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Estimates are based on data from the Canadian Health Measures Survey (CHMS). During an in-home interview, demographic and socioeconomic data and information about lifestyle, medical history, current health status, the environment, and housing characteristics are collected. At a mobile clinic, physical measurements, such as blood pressure, height, weight and physical fitness are assessed. Blood and urine samples are taken to test for infectious diseases, nutritional status, cardiovascular disease risk factors, and levels of environmental chemicals, including heavy metals.

The CHMS covers the population aged 6 to 79 living in private households at the time of the interview. Residents of Indian Reserves or Crown lands, institutions and certain remote regions and full-time members of the Canadian Forces are excluded. Approximately 97% of Canadians are represented. Data are collected from 15 sites across Canada from March, 2007 through March, 2009. Ethics approval was obtained from Health Canada's Research Ethics Board. Informed written consent was obtained from respondents older than 14. For younger children, a parent or legal guardian provided written consent, and the children provided written assent. Participation is voluntary; participants can opt out of any part of the survey at any time. Additional information about the CHMS is available in print and online.2-7 

The data in this article pertain to 2,678 participants from the first 8 data collection sites. Blood samples were drawn at the mobile clinic by a certified phlebotomist into 6 mL EDTA Becton Dickinson Vacutainer BD367863. Samples were stored in the mobile clinic freezer at -20°C, and once a week, were shipped on ice packs to the reference laboratory, the Centre de toxicologie du Québec (CTQ) of L'Institut national de santé publique du Québec (INSPQ). Blood samples were diluted in a basic solution containing octylphenol ethoxylate and ammonia, and analyzed for lead, total mercury and cadmium using inductively coupled plasma mass spectrometry (ICP-MS, DRC). INSPQ compiled, encrypted and electronically transmitted the results to Statistics Canada's Head Office. The laboratory is accredited under ISO 17025 and uses numerous external quality control programs, including the German External Quality Assurance Scheme (EQAS) and Lead and Multi-element Proficiency Testing (LAMP). Periodically, blind replicates and commercial controls are tested to monitor the precision of the assay.

For each heavy metal, the proportion above the limit of detection (LOD), proportion equal to or above the guidance value (where guidance values exist), geometric mean, selected percentiles, and 95% confidence intervals for the geometric mean and percentiles were calculated. The LOD is the level at which the blood concentration was so low that it could not be reliably or accurately determined by the laboratory test method. For lead, the LOD was 0.02µg/L; for mercury, 0.10µg/L; and for cadmium, 0.04µg/L. To calculate the geometric mean and percentiles, blood concentrations less than the LOD were assigned a value equal to half the LOD.8-10

Geometric means were calculated by taking the log of each individual result, calculating the mean of those log values, and then taking the antilog of that mean. Compared with the arithmetic mean, the geometric mean is less influenced by high values, and therefore, provides a better estimate of central tendency for data that are distributed with a long tail at the upper end of the distribution—a common distribution when measuring environmental chemicals in blood.

The use of preliminary data limited the analyses that were possible for this article. Because data were available for only 8 of the 15 collection sites, the sample size was too small and/or the variability was too high to calculate statistically reliable estimates of the percentile distribution for mercury and selected percentiles for lead and cadmium, or to perform analyses by age groups, sex and smoking status. These analyses and others will be possible upon survey completion. For example, previous research suggests that heavy metal exposure is unlikely to be equal across the population; lead exposure varies by socioeconomic status,11, 12 and cadmium exposure13-15 varies by smoking status. Future CHMS studies will be able to explore issues such as these.

Statistical analyses were based on weighted data. To account for survey design effects, standard errors, coefficients of variation and 95% confidence intervals were estimated using the bootstrap technique.16-18