It’s good they took this big picture.”

About 57% of the mercury released since 1850 continues to circulate in the environment; the remainder is locked away in sediments or landfills. Horowitz says it was “really difficult” to come up with an error range around the estimates. “I think the uncertainty is quite large,” she says. Indeed, in the latest Global Mercury Assessment, by the United Nations Environment Programme (UNEP), the estimate of annual emissions ranged by a factor of 4.

This research "helps to fill some of the many holes in our understanding of mercury flows historically," says Peter Maxson, a consultant based in Brussels who advises the European Commission, UNEP, and others on mercury issues. "Once mercury is brought into the environment, it doesn't just go away but becomes everyone's problem for a very long time," says Maxson, who was not an author of the new paper.

Michael Bender, who directs the Mercury Policy Project, an advocacy organization in Montpelier, and who did not participate in the study, and others say it is crucial to continue encouraging companies to phase out their use of mercury and to tighten regulations for dealing with mercury wastes.

One of the largest concerns today with mercury is small-scale gold mining in the developing world. Environmentalists and public health advocates are pinning their hopes on an international treaty, agreed to in 2013, called the Minamata Convention, after a town in Japan where citizens suffered from severe mercury poisoning. The treaty would require countries that ratify it to ban mercury in batteries, light bulbs, and other products, and cut emissions from power plants, incinerators, and factories.