Riskcom - Framework - Health Risk Science

Recent advances in analytical chemistry allow us to better detect both natural and synthetic compounds in human tissues such as blood, urine, breast milk or hair through an advanced technology called biomonitoring. Results of biomonitoring studies are now broadly available and are more widely used for many applications other than occupational exposures, but the quality of the results, the design, and objectives have been questioned by scientists, policy makers and the public. Thoughts are that our interpretation of biomonitoring data as been surpassed by our technical ability to detect very low levels of chemicals in the human body.

The Centre for Disease Control and Prevention (CDC) in the United States defines biomonitoring as one method for assessing human exposure to chemicals by measuring the chemicals¹ or their metabolites² in human tissues or specimens, such as blood or urine (CDC 2005). This promising public health tool can help us better understand human exposure to a wide range of substances. Biomonitoring is incredibly useful because chemicals that have entered the human body can be measured in any events along the exposure-disease continuum through the use of different types of biomarkers; exposure, susceptibility, effect and early disease. The U.S. National Research Council has developed a framework for the use and selection of different types of biomarkers. This framework is helpful in understanding the strengths and limitations of various biomarkers based on the scientific knowledge we have about them http://www.nap.edu/catalog/11700.html. Biomonitoring can be applied in various ways, including evaluation of the presence of chemicals in body tissues, and to track spatial variation and temporal trends in chemical exposure. Biomonitoring may also be helpful in identifying at-risk populations.

By measuring these compounds, scientists can gain invaluable information about individual exposures at a specific point in time. This information can then be used to determine current exposure levels within the general population; make comparisons in exposures among subgroups; and measure trends in exposure over time. A large ongoing study by the U.S. Centers for Disease Control (CDC)- the National Health and Nutrition Examination Survey (NHANES)- gave the first evidence that Americans had too much lead in their blood. Results helped policy makers to phase out the use of lead as an additive in gasoline. CDC is also monitoring through the National Report on Human Exposure to Environmental Contaminants and released their third report in July 2005. The CDC has undertaken the endeavour of measuring over 148 chemicals in approximately 5000 individuals per year. These chemicals range from a variety of metals, to polycyclic aromatic hydrocarbons (PAH’s), polychlorinated biphenyls (PCB’s), phthalates, phytoestrogens, to a variety of pesticides and herbicides. By measuring a wide range of environmental chemicals in the civilian population, they have been able to report differences based on age, gender and ethnicity as well as track decreases in chemicals such as lead and cotinine (a marker of tobacco smoke exposure) following public health intervention strategies. Unfortunately, biomonitoring data is limited in that it can not determine the source and route of an exposure, how long it has been in the body or what effect the exposure may have on human health. Also, of the 148 chemicals measured in the latest 2005 CDC report, only 25 of them have established EPA reference values, reference concentrations, reference doses, or cancer potency factors.

Concerns over results of biomonitoring studies pressed the U.S. Congress to request that a committee on human biomonitoring be established and recommend how future research can better assess how chemicals in the environment affect human health. The National Academy of Science (NAS) Committee on Human Biomonitoring for Environmental Toxicants of the U.S. National Research Council assembled experts on a committee which later released in 2006 a report on Human Biomonitoring of Environmental Chemicals http://books.nap.edu/catalog.php?record_id=11700. The committee’s role was to review current practices and make recommendations including; how to design biomonitoring studies, to improve interpretation and uses of human biomonitoring data, and communicate results. A 2006 workshop on “Understand Human Biomonitoring” http://www.mclaughlincentre.ca/events/biomonitoring_WS.shtml was held at the University of Ottawa to increase the understanding of human biomonitoring and the importance of robust study designs, scientifically sound interpretations of results and effective communication of new results.

Biomonitoring programs often rely on one or few measurements/person and as a result can only provide a ‘snapshot’ of a person’s exposure. Because of this, biomonitoring is best at detecting persistent chemicals. Persistent chemicals are chemicals that are either fat-soluble or those that can bind to various proteins in the body. Fat-soluble chemicals accumulate in fat and equilibrate with blood serum over time, and as a result, can persist in the body for months or years. These chemicals include PCB’s, dibenzodioxis, dibenzofurans, and polybrominated diphenyl ethyers (PBDEs), and are of particular concern because of their ability to be passed onto infants through breast milk. Non-persistent chemicals on the other hand often leave the body very quickly, as either they or their metabolites are water-soluble and can be directly excreted in urine. The transient nature of non-persistent chemicals can make accurate assessment of exposure from biomonitoring difficult since measurements may not be indicative of a person’s exposure a week ago or even today.

Interpreting the results from biomonitoring is often problematic since the presence of a chemical does not necessarily indicate an increased risk of adverse health effects. With the advancement of technology over past years, scientists now have the ability to measure contaminants in very low levels from human samples- often in parts per million, parts per billion or parts per trillion. To put this into context, one pert per billion (ppb) is the equivalent of approximately one drop of food dye in 16,000 gallons of water. In today’s society, everyone is exposed to chemicals in daily life. Chemicals have allowed us to have access to safe drinking water and effective medical treatments. Just because a chemical is found in someone’s body does not mean that it is unsafe, in fact, most people have detectable levels of a large number of chemicals in their body. The concentrations of chemicals are more important to determine the relation with a specific disease than only the detection of a chemical. It is critical that biomonitoring study designs be carefully constructed in order to ensure that informative biomonitoring results be obtained.

To better understand the levels of environmental contaminants found in Canadians, Health Canada and the Public Health Agency of Canada have supported Statistics Canada’s in achieving funding for the Canadian Health Measures Survey (CHMS) http://www.statcan.ca/english/about/pia/canadian.htm. The objectives of the CHMS biomonitoring component are to establish current population-representative levels for a range of environmental chemicals, to provide a baseline for assessing emerging trends, and to allow comparisons of data from other sub-populations and geographic regions in Canada, and with other countries. This study starting in the winter 2007 aims to collect health related information through both direct physical measurements and household questionnaires for approximately 5,000 participants aged between 6 and 79 years of age living in privately occupied dwellings representing 97% of Canadians. Results of the survey are expected to be released in early 2010.

Biomonitoring is an important step to understand the risks posed by environmental chemicals. It constitutes a very effective method of exposure assessment, and as a result has the potential to reduce exposure misclassification in epidemiological studies. Exposure misclassification occurs when an exposed individual is classified as unexposed or vice versa. For example, an individual may be defined as occupationally exposed given their job title However, not all individuals with that job title may actually be exposed or they may use protective equipment that significantly reduce their exposures. Because biomonitoring uses direct measurements from body tissues, it confirms that an individual has in fact been exposed to the contaminant of interest, and that the contaminant has penetrated the absorption barrier. Although some sources of error still exist with biomonitoring data, it is a much more accurate method than other methods of estimating exposure. Human biomonitoring is a tool with great potential to contribute to our understanding of human exposure to environmental substances.

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¹Chemical refers to a chemical compound or element present in air, food, soil, dust or other environmental medium (CDC 2003).

² Metabolite is a chemical alteration, produced by body tissues of the original compound (CDC 2005)