TCS Daily

The Dose Makes the Poison

By Harold M. Koenig - July 7, 2003 12:00 AM

Humans have many uses for mercury: in light bulbs, pesticides, batteries, paint, thermometers and barometers. Over the past few decades, the ubiquitous and persistent nature of mercury has become known and made it an environmental and human health concern.

During the 1950s and 1960s, two mercury-poisoning episodes occurred in Japan resulting from long-term consumption of mercury-contaminated fish. The first occurred in Minamata City where 111 people died or suffered nervous system damage. Children displayed severe psychomotor retardation while their mothers showed either minor manifestations of poisoning or none at all. The second incident occurred in Niigata where 120 people were poisoned.

Mercury poisoning occurred in two separate incidents in Iraq involving consumption of seed grains contaminated with a pesticide containing mercury. More than 6,500 Iraqis were hospitalized and 459 died. The Japanese and Iraqi episodes are the only known large scale cases of mercury poisoning.

Because of concerns raised by these events, our nation's use of mercury has significantly declined. Mercury extraction from U.S. mines ceased in 1991. Mine closures have significantly reduced mercury release into the environment. However, a considerable amount of mercury is still produced and used internationally.

Mercury gets into the environment by both natural and man-made processes. The natural bio-geochemical global cycling of mercury involves volatilization from surface waters and soils, transport through the atmosphere, deposit back onto land and water and absorption into soil and sediment.

Up to 6,000 tons of mercury are released annually into the atmosphere from the naturally occurring volatilization from the Earth's oceans and crust. U.S. coal-fired electric utilities, our largest source of human-related mercury emissions, release approximately 40 tons annually. This is about one-third of the man-made mercury released in our nation. Approximately 60 percent of the total mercury deposited on the nation's soils and water bodies comes from U.S. man-made emissions. The remaining 40 percent comes from international man-made emissions and natural sources. The amount of mercury deposited over the United States increased rapidly from 1900 to 1950, and then declined about two- to three-fold between 1950 and the 1990s. Since 1995 mercury emissions have continued to decrease, but mercury deposition has remained fairly constant.

Mercury deposited on bodies of water is readily absorbed by microorganisms where it is methylated and becomes bio-active. Small fish ingest these microorganisms; these fish are then eaten by larger fish, and so on up the aquatic food chain. The amount of mercury bioaccumulates at each level and reaches high concentrations in fish at the top of the chain. In general, mercury levels in fish range from less than 0.01 parts per million (ppm) to 0.5 ppm.

Attempts to establish dose-response relationships between the amounts of fish consumed and mercury poisoning have been undertaken in New Zealand, the Faroe Islands and the Republic of the Seychelles. These studies examined prenatal mercury exposures within the range of the general U.S. population exposures, and evaluated subtle neurotoxicity. Both the Faroe Islands and New Zealand studies found that increased prenatal mercury exposure was associated with lower performance on neuropsychological tests. However, a large study recently reported in the May 17, 2003 issue of Lancet of 643 children followed from before birth to age 9 years showed no detectable risk from maternal mercury levels in women who ate an average of twelve fish meals a week. This study is the latest in a series of updates on these children who have been followed since their birth in 1989 and 1990 and have been re-evaluated five times.

There is no doubt that prolonged and intense mercury exposure can cause toxic effects, but like for any other substance, the dose makes the poison. Arguments calling for further regulation of emissions of mercury from U.S. coal-fired electric utilities need to be based upon evidence that current mercury concentrations in fish are harmful and that continued emissions from these sources could eventually contribute significantly to yet higher mercury levels.

Further complicating the relationship of mercury concentrations in fish is the global nature of mercury emissions. Mercury travels great distances before leaving the atmosphere. Much of the mercury deposited on our nation's waterways comes from natural or international sources. Any attempt to reduce mercury levels in fish in bodies of water will require actions to reduce mercury emissions on a global scale, rather than a point source scale. The need for burdensome and costly regulations to reduce a yet uncertain risk to public health is neither necessary nor prudent.

Dr. Harold Koenig, Vice Admiral and Former Surgeon General, U.S. Navy, Retired, is Chair and President of The Annapolis Center for Science-Based Public Policy.

To read the Executive Summary and/or the complete report "Mercury in the Environment: The Problems, the Risks, and the Consequences," go to

TCS Daily Archives