Biomonitoring

Bio-monitoring is a scientific technique which allows researchers to determine concentrations of natural and synthetic compounds present in human tissues such as blood, urine, breast milk or hair. This is an incredibly useful technique because it takes much of the guesswork out of assessing how much of a person’s environmental exposure actually gets into the body and can confirm that an exposure to a certain chemical has taken place.

Bio-monitoring can measure the presence of a chemical directly, or it can measure the presence of its various metabolites – that is, the presence of chemicals formed as a result of the breakdown of the original compound. For example, often p,p-DDE (1,1'-(2,2-dichloro-ethenylidene)-bis[4-chlorobenzene]) is used as a biomarker for DDT (dichlorodiphenyltrichloroethane) exposure. In other cases, a measurable change in cellular or biological activity may be used as a biomarker for exposure. An example of this would be increased enzyme levels within a person’s body due to exposure to poly-aromatic hydrocarbons (PAH’s) or production of the protective protein metallothioein after exposure to cadmium.

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) has illustrated the usefulness of such monitoring through the National Report on Human Exposure to Environmental Contaminants; http://www.cdc.gov/exposurereport/. The CDC has undertaken the endeavor 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, bio-monitoring data is limited in that it can not determine the source of an exposure, how long it has been in the body or what effect the exposure may have on human health.        

Bio-monitoring 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, bio-monitoring 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, dibenzodioxins, 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 bio-monitoring difficult since measurements may not be indicative of a person’s exposure a week ago or even yesterday. 

Interpreting the results from bio-monitoring 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 part 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 a large number of chemicals in their bodies.

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). This study aims to collect health related information through both direct physical measurements and household questionnaires for approximately 5071 participants between winter 2007 through to winter 2009.  Combining information collected from the household questionnaire related to nutrition, smoking habits, alcohol use, medical history, current health status, sexual behavior, lifestyle and physical activity, the environment and housing characteristics, as well as demographic and socioeconomic variables, with direct measurements such as blood pressure, height, weight, and  biomarkers for environmental contaminants, this study will provide valuable information that can be used to create national baseline data on a variety of health conditions as well as exposure to infectious disease and environmental contaminants.  By targeting individuals between the ages of 6 and 79 living in privately occupied dwellings in the ten provinces and the three territories, approximately 97% of the Canadian population will be represented by this study.

Bio-monitoring is an important step to understanding 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 vise versa. For example, an individual may be defined as occupationally exposed given that their job title, however, not all individuals with that job title may be actually be exposed or they may use protective equipment that significantly reduce their exposures. Because bio-monitoring 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 exists with bio-monitoring data, it is a much more accurate method than other methods of estimating exposure.  

Exposure assessment is an important component of risk assessment that when  combined with other inputs such as hazard identification, dose-response assessment, and risk characterization, allows scientists to quantify the probability of a harmful effect to individuals or populations from an contaminant of interest. Although, bio-monitoring does not yield a complete picture of risk, it is a very informative and useful step to reducing uncertainty and in turn better understanding the health effects of chemicals in our environment.