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- What is Biomonitoring?
- What is measured in a biomonitoring study?
- How do chemicals enter the body?
- Can very minute amounts of a chemical in the body be measured?
- What is meant by "body burden?"
- How is a biomonitoring study conducted
and can any lab do it?
- Why are biomonitoring data important?
- What do biomonitoring data say about
human exposure?
- What can be learned about risk
to humans from the results of biomonitoring?
- Should people be concerned when
biomonitoring studies find new chemicals that have not been previously
detected?
- Where is biomonitoring heading?
What is Biomonitoring?
Biomonitoring is a scientific technique for assessing human exposures to natural and synthetic chemicals, based on sampling and analysis of an individual's tissues and fluids. While blood, urine, breast milk and expelled air are most commonly measured, hair, nails, fat, bone and other tissues may also be sampled. This technique takes advantage of the knowledge that chemicals that have entered the human body leave markers reflecting this exposure. The marker may be the chemical itself. It may also be a breakdown product of the chemical or some change in the body that is a result of the action of the chemical on the individual. For example, alterations in the levels of certain enzymes or other proteins may serve as markers and so might modifications of normal body processes, such as the transfer of oxygen to tissues.
What is measured in a biomonitoring study?
Biomonitoring may be used to assess the levels of any chemicals, natural or synthetic, that are or have been present in the environment. Natural chemicals that have been the subjects of biomonitoring include those found in the earth's crust, such as the elements lead and arsenic, as well as a large variety of more complex compounds that are part of the food that we eat, the water that we drink and the air that we breathe. These complex chemicals include substances that are a part of a number of plants and animals and that have biological activity, including natural pesticides, carcinogens, and hormonally active chemicals.
Synthetic, or manufactured chemicals, often the focus of public concern, include a large range of compounds that have been or are being manufactured for a variety of applications. Also included are substances that are produced as by-products of the synthesis and use of these compounds. In the modern world, synthetic chemicals are a part of every aspect of human life; they are critical to preventing and treating disease, to transportation, to agricultural production and to the many consumer products used for supporting the standard of living that we enjoy. Therefore, it is not surprising that many of these find their way into the soil, air, water and food and thus ultimately into the fluids and tissues of individuals.
How do chemicals enter the body?
Biomonitoring measures which substances humans have been exposed to and the relative amounts of exposure to those compounds. This technique detects both naturally occurring and synthetic substances that, for example, are part of the food that we eat, the water that we drink, the air that we breathe and the surroundings we contact with our skin.
Can very minute amounts of a chemical in the body be measured?
For both natural and synthetic chemicals, body levels measured by biomonitoring are generally very low, typically in the parts per million (ppm), parts per billion (ppb) or parts per trillion (ppt) range. If one ppm was expressed as time instead of as concentration, it would be the equivalent of 1 second in 11.5 days; similarly, 1 ppb would be equivalent to 1 second in 31.7 years, and 1 ppt would be equivalent to 1 second in almost 32,000 years.
One situation in which high levels of exposure may occur is from the ingestion of foods containing significant amounts of environmental chemicals. This may happen if there is bioaccumulation and biomagnification of the chemical as it is passed up the food chain from organism to organism, e.g., from very small creatures up to large fish. Higher levels of chemicals in food may also occur as a result of unusual environmental conditions, e.g., extreme climatic conditions. For example, certain fungi that make mycotoxins may be produced on grain in greatly increased amounts when the weather is unusually hot and damp for a long period of time.
What is meant by "body burden?"
The levels of natural and synthetic chemicals detected through biomonitoring are sometimes referred to as body burdens. This is a misleading term in that it suggests that the detection of a substance always means that it is causing adverse effects. However, biomonitoring only measures exposure; it does not provide information about toxicity or risk. Indeed, many natural chemicals, such as the metal manganese, found in tissues and fluids are essential for the proper functioning of the human body.
How is a biomonitoring study conducted and can any lab do it?
The process of biomonitoring involves three steps: (1) selecting who will be monitored, as well as when and where, (2) collecting tissue and/or fluid samples, and (3) deciding which chemicals to study and analyzing for those chemicals in the samples that are collected. This is a complex and expensive process, especially if the goal is to obtain results that reflect how body levels vary by age, sex, ethnic group, geographical location, and state of health of the individual. Biomonitoring also depends on the ability of analytical chemists to detect minute amounts of chemicals, an ability that has increased significantly in the past decade.
In recent years, however, the U.S. Centers for Disease Control and Prevention (CDC) has embarked on an ambitious program of monitoring. The actual analyses are based on commonly accepted laboratory techniques. However, they do generally require sophisticated analytical instruments and techniques, because it is only the application of very sensitive test methods that provides the opportunity to detect the very small amounts of many of the environmental chemicals found in humans. These special tests cannot be performed by the medical laboratories that routinely do the blood and urine analyses ordered by doctors.
Why are biomonitoring data important?
Perhaps the most important strength of biomonitoring is that it is the only technique that can provide a direct measure of the exposure of human individuals and populations. However, because it requires a great deal of resources, only a relatively small number of individuals and compounds can be monitored. This, in turn, limits the amount and applicability of the analytical data collected.
In the absence of biomonitoring, exposure assessments are performed indirectly - based on a combination of: (1) measurements of concentrations of chemicals in the environment, e.g., in soil, water, or food, and (2) estimates of human behaviors, e.g., food consumption or time spent at various activities. This indirect approach has several drawbacks. First, environmental analyses are typically limited in both space and time, i.e., concentrations are known for only limited numbers of places at a few times. Second, human behaviors are quite variable, and this adds significantly to the uncertainties in the calculations. Due to these limitations, it is difficult to know how accurately such indirect assessments reflect human exposures.
In addition to being a direct measure, biomonitoring has the advantage that it integrates, or adds together, exposures from multiple sources, e.g., air, water, and food, to provide a reflection of total exposure. Thus, it is a measure of total exposure by all routes and from all sources.
What do biomonitoring data say about human exposure?
Biomonitoring provides exposure information that can be used in a number of ways. These data help in understanding which chemicals are in the environment and the relative levels of each, how these levels change over time, and which sectors of the population may have unusually high exposures to particular compounds. As a result of this understanding, it may be possible to assess the effectiveness of steps taken to reduce exposures, to identify new research that is needed and to help physicians diagnose and treat patients who may have had unusually high exposures to particular substances.
What can be learned about risk to humans from the results of biomonitoring?
Whether a chemical found in the body poses any risk depends on two factors: (1) the magnitude, time course, and route (ingestion, inhalation or dermal contact) of the exposure, and (2) its toxicity, i.e., what, if any, adverse effects are associated with this type of exposure. Risks can occur if people are very highly exposed for a short time, less highly exposed for a long period of time, or exposed at lower levels to compounds with high toxicity. Thus, knowledge of both toxicity and the characteristics of the exposure is critical in assessing the possible risk.
Biomonitoring provides only one part of the data needed to assess risk -- it cannot be used as a surrogate for risk. Even if extensive toxicity data for a chemical are available, they are almost always in a form that is difficult to combine with biomonitoring-generated exposure values to assess risk. It is only in a small number of special cases, such as that of lead, that inferences about risk can be made readily from biomonitoring data.
Unfortunately, if individuals do not understand the limitations of biomonitoring in providing risk information, they may take steps to reduce exposures that increase rather than decrease their overall risk. A good example is mothers refraining from breast feeding when informed that certain chemicals have been found, or are likely to be found, in their breast milk. In almost all cases, the benefits of breast feeding outweigh any possible risks from these chemicals -- a conclusion that is reflected in the advice given to nursing mothers by public health authorities.
Should people be concerned when biomonitoring studies find new chemicals that have not been previously detected?
Scientists can now detect minute amounts of chemicals -- an ability that has increased significantly in the past decade. Body levels of environmental chemicals measured by biomonitoring are generally very low, typically in the parts per million (ppm), parts per billion (ppb) or parts per trillion (ppt) range.
With new biomonitoring techniques, a chemical that has previously been below levels of detection may now be found in trace amounts, but finding it does not necessarily mean that exposure is increasing.
As discussed above, the mere presence of a substance in the body, at any level, cannot be interpreted to mean that adverse effects are likely to occur. Knowledge of both toxicity and exposure is necessary for assessing possible risk.
Where is biomonitoring heading?
Biomonitoring of large numbers of chemicals in representative human populations is just beginning. In these early stages, the aim is to both assess the current levels of natural and synthetic chemicals in human tissues and fluids and to provide the data for decision-making about future research needs. The information that is being gathered will be used mainly to: (1) establish baseline and reference levels for environmental chemicals, (2) identify chemicals for which appropriate toxicological and environmental data are lacking, and (3) refine future biomonitoring efforts.