Defining Pesticide Biomarkers
Biomarkers are measurable substances or characteristics in the human body that can be used to monitor the presence of a chemical in the body, biological responses or adverse health effects. The use of biomarkers will help us evaluate potential exposures to pesticides as well as predict effects that may result, allowing us to make decisions that are more protective of human health. Biomarkers are commonly grouped into biomarkers of exposure, effect and susceptibility. This Web page describes these groups of biomarkers and provides examples.
- Biomarkers of exposure
- Biomarkers of exposure categories
- Biomarkers of effect
- Biomarkers of effect categories
- Biomarkers of susceptibility
Biomarkers of exposure are used to assess the amount of a chemical that is present within the body. Many chemicals can be measured in urine, blood, saliva, and, if they are fat soluble, in body fat and breast milk (e.g., DDT). Biomarkers of exposure provide information on
- chemical exposures in individuals
- changes in levels over time; and
- variability among different populations.
They may also provide information on the relative importance of different exposure pathways and associated risk. It is important to note that the measurement of a chemical in someone’s body does not by itself mean that chemical has caused adverse health effects.
Additionally, there are a number of uses related to the interpretation of biomarkers of exposure. For example, the measurement of 3-phenoxybenzioc acid (3-PBA) in urine is considered a non-specific biomarker of exposure because 3-PBA is a common metabolite of several pyrethroid pesticides. Therefore, additional information is needed to resolve which pyrethroid was the parent chemical.
- Chemical –The most specific exposure biomarker is direct measurement of the chemical of interest in the body. Typically, measurement of the chemical is made in an accessible biological matrix (e.g., blood, urine). While some pesticides can be directly measured in the body, it is generally more common to measure metabolites of pesticides.
- Metabolite – Many chemicals are rapidly metabolized or difficult to measure. In these cases, a more stable breakdown product (metabolite) of the chemical may be measured to estimate exposure to the chemical. When a metabolite may derive from a number of different chemicals (as in the 3-PBA example above), additional information is needed to resolve to which chemical the person was exposed.
- Endogenous surrogate – In some cases, a chemical or class of chemicals may result in an endogenous response (response within the body) that is highly characteristic of that chemical or class. Measures of that response can be used as a surrogate in lieu of direct measurement of the chemical or metabolite concentration when sufficient additional information is available. Since there are many factors that can influence endogenous responses, this type of exposure biomarker is accompanied by many uncertainties that should be identified and discussed.
See examples in Table 1
Biomarkers of effect are indicators of a change in biologic function in response to a chemical exposure1. Thus, they more directly relate to insight into the potential for adverse health effects compared with biomarkers of exposure.
One example of a biomarker of effect is blood cholinesterase, which can become depressed following exposure to organophosphate and N-methyl carbamate pesticides. Measuring cholinesterase levels can be a useful tool for monitoring agricultural workers and identifying workers that may potentially be overexposed to pesticides.
Bioindicator – An ideal biomarker of effect has an explicitly known mechanism that links the marker and an adverse outcome. In most cases, this is achieved by a sufficient understanding of the adverse outcome pathway or mode of action of the chemical, and the causal or correlative relationship of biological events between the marker and the adverse outcome.
Bioindicators provide a high degree of confidence in predicting the potential for adverse effects in an individual or population based on marker levels. An understanding of the adverse outcome pathway also supports development of a variety of bioindicators for different key events or outcomes of interest (e.g., markers for precursor events leading to a clinically detectable adverse outcome to support early detection and prevention).
When cellular or molecular initiating events can be identified as critical steps in an adverse outcome pathway, bioindicators can be developed in conjunction with high throughput screening assays to provide a rapid and efficient means for early detection of adverse outcomes in target population. EPA researchers are actively developing this class of biomarkers of effects in support of Toxicity Testing in the 21st Century.
Undetermined consequence – This subgroup of biomarkers provides more limited and uncertain indication of the potential for adverse effects, because the events or deterministic linkages in an adverse outcome pathway are less well known. An example would be markers of oxidative stress where elevations have been associated with a variety of adverse outcomes, but the explicit relationships have yet to be defined. As the role of oxidative stress in different disease processes (and adverse outcome pathways) becomes more clearly defined, there will be increasing certainty in the use of oxidative stress biomarkers to predict the potential for organism/population-level effects. Meanwhile, these biomarkers can be used in conjunction with other biomarkers in this or other subgroups to improve the specificity and sensitivity of the overall set of markers for development of an adverse outcome.
Exogenous surrogate – Some chemicals have well known adverse effects, which are accompanied by other effects that can be used as surrogate indicators of the main adverse effect of interest. A common example is paranitrophenol, a metabolite of methyl parathion. Measurement in humans of paranitrophenol in the urine has been used as an exogenous surrogate biomarker of exposure to methyl parathion and as an indicator of the potential for toxicity due to methyl parathion-induced acetylcholinesterase inhibition.
Exogenous surrogate biomarkers are suboptimal as effects markers because they do not directly capture the contribution of additional factors (intrinsic and extrinsic) that may influence the incidence or severity of an adverse outcome. Given these limitations, use of exogenous surrogates as biomarkers is mostly limited to measurement of those effects that are predominantly due to the chemical of interest (i.e., to reduce the number of potentially confounding effects and to decrease uncertainty associated with the measured surrogate biomarker).
See examples in Table 2
Biomarkers of susceptibility are factors that may make certain individuals more sensitive to chemical exposure. Biomarkers of susceptibility include:
- Genetic factors that may influence how the body interacts with a chemical.
- Other biological factors related to nutritional status, health status, lifestyle, and life stage that may affect an individual’s susceptibility to chemical exposure.
3-PBA: 3-phenoxybenzioc acid
DiPap: Polyfluoroalkyl phosphate ester
PFOA: Perfluorooctanoic acid
T4: thyroxine (a thyroid hormone)