Introduction to Mercury: Building Background Understanding

By the end of this Unit your students will understand the basics of the mercury cycle.

Unit Overview

This first unit will help students understand what mercury is, why it presents a health concern, and where it comes from. The classroom activities cover background information, mercury concentrations, mercury movement in the environment, and mercury toxicity. In the Links section you will find excellent background reading materials including the BioDiversity Research Institute's Mercury Connections report that many students have found very helpful.

Mercury is an element on the periodic table. It occurs naturally in the environment in some types of rocks. It can be released rapidly by the eruption of volcanoes and slowly by rocks and soils. Humans have been speeding up the release of mercury from these sources by digging up and burning fossil fuels. On average, scientists estimate that about three times more mercury falls from the sky now than before the Industrial Revolution.

It is useful to have a general picture of the sources of mercury. In the U.S. in 1999, anthropogenic (caused by humans) sources of Hg to the atmosphere were emissions from coal-burning power plants (40%), industrial boilers (10%), burning hazardous waste (5%), and chlorine production (5%).

The U.S. and other developed countries have worked to decrease these releases of mercury since 1999. However, over the past few decades, emissions of mercury from developing countries have increased, offsetting decreases from developed countries.

Once it’s emitted to the atmosphere from smokestacks and the like, mercury has a long residence time in the atmosphere, estimated at up to a year. This makes mercury a global problem, with emissions able to circle the globe and land in remote areas that don’t emit large amounts of mercury such as the arctic – or Maine. Though Maine releases less than a thousandth of a percent of US emissions, many ecosystems and biota in Maine and the Northeast have been found to be contaminated with mercury.

There are a few factors that contribute to the mercury contamination problem. Scientists believe that it’s because Maine and the Northeast are downwind of a lot of the large coal-burning power plants in the mid-western US. Also, some of our ecosystems are probably more sensitive to mercury contamination – they have a lot of forested areas, which capture mercury from air, and a lot of wetlands, where naturally-occurring bacteria convert mercury to its toxic form. However, almost all US states have some kind of mercury fish consumption advisory, and it’s an issue for many National Parks, including Everglades in Florida, Isle Royale & Voyageurs in the upper Midwest, Yellowstone, Kejimkujik in Nova Scotia, and many more. Scientists are still learning the extent of the mercury problem, and it’s hard to compare Maine to other places though the Northeast is thought to have many ‘hotspots’ of mercury contamination.

Finally, it only takes a very small amount of mercury to contaminate an ecosystem. It’s what’s called an ‘ultra-trace’ substance – usually measured at the parts per billion or parts per trillion levels. A little mercury goes a long way, because it is eventually passed up the food chain and bioaccumulates.

Key Ideas introduced in this Unit

  • Mercury is toxic in small quantities and is toxic to humans.
  • There are a good number of environmental sources and hazards associated with mercury.
  • Mercury occurs in three forms in nature—elemental (Hg0), gaseous (Hg2+), methylmercury (MeHg or CH3Hg).
  • Methylmercury is the byproduct of metabolism by anaerobic bacteria, typically living in an acidic environment with access to carbon and sulfur. Methylmercury can be absorbed by living things and it bioaccumulates.
  • In Maine and the Northeast most of the mercury that gets into the food web comes through the air from coal burning plants used to generate electricity.

Concepts and Relationships

Jumping down to a lower level of detail, students should have command of a number of concepts and relationships at the end of this unit. They will need this understanding in order to work successfully in the units that follow. 

Students should be able to:

  • Compare mercury concentrations measured in ppb and ppm and know the difference
  • Convert units such as μg/g to ppb or ppm
  • Have a general idea of typical mercury concentrations in fish muscle tissue
  • Have a general notion of what a high level of mercury would be in a fish or in human hair
  • Identify the different forms of mercury and where they come from
  • Connect the principal factors in mercury transport and deposition in a way that enables them to answer simple questions about the kinds of conditions that typically result in more mercury deposition in a watershed


  • Anaerobic bacteria—bacteria which live in an environment devoid of oxygen
  • Biomagnification—the amount of a substance (such as mercury) magnifies as it moves up the food chain
  • Concentration—a measure of the amount of dissolved substance contained per unit of volume
  • Emissions—discharges, such as from a smokestack
  • Methylmercury—the byproduct of metabolism by anaerobic bacteria, typically living in an acidic environment with access to carbon and sulfur
  • Toxicity--the degree to which something is poisonous

Student Prerequisites

How to measure volume and square inches
Parts of a circle
Computer skills
Library skills

Misconceptions about mercury

  • It’s OK to play with it
  • It isn’t a problem in Maine
  • A little bit is not dangerous
  • Mercury in the environment is not related to personal health
  • It’s OK to eat all kinds of fish any time

Activities - Introduction: 

The objective for the activities in this first unit is to provide students with the key information about mercury and about how mercury moves around in forests, ponds, and other areas.  Our goal is to give the students the background they need to investigate the amounts of mercury in plants, invertebrates, fish, and people.  Later on, we will expect them to consider questions such as “Would caterpillars have a higher mercury burden than beetles?”  or “What is the relationship between fish diet and fish mercury burden?”

In order for the students to be able to work with questions such as these in some way other than blind guessing, they need to develop a basic understanding of how mercury gets into their pond, field, forest, stream, or other area that they are studying.  They also need to have a general feel for the factors that need to be present in order for the mercury to change into a form in which it can be absorbed by living things.  In short, the students need a basic understanding of mercury chemistry.

The students need to understand more than the chemistry, of course.  They also need to understand how bioaccumulation and biomagnification work, which means understanding the operation of food chains and food webs.  We will focus on food chains and food webs in Unit 2.   Here, in this first unit, our objective is just to introduce the following basic ideas:

    Mercury is toxic in very small quantities.
    Mercury occurs in a number of forms in nature.  One of these, forms, methylmercury, can be absorbed by living things and is very toxic.
    In the Northeast, most mercury arrives through the air.
    Methylmercury is the byproduct of metabolism by anaerobic bacteria, typically living in an acidic envi-ronment with access to carbon and sulfur.

The Essential Activity, “Why Do We Care About Mercury?,” focuses on the first of these ideas and introduces the second one.  The Optional Activity, “Mercury Concentrations,” reinforces the second idea and develops the third and fourth ones. Optional Activity "Mapping U.S. Mercury Emissions" helps develop the second idea, and "The Road to Toxicity" develops the last of these ideas.