Mercury Levels and Cognitive Function Investigated in Adults

Although the omega-3 fatty acids found in fish are well known for their health benefits, many fish are also the primary source of mercury in the general population. Researchers from the Johns Hopkins Bloomberg School of Public Health recently completed the first study of mercury and cognitive function in urban, U.S. adults between the ages of 50 and 70 years. They found that blood mercury levels were not consistently associated with adverse performance on a broad range of tests of cognitive function. This study may help policy makers with future decisions about mercury emissions from power plants as well as fish consumption recommendations for older adults. The study is published in the April 20, 2005, issue of the Journal of the American Medical Association (JAMA).

“Our study provides no evidence to challenge the government’s current recommendations for blood mercury levels, but neither does it indicate that they are safe. The key point is that the aging population may be more sensitive to toxic chemicals and this is the first study to examine mercury exposure in the older U.S. population,” said Megan Weil, MHS, lead author of the study and a PhD-candidate in the Bloomberg School of Public Health’s Department of Environmental Health Sciences.

Weil explained that the Environmental Protection Agency bases its recommendations on studies of children and women of childbearing age. From these studies, they determined that blood mercury levels should not exceed 5.8 µg/L. The median mercury level in study participants was 2.1 µg/L and only approximately 10 percent of subjects exceeded the government guideline.

The researchers analyzed the blood samples of 474 study participants, who were part of the larger Baltimore Memory Study. They also administered 12 cognitive function tests, which measured language, memory, attention, concentration, reaction time and other brain functions. The study authors found that higher blood mercury levels were associated with poorer performance on a memory test that measured the ability to recall a complex, two-dimensional line drawing. However, they saw improved performance on tests measuring motor speed, eye-hand coordination and manual dexterity. Because most of the large number of tests showed no correlation with mercury and there was a lack of consistency of mercury effects in the different aspects of brain function, the researchers concluded that overall the data did not provide strong evidence that mercury at these levels had an adverse impact on cognitive function.

“Mercury is a major global public health challenge. Global emissions will continue to increase for the foreseeable future and mercury released in one area can reach people at very great distances. Studies of this type are critical to the development of effective regulatory policies,” said Brian Schwartz, MD, senior author of the study and a professor in the Bloomberg School of Public Health’s Department of Environmental Health Sciences.

The study was supported by the National Institute of Environmental Health Sciences and the National Institute of Occupational Safety and Health.

Additional co-authors of the study were Joseph Bressler, PhD, Patrick Parsons, PhD, Karen Bolla, PhD, and Thomas Glass, PhD.