Uses of human biomonitoring data in risk assessment 

Fact sheet series: Topics in risk assessment of substances under the Canadian Environmental Protection Act, 1999 (CEPA)

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• Human biomonitoring and substance exposure
• Human biomonitoring surveys
• Human health assessment and biomonitoring

Human biomonitoring and substance exposure

Biomonitoring is the measurement of chemical substances as parent compounds, their metabolites or their reaction products in human tissues and fluids. These measurements are usually taken from blood and urine and sometimes in other tissues and fluids, such as human milk. The measurements are often referred to as biomarkers of exposure. The purpose of human biomonitoring is to estimate chemical exposure levels within a population. The biomonitoring data represents exposure of the substance from multiple sources. This includes indoor and outdoor air, soil, dust, water, food or potential exposures from products used frequently by consumers, such as cosmetics and health products. Biomonitoring data also incorporates exposures from all routes (oral, dermal and inhalation). Although biomonitoring data may provide an estimate of overall exposure to a substance, its presence in the body does not necessarily mean that it is causing harm. In addition, absence of a substance measured in blood or urine, does not necessarily mean the individual was not exposed to a given substance.

Biomonitoring data, which is the concentration of a substance in the bodily fluid, does not identify the sources of exposure (source attribution) or how long a substance has been in the body. There are also limitations in identifying the duration of exposure, especially for substances with longer half-lives. Careful consideration of the study design, biomarker selection, toxicokinetics (that is, how a substance moves through the body through absorption, distribution, metabolism, and excretion), toxicology, and populations examined are critical when interpreting and communicating biomonitoring data. When interpreting biomonitoring results it is important to take into consideration background levels, complex metabolic interactions and/or difficulty correlating measured concentrations of multiple metabolites to exposures from a single substance.

Human biomonitoring surveys

In Canada, biomonitoring data in the general population is being collected on an ongoing basis since 2007 as part of Statistics Canada's Canadian Health Measures Survey (CHMS). Canadian biomonitoring data are also available in various subpopulation studies, such as the Maternal-Infant Research on Environmental Chemicals (MIREC) study, the Alberta Biomonitoring Program, the First Nations Biomonitoring Initiative (FNBI) and the Northern Contaminants Program (NCP). Other population studies, such as the United States National Health and Nutrition Examination Survey (NHANES), are also available.

Human health assessment and biomonitoring

Human health assessments often use predictive models or algorithms to estimate potential exposure of people in Canada to substances. Measurements such as the levels of a substance in the environment (air, water, dust) and the concentration of a substance found in products are used to feed into the models and algorithms. Various assumptions are applied when estimating substance exposure to the general or susceptible populations. These include assumptions about the route of exposure, intake rate, fraction that is absorbed or metabolised, and the potential sources of exposure. These assumptions may add uncertainty to the risk characterization, especially for substances that are used in many different ways. In some circumstances, the use of human biomonitoring data may provide a more accurate estimate of total exposure for risk characterization relative to use of predictive models. Further, biomonitoring data incorporate individual variability in exposure and toxicokinetics.

Human biomonitoring data can be used in human health assessments in a number of ways, including:

• for quantitative and/or qualitative exposure and risk characterization
• for tracking trends in how levels of substances are changing in the general population over time
• for identifying subpopulations with higher exposure levels, such as by sex or age
• for comparing estimates of exposure derived from predictive models (for example, dietary intakes derived using monitoring data in foods and dietary consumption rates)

As part of the amendments made to CEPA in June 2023, the role of the Minister of Health to conduct research and studies, including biomonitoring activities, in relation to the health effects of substances has been noted in the Act. This is to support the Government in protecting a right to a healthy environment and may also relate to vulnerable populations (as defined in CEPA). The use of human biomonitoring data in an assessment is feasible if it is determined that a biomarker of exposure is specific and sensitive, such that concentrations measured reflect exposure to the substance of interest. In addition, population-level biomonitoring data generated from outside Canada may also be used. These data would be evaluated on a case-by-case basis.

Human biomonitoring data can be interpreted in health assessments using both qualitative and quantitative approaches, which are explained below.

Qualitative approaches have been used in the existing substances risk assessment program for substances which are detected at very low frequencies in Canadian biomonitoring studies and are, accordingly, considered to be of low concern to human health. This approach was published in the biomonitoring-based approach 1 science approach document.

When biomonitoring data indicate that the general population has potential for exposure, a quantitative approach may be applied in assessment of substances. Quantitative approaches include direct comparison, reverse dosimetry and forward dosimetry, which are explained below.

In direct comparison, if the concentration found in a specific medium (for example, blood or urine) associated with a critical health effect is known, then concentrations reported in human biomonitoring studies can be compared directly to levels associated with that critical health effect. Generally, this approach is used less frequently due to its extensive need for data.

In reverse dosimetry (or exposure reconstruction), biomonitoring data, for example measured in mg/L of a substance or mg/g creatinine, are converted to external exposure levels (for example, mg/kg/day). The triclosan assessment provides details of the application of reverse dosimetry for the external exposure estimates using biomonitoring data.

In forward dosimetry, biomonitoring data are compared with a human biomonitoring health-based guidance value, such as a biomonitoring equivalent (BE). A BE is defined as the concentration (or range of concentrations) of a substance or its metabolites in a biological medium (blood, urine, or other medium) that is consistent, or directly related, with an existing health-based exposure guidance value, such as a reference dose (RfD) or tolerable daily intake (TDI)^{Footnote 1}. This quantitative approach may be applicable when there is widespread presence of the substance in the general population and allows a comparison of the biomonitoring data (as a surrogate for estimates of exposure) against effect levels represented by a BE or other human biomonitoring health-based guidance values. The human biomonitoring health-based guidance values (for example, BEs) in forward dosimetry can be derived using toxicokinetic models (models that simulate substance behaviour after it is incorporated into the human body) or correlation between the concentration of the substance in a biological matrix and daily intake levels. For example, the Government of Canada's risk assessment of Silver and its Compounds used a toxicokinetic model to derive whole blood BE values associated with health effects. Similarly, in the assessment of Selenium and its Compounds, a correlation between selenium whole blood concentrations and dietary intake levels was used to derive whole blood BE. The simple mass balance approach, which is based on the fraction of target substance excreted from the body and urinary flow rate (for example, L/Kg bw/day), is the common method used for the derivation of urinary human biomonitoring health-based guidance values (for example, urinary BE derivation in Zinc and its Compounds assessment). Details of the forward dosimetry method can be found in the biomonitoring-based approach 2 science approach document.

In addition to use in assessments, tracking biomonitoring data over time can also be an important tool in substance-based performance measurement of the effectiveness of risk management actions taken on toxic substances. Risk management measures that decrease the levels of substances in the general population, as shown through human biomonitoring, would be seen as highly effective. For example, through risk management actions already in place, the levels of lead in the blood of people in Canada have declined over 70% since the 1970s.

Advances in the field of analytical chemistry have improved the availability of human biomonitoring data with over 500 substances being monitored in various human biological matrices.