Biomedical subjects

Melissa C Friesen

Publications and source records attributed to Melissa C Friesen.

6 recordsLinked to original sources

PubMed2007

Impact of the specificity of the exposure metric on exposure-response relationships.

BACKGROUND: Exposure misclassification may occur when nonspecific exposure indicators are used. Developing estimates of more specific measures may be difficult due to sampling limitations or a paucity of historical measurements and, thus, often requires substantial effort. We examine the impact on exposure-response relationships of moving from 2 measures of exposure mixtures (dust, chlorophenols) to more specific exposure indicators (wood dust, pentachlorophenol, tetrachlorophenol) in a retrospective cohort. METHODS: The study population consisted of 26,847 male sawmill workers (> or =1 year employment between 1950 and 1995) with linkage to national cancer registries. A subcohort (n = 11,273 employed more than 1 day between 1985 and 1995) was linked to hospital discharge records. We evaluated the shape (log-linear vs log-log models), goodness of fit, precision, and expected versus observed attenuation of the exposure-response relationships. RESULTS: The correlation between the cumulative exposure indices was moderately high (dust/wood dust, r = 0.68; total chlorophenol/pentachlorophenol, r = 0.88; total chlorophenol/tetrachlorophenol, r = 0.78). An increase in chronic obstructive pulmonary disease hospitalizations was found with wood dust but not with total dust. Stronger associations for non-Hodgkin lymphoma and kidney cancer incidence were observed with pentachlorophenol than with total chlorophenol; no association was observed with tetrachlorophenol. We observed greater attenuation than expected using total dust, but less than expected using total chlorophenol. CONCLUSIONS: The relationships between health outcomes were substantially attenuated when nonspecific exposure indicators were used. This study demonstrates the importance of developing exposure metrics as specific to the disease-causing agent as possible, particularly when the composition of mixed exposures varies by work areas.

British Columbia↗

PubMed2006

Utilitarian bicycling: a multilevel analysis of climate and personal influences.

BACKGROUND: Increasing utilitarian bicycling in urban areas is a means to reduce air and noise pollution, increase physical activity, and reduce the risk of chronic diseases. We investigated the impact of individual- and city-level characteristics on bicycling in Canadian cities to inform transportation and public health policies. METHODS: The study population included 59,899 respondents to the 2003 Canadian Community Health Survey (CCHS) living in cities with populations greater than 50,000. In 2005, data on individual characteristics were drawn from the CCHS, and city-level climate data from Environment Canada records. Separate multilevel logistic regression models were developed for the general (nonstudent) and student populations. RESULTS: The proportion of the urban population reporting bicycling in a typical week was 7.9%, with students cycling more than nonstudents (17.2% vs 6.0%). In the general population, older age, female gender, lower education, and higher income were associated with lower likelihood of cycling. More days of precipitation per year and more days of freezing temperatures per year were both associated with lower levels of utilitarian cycling (odds ratios [ORs] for every 30-day increase in precipitation=0.84, 95% confidence interval [CI]=0.74-0.94, and for every 30-day increase in freezing temperatures OR=0.91, 95% CI=0.86-0.97). There was less variation in the proportion of students who cycled by age and income, and only the number of days with freezing temperatures influenced bicycling. CONCLUSIONS: Bicycling patterns are associated with individual demographic characteristics and the climate where one lives. This evidence might be useful to guide policy initiatives for targeted health promotion and transportation infrastructure.

Adolescent↗

PubMed2006

Comparison of two indices of exposure to polycyclic aromatic hydrocarbons in a retrospective aluminium smelter cohort.

BACKGROUND: The association between coal tar-derived substances, a complex mixture of polycyclic aromatic hydrocarbons, and cancer is well established. However, the specific aetiological agents are unknown. OBJECTIVE: To compare the dose-response relationships for two common measures of coal tar-derived substances, benzene-soluble material (BSM) and benzo(a)pyrene (BaP), and to evaluate which among these is more strongly related to the health outcomes. METHODS: The study population consisted of 6423 men with > or =3 years of work experience at an aluminium smelter (1954-97). Three health outcomes identified from national mortality and cancer databases were evaluated: incidence of bladder cancer (n = 90), incidence of lung cancer (n = 147) and mortality due to acute myocardial infarction (AMI, n = 184). The shape, magnitude and precision of the dose-response relationships and cumulative exposure levels for BSM and BaP were evaluated. Two model structures were assessed, where 1n(relative risk) increased with cumulative exposure (log-linear model) or with log-transformed cumulative exposure (log-log model). RESULTS: The BaP and BSM cumulative exposure metrics were highly correlated (r = 0.94). The increase in model precision using BaP over BSM was 14% for bladder cancer and 5% for lung cancer; no difference was observed for AMI. The log-linear BaP model provided the best fit for bladder cancer. The log-log dose-response models, where risk of disease plateaus at high exposure levels, were the best-fitting models for lung cancer and AMI. CONCLUSION: BaP and BSM were both strongly associated with bladder and lung cancer and modestly associated with AMI. Similar conclusions regarding the associations could be made regardless of the exposure metric.

Air Pollutants, Occupational↗

PubMed2006

Mixed models and empirical bayes estimation for retrospective exposure assessment of dust exposures in Canadian sawmills.

OBJECTIVES: Data on job histories is commonly available from study subjects and worksites, therefore jobs are often used for assigning exposures in historical epidemiological studies. Exposure estimates are often derived by offering jobs as fixed effects in statistical models. An alternative approach would be to offer job as a random effect to obtain empirical Bayes estimates of exposure. This approach is more efficient since it weights exposure estimates according to the within-job and between-job variability and the number of measurements for each job. We assess three models for predicting historical dust exposures of sawmill workers. METHODS: Models were developed using 407 inhalable dust measurements collected from 58 jobs in four sawmills. The first model incorporated all variables as fixed effects; the second added a random term to account for correlation within workers; and the third offered random terms for worker, job and mill (hierarchical model). Empirical Bayes estimates were used to calculate job-specific exposures from the hierarchical model. RESULTS: The fixed effects and random worker mixed models performed nearly identically because there was low within-worker correlation (r = 0.26). The Bayesian exposure predictions from the hierarchical model were slightly more correlated with the observed mill-job arithmetic means than those from the models where jobs were fixed effects (0.74 versus 0.70). CONCLUSIONS: While we observed no large differences in exposure estimates by treating job as a fixed or random effect, treating job as a random effect allowed for job-specific coefficients to be estimated for every job while borrowing strength in the presence of sparse data by assuming that the job means are normally distributed around the group mean. In addition, empirical Bayes job estimates can be used for a posteriori job grouping. The use of this method for retrospective exposure assessment should continue to be examined.

Air Pollutants, Occupational↗

PubMed2006

Cancer and occupational exposure to pentachlorophenol and tetrachlorophenol (Canada).

OBJECTIVE: The objective of this study is to assess the carcinogenicity of pentachlorophenol and tetrachlorophenol using data from the BC sawmill workers cohort study. METHODS: The cohort consisted of 27,464 men employed by 14 sawmills for 1 year or more between 1950 and 1995. Fatal (1950-1995) and incident (1969-1995) cancers were identified using national registries. Plant records and systematic interviews with senior employees were used to estimate dermal exposure. Comparisons were made with the general BC population and dose-response relationships were assessed using Poisson regression. RESULTS: There were 1,495 fatal cancer and 2,571 incident cancers. There were no large or statistically significant excesses of any of the specific cancers were observed compared to the general population. Internal analyses showed strong dose-response relationships for non-Hodgkin's lymphoma, multiple myeloma, and kidney cancer. These relationships were strongest when exposure was restricted to pentachlorophenol. The strength of the dose-response increased when exposure was lagged by 20 years. CONCLUSIONS: Dermal exposure to pentachlorophenol was associated with non-Hodgkin's lymphoma, multiple myeloma, and kidney cancer, but not with other cancers of a priori interest.

British Columbia↗

PubMed2005

Twenty-four-hour light exposure and melatonin levels among shift workers.

OBJECTIVES: We sought to measure melatonin levels and 24-hour light intensity exposure in health care workers over a 7-day period in natural occupational and residential settings. METHODS: Five office workers and 17 nurses working either days or rotating night and day shifts wore a device to record light intensity exposure for one or two 7-day periods, completed a questionnaire, and provided three saliva samples for melatonin. RESULTS: Rotating shift workers had irregular light exposure patterns and abnormal melatonin levels compared with those working days. In addition to lower-than-normal melatonin levels during sleep periods, rotating shift workers exhibited higher-than-normal melatonin levels on arising and during work. Self-reported years of shift work were correlated with measured melatonin and light. CONCLUSIONS: Rotating shift work is supported as a surrogate for exposure to light-at-night and circadian disruption.

Adult↗