Glossary of Terms

Science and Regulation of Mercury Levels in Seafood

Although there is extensive science behind how trace levels of methylmercury are regulated in fish, the terms associated with this issue are usually complicated and oftentimes misunderstood. To end the confusion, what follows are simple definitions of some of the more frequently used terms regarding the science and regulation of mercury levels in seafood.

Absolute Risk		The chance of a specific occurrence happening over a specified time-period.
Adverse Effect		Any effect that produces functional impairment.
Analytical study		A type of epidemiological study where scientists observe certain behaviors, such as food choices, and track whether certain outcomes, such as the development of disease, occur.
Anthropogenic Emissions		Atmospheric emission caused by human activity, such as the emission of mercury caused by burning coal and other fossil fuels.
Bias		Problems in study design that can lead to effects that are not related to the variables being studied. An example is selection bias, which occurs when study subjects are chosen in a way that can misleadingly increase or decrease the strength of an association.
Bioaccumulation		An increase in concentration in living organisms as they take in contaminated air, water, or food because the substances are very slowly metabolized and/or excreted.
Blind - Single or Double		In a single-blind experiment, the subjects do not know whether they are receiving an experimental treatment or a placebo. In a double-blind experiment, neither the researchers nor the participants are aware of which subjects receive the treatment until after the study is completed.
Boston Naming Test		Originally developed to help diagnose aphasia (i.e., difficulty comprehending spoken or written language) in adults after strokes, brain trauma or dementia, the Boston Naming Test is used as a determining endpoint in the Faroe Islands study to suggest a subtle change in children’s cognitive development. The developers of the Boston Naming Test have noted problems with using it in young children because there is no way to tell if lower scores are caused by a lack of word knowledge and language skills - and by a memory deficit.
Clinical Trial		This type of experimental research uses people as subjects to evaluate the effectiveness and safety of a nutrient or medical treatment by monitoring its effect on the people participating in the clinical trial. Clinical trials may be small with a limited number of participants - or they may be large intervention trials seeking to discover the outcome of treatments on entire populations. The more participants in a study, the greater the likelihood that the study results can be replicated in the general population.
Cohort Study		An epidemiological study where scientists select the study population according to their exposure, regardless of whether the group has the disease or health outcome being studied. The researchers then determine the outcomes and compare the results on the basis of the individuals' exposure(s). Cohort studies are often referred to as prospective studies because they follow the study population forward in time.
Confounding Variable or Confounding Factor or Hidden Variable		A "hidden" variable that may cause an association that the researcher attributes to other variables. In some situations, a confounding factor may wrongly increase the effect of a substance.
Control Group		The group of subjects in a study to whom a comparison is made to determine whether an observation or treatment has an effect. In an experimental study, it is the group not receiving any treatment(s).
Cord Blood		A sample of blood collected from the umbilical cord at birth.
Correlation		An association. When one phenomenon is found to be accompanied by another. A correlation does not prove cause and effect.
Cross-sectional Study		This type of epidemiological study is basically the same as a survey. Here, the epidemiologist defines the population to be studied and then collects information from members of the population about their disease and exposure status. Since the data represent a point in time, it's like taking a "snapshot" of the population.
Defect Action Level		The level at which the U.S. Food and Drug Administration (FDA) can take action to remove a food product from the market. For fish, the mercury action level is set at 1.0 part per million (ppm) methylmercury in edible fish tissue.
Descriptive Study		A type of epidemiology study where scientists collect information to characterize and summarize the health event or problem.
Dose		The amount of a risk agent that enters or interacts with organisms. An administered dose is the amount of a substance given to an animal or person, usually measured in milligrams per kilogram of body weight - or in parts per million of the diet, drinking water, or ambient air. An effective dose is the amount of the substance reaching the target organ.
Endpoint		The measure of an effect that may correlate with exposure to a chemical or drug - but there is no guaranteed relationship.
Epidemiologic Study		Considered the basic science of public health, this type of observational research usually focuses on studying large groups --sometimes tens or hundreds of thousands of people. Epidemiology seeks to identify possible factors that increase the risk or probability of a disease.
Experimental Group		The group of subjects in an experimental study receiving treatment(s).
Experimental Research		Research that generates data by investigating biochemical substances or biological processes. Experimental research is often conducted in vitro, such as in test tubes. It is also conducted in vivo in both animals and humans.
Faroe Islands Study		Supported by the National Institute of Environmental Health of the National Institutes of Health (NIEHS / NIH / DHHS) and the European Commission, this was the first large-scale study of 7-year-old children to show subtle delays in neurological development if their mothers had high intakes of methylmercury from eating frequent seafood dinners during pregnancy. The study was published in the journal Neurotoxicology and Teratology in 1997 and was carried out in the Faroe Islands, in the North Atlantic north of Scotland, where dietary mercury exposure is primarily from eating pilot whale meat. The whale meat and blubber also contained significant levels of the known neurotoxicant(s) PCBs. The study established the lowest benchmark dose level (BMDL) for neurodevelopmental effect at 58 parts per billion (ppb) in blood.
Generalizability		The extent to which the results of a study can be applied to the general population.
In Vivo		Literally “in glass.” Used to mean outside the living body and in an artificial environment.
In Vitro		Literally “in the living.” Used to mean in the living body of a plant or animal.
Incidence		The number of new cases of a disease during a given period of time in a defined population.
Ingested Dose		The amount of a substance consumed by an individual, usually expressed as the amount of the substance per kilogram of body weight over a given time period.
Intake		The amount of material inhaled, absorbed through the skin, or ingested during a specific time period.
Mechanism of Action		The way in which a substance, such as a chemical, exerts its effects.
Mercury		A naturally occurring element and a part of the earth’s crust, oceans, and atmosphere. Elemental mercury is the unique silver-white metal that is a liquid at room temperature and easily vaporizes. Most environmental mercury is released through natural processes, including surface volcanic eruptions, deep-sea vents, underwater volcanic eruptions, and erosion. The chemical abbreviation for mercury is Hg.
Mercury Poisoning		An extremely rare situation that has only occurred in people four times globally when very large amounts of methylmercury was introduced into consumed food and eaten in large doses. These four situations occurred two times each in Japan and in Iraq. No case of mercury poisoning from fish consumption has ever occurred in the United States.
Meta-analysis		A quantitative technique in which the results of several individual studies are pooled to yield overall conclusions.
Methylmercury		This organic form of mercury is produced when mercury enters oceans, lakes, rivers, and streams and is converted to methylmercury by certain bacteria. In large doses, methylmercury is a human neurotoxin. Methylmercury is rapidly absorbed through the gastrointestinal (GI) tract to then possibly enter the adult and fetal brain. Methylmercury is not harmful to people in trace amounts. The chemical abbreviation for methylmercury is MeHg.
Minimum Risk Level		This level was developed by the federal government’s Agency for Toxic Substances and Disease Registry (ATSDR / DHHS). The minimal risk level is an estimate of the level of daily human exposure to a hazardous substance likely to not cause appreciable risk of adverse non-cancer health effects over a specified duration and route of exposure. ATSDR’s level for methylmercury is 0.3 micrograms per kilogram of body weight per day (µg/kg/day).
National Health and Nutrition Examination Survey (NHANES)		A continuous survey of the health and nutritional status of the U.S. population conducted by the Centers for Disease Control and Prevention (CDC) / DHHS.
Neurotoxicity		The occurrence of adverse effects on the nervous system following exposure to a chemical and/or some other toxic substance(s).
No Observable Adverse Effect Level (NOAEL)		The No Observable Adverse Effect Level is used to calculate the risk for many toxic substances. For methylmercury, the NOAEL is calculated based on the most sensitive endpoint tested - the neurological development in children exposed in utero to methylmercury as a result of their mothers’ consumption of seafood containing methylmercury.
Observational Research		May be used in the laboratory but it is primarily conducted in a natural setting to study the relationship between a specific factor and some aspect of health or illness. For this reason, observational research may suggest an association but does not determine cause and effect.
Outcomes Research		A type of research that provides information about how a specific procedure or treatment regimen affects the subject (clinical safety and efficacy), the subject's physical functioning and lifestyle, and economic considerations such as saving or prolonging life - and avoiding costly complications.
Phase I Study (Clinical Trial)		Researchers initially test a new treatment or intervention in a small group of people to evaluate its safety, determine a safe dose range, and identify side effects.
Phase II Study		The treatment is given to a larger group of people to see if it is effective and to further evaluate its safety.
Phase III Study		The treatment is given to large groups of people to confirm its effectiveness, monitor side effects, compare it to commonly used treatments, and collect information that will allow the drug or treatment to be used safely.
Phase IV Study		Studies are done after the treatment has been marketed to gather information about its effect in various populations and any side effects associated with long-term use.
Placebo		Sometimes casually referred to as a "sugar pill," a placebo is a "fake" treatment that seems to be identical to the real treatment. Placebo treatments are used to eliminate bias that may arise from the expectation that a treatment should produce an effect.
Prevalence		The number of existing cases of a disease in a defined population at a specified time.
Prospective Study		Epidemiological research that follows a group of people over a period of time to observe the potential effects of diet, behavior, and other factors on health and/or the incidence of disease.
Provisional Tolerable Weekly Intake (PTWI)		The World Health Organization (WHO) Joint Expert Committee on Food Additives (JECFA) reviewed data in 2003 to establish a Provisional Tolerable Weekly Intake (PTWI) level for methylmercury. WHO calculated the PTWI at 0.23 micrograms per kilogram of body weight per day (µg/kg/day).
Randomization or Random Assignment		A process of assigning subjects to experimental or control groups in which the subjects have an equal chance of being assigned to each group. Used to control for known, unknown, and difficult-to-control-for variables.
Random Sampling		A method to select subjects to participate in a study in which all individuals in a population have an equal chance of being chosen. Helps to ensure the generalizability of the study results.
Reference Dose (RfD)		The highest daily dose that the most sensitive member(s) in the population can be exposed to over a lifetime without experiencing any adverse effect(s). The U.S. Environmental Protection Agency (EPA) is responsible for setting the methylmercury RfD at 0.1 microgram per kilogram of body weight per day (µg/ kg-day). This RfD builds in a ten-fold (10x) safety factor, making it the world’s most restrictive mercury standard.
Relative Risk		Puts the chance in comparative terms by describing the outcome rate for people exposed to the factor in question compared with the outcome rate for people not exposed to the factor. Although relative risks are the most commonly used measure of morbidity or mortality in the current medical literature, in most cases absolute risk is more relevant for the public.
Reliability		Whether a test or instrument used to collect data, such as a questionnaire, gives the same results if repeated with the same person several times. A reliable test gives reproducible results.
Research Design		How a study is set up to collect information or data. For valid results, the research design must be appropriate to answer the question or hypothesis being studied.
Residual Confounding		The effect that remains after attempting to statistically control for variables that cannot be perfectly measured. This is an important epidemiological studies concept because knowledge of human biology is still developing. Unknown variables may exist that could change conclusions based on epidemiological research.
Retrospective Study		Research that relies on recall of past data or on previously recorded information. Often, this type of research is considered to have limitations because the number of variables cannot be controlled - and because people’s memories are fallible.
Risk		A term encompassing a variety of measures of the probability of an outcome. It is usually used in reference to unfavorable outcomes such as illness or death.
Risk Factor		Anything statistically shown to have a relationship with the incidence of a disease. Does not necessarily infer cause and effect.
Seychelles Child Development Study (SCDS)		A 66-month study conducted by researchers at the University of Rochester School of Medicine and Dentistry with funding from the Department of Health and Human Services (DHHS). This study found no adverse effect from methylmercury exposure from consuming fish. Published in the Journal of the American Medical Association (August 1998), this was a prospective study of 711 mother-child pairs living in the Republic of Seychelles located in the Indian Ocean. Mothers there reported eating approximately 12 meals of saltwater fish per week.
Statistical Power		A mathematical quantity that indicates the probability a study has of obtaining a statistically significant effect. A high power of 80 percent or 0.8 indicates that the study would produce a statistically significant effect 80 percent of the time if conducted repeatedly. A power of only 0.1 means that there would be a 90 percent chance that the research missed any effect(s).
Statistical Significance		The probability of obtaining an effect or association in a study sample as or more extreme than the effect or association observed if there was actually no effect.
Subclinical Effects		Relating to the stage in the development of a disease or condition before any signs are observable.
Uncertainty Factor		Derived to determine a margin of safety consistent with adequate precautionary approaches and good public health practices.
Validity		The extent to which a study and/or study instrument(s) measures what it is intended to measure. Validity refers to accuracy or truthfulness of a study's conclusion(s).
Variable		Any characteristic that may vary in study subjects including gender, age, body weight, diet, behavior, attitude, or other attribute. In an experiment, the treatment being investigated is called the independent variable. The variable that is influenced by the treatment is the dependent variable; it may change as a result of the effect of the independent variable.