Methylmercury is an organic compound that forms when mercury, a naturally occurring element, transforms in the environment. This form of mercury is concerning due to its ability to accumulate within living organisms. It is a significant environmental contaminant, primarily because of its prevalence in the food chain, making fish consumption the main route of human exposure.
How Methyl Mercury Forms and Enters the Food Chain
Elemental mercury, released into the environment from both natural sources like volcanic activity and human activities such as industrial processes and coal burning, undergoes a transformation in aquatic environments. In lakes, streams, and sediments, certain anaerobic bacteria, particularly sulfate-reducing bacteria, convert inorganic mercury into methylmercury through a process called methylation. This conversion occurs under oxygen-free conditions, often at the sediment-water interface where these bacteria thrive.
Once formed, methylmercury enters the aquatic food web and accumulates in organisms. This process begins when small aquatic organisms absorb methylmercury from the water or by consuming other contaminated food sources. Methylmercury is not easily eliminated from their bodies, leading to its buildup in individual organisms, a process known as bioaccumulation.
As smaller fish are consumed by larger predatory fish, the concentration of methylmercury significantly increases up the food chain, a phenomenon called biomagnification. For instance, large carnivorous fish like tuna, mackerel, and swordfish tend to have higher levels of methylmercury because they consume many smaller, contaminated fish over their lifespan. Methylmercury concentrates in the muscle tissue of fish, which is the part typically eaten.
Health Impacts and Who is Most Susceptible
Methylmercury is a neurotoxin that damages nerve tissue, primarily targeting the central nervous system. Exposure can lead to neurological problems, including impaired coordination, vision difficulties, and speech impediments. Severe exposure has been linked to conditions like cerebral palsy and deafness.
The developing brain is vulnerable to methylmercury’s toxic effects. Exposure during fetal development can cause widespread neurological damage, even if the pregnant mother shows no symptoms. Children exposed in utero may experience developmental delays, including issues with motor skills, language acquisition, and memory function. These neurodevelopmental effects can manifest as lowered IQ points and attention deficits.
Certain populations face a higher risk from methylmercury exposure. Pregnant women, nursing mothers, and young children are particularly susceptible. Methylmercury can easily cross the placenta and accumulate in the fetus at higher concentrations than in the mother’s blood. The severity of damage depends on the amount ingested and the timing of exposure during development.
Practical Steps to Reduce Exposure
To minimize methylmercury intake, individuals can make informed choices about the fish they consume. The most effective way to reduce exposure is to primarily eat types of fish with lower mercury levels, while limiting consumption of fish species higher in mercury.
Fish such as salmon, shrimp, pollock, catfish, tilapia, and canned light tuna have lower mercury content. These can be eaten more frequently, with recommendations suggesting 8 to 12 ounces (about two average meals) per week for adults, including pregnant women. In contrast, fish with higher mercury levels, such as shark, swordfish, king mackerel, and tilefish (especially from the Gulf of Mexico), should be avoided by pregnant women, nursing mothers, and young children, and limited by others.
Federal agencies like the U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) provide joint advisories on fish consumption to help consumers make safer choices. These advisories categorize fish based on their mercury content and provide guidance on consumption frequency for different population groups. Checking local fish advisories, issued by state or local health departments, is also beneficial, especially for fish caught recreationally, as mercury levels can vary by specific water bodies.