Assessment of selection bias in the Canadian case-control study of residential magnetic field exposure and childhood leukemia

The authors evaluated the role of selection bias in the 1999 Canadian case-control study of residential magnetic field exposure and childhood leukemia. They included cases, participating controls, and first-choice nonparticipating controls in their analyses. Exposure was assessed by wire coding, a classification system based on the distribution line characteristics near homes. Although an imperfect measure of magnetic field exposure, wire coding is the only method applicable to nonparticipating subjects. First-choice nonparticipant controls tended to be of lower socioeconomic status than their replacements (non-first-choice participant controls), and lower socioeconomic status was related to higher wire code categories. The odds ratios for developing childhood leukemia in the highest exposure category were 1.6 (95% confidence interval: 1.0, 2.6) when the actual participating controls were used and 1.3 (95% confidence interval: 0.8, 2.1) when the first-choice ideal controls were used, regardless of their participation. Overall, the authors conclude that, although there is some evidence for control selection or participation bias in the Canadian study, it is unlikely to explain entirely the observed association between magnetic field exposure and childhood leukemia. Inherent problems in exposure assessment for nonparticipating subjects, however, limit the interpretations of these results, and the role of selection bias cannot entirely be dismissed on the basis of these results alone.     

Occupational magnetic field exposure among women in Stockholm County, Sweden

Background: Most epidemiological studies on adverse health effects among women in relation to occupational magnetic field exposure have been based on information about men's exposure.

Aims: To create a job-exposure matrix for occupational exposure to extremely low frequency magnetic fields among women.

Methods and Results: Measurements were performed using personal magnetic field meters (Emdex Lite) carried by the subjects for 24 hours on a normal workday. Subjects were volunteer women working in the occupations identified as common among women in Stockholm County based on the 1980 census. A total of 471 measurements were made in 49 different occupations, with a minimum of 5 and a maximum of 24 measurements in each occupation. The included occupations cover about 85% of the female population gainfully employed in 1980. Parameters representing average and peak magnetic field exposures, temporal change in the exposure, and proportion of time spent above certain exposure levels were calculated both for the workday and for the total 24 hour period grouped by occupational titles. The occupations with higher than average exposure were cashiers, working proprietors in retail trade, air stewardesses, dental nurses, cooks, post-office clerks and kitchen maids.

Conclusions: This new job-exposure matrix substantially increases the knowledge about magnetic field exposure among women and can be used for exposure assessment in future studies.

     

Extremely low-frequency magnetic fields and childhood acute lymphoblastic leukemia: an exploratory analysis of alternative exposure metrics

Data collected by the National Cancer Institute-Children's Cancer Group were utilized to explore various metrics of magnetic field levels and risk of acute lymphoblastic leukemia (ALL) in children. Cases were aged 0-14 years, were diagnosed with ALL during 1989-1993, were registered with the Children's Cancer Group, and resided in one home for at least 70 percent of the 5 years immediately prior to diagnosis. Controls were identified by using random digit dialing and met the same residential requirements. With 30-second ("spot") measurements and components of the 24-hour measurement obtained in the subject's bedroom, metrics evaluated included measures of central tendency, peak exposures, threshold values, and measures of short-term temporal variability. Measures of central tendency and the threshold measures showed good-to-high correlation, but these metrics correlated less well with the others. Small increases in risk (ranging from 1.02 to 1.69 for subjects in the highest exposure category) were associated with some measures of central tendency, but peak exposures, threshold values, measures of short-term variability, and spot measurements demonstrated little association with risk of childhood ALL. In general, risk estimates were slightly higher for the nighttime (10 p.m.-6 a.m.) interval than for the corresponding 24-hour period.     

60 hertz magnetic field exposure assessment for an investigation of leukemia in telephone lineworkers

The purpose of this paper is to present the assessment of magnetic field exposure conducted as a part of a nested case-control investigation of leukemia mortality in telephone lineworkers. For the purposes of exposure classification, telephone company jobs were initially divided into two classes: those with potential for working in an electric environment, referred to as linework jobs, and those not working in an electric environment, referred to as nonlinework jobs. Linework jobs were further divided into the following four categories: outside plant technicians (OPT), installation/maintenance/repair (IMR) technicians, central office technicians (COT), and cable splicing technicians (CST). These job groupings were based on similarity of work tasks and exposure environments. Emdex data-logging dosimeters were used to measure personal exposures to ELF magnetic fields for 204 telephone company workers. Three general classes of exposure indices were calculated for each exposure record: measures of central tendency, measures of peak or maximum exposure, and measures of exposure variability. CSTs had the highest full-shift mean and median exposure, 4.3 and 3.2 mG, respectively. CSTs also ranked the highest, with average peak, average 95th percentile, and average time above background equal to 99.2 mG, 11.1 mG, and 156 min, respectively. In addition, the results suggest the OPT and IMR technicians have exposures similar to nonlineworkers. Exposure classifications, therefore, which misclassify all lineworkers into one “telephone lineworker” job grouping are not appropriate and future studies should concentrate on cable splicing technicians.     

Refinements in magnetic field exposure assignment for a case-cohort study of electrical utility workers

This study examined the effect of refinements in exposure assignment on annual and career exposure to 60 Hz magnetic fields, using all deaths from brain cancer (145) and leukemia (164) and a random sample of 800 workers from a cohort of 138,905 men. Reassessment of 1060 job titles in the measurement database generated 20 subcategories in addition to 28 occupational categories used in the original cohort mortality study. Furthermore, previously misclassified jobs were corrected. The complete work history of each sub-cohort member was re-examined. Original and refined average annual exposures were 0.086 and 0.088 microT, respectively. The average career cumulative exposures were 1.40 and 1.44 microT-years, respectively. Spearman correlation coefficients between the original and refined methods across the companies were 0.81 for annual exposure and 0.93 for career cumulative exposure. 23% of the workers were assigned to another exposure ranking after refinement, but 85% of these moved to an adjacent group, suggesting that the differences in exposure ranking are small. The results of this study indicate that refinements have modest influence on the average annual and career exposures. However, the refinements may only change a very rough exposure assessment into one that is slightly less crude. The proportion of workers assigned to another exposure ranking indicated that nondifferential exposure misclassification in the original cohort mortality study may have occurred. Implications of these changes for the risk estimates of brain cancer and leukemia cases will to be examined.     

Lymphoma and multiple myeloma mortality in relation to magnetic field exposure among electric utility workers

Associations between occupational magnetic field exposure and non-Hodgkin's lymphoma (NHL), Hodgkin's disease, and multiple myeloma mortality were evaluated in 138,905 electrical utility workers. A job-exposure matrix based on measured magnetic fields was used to derive individual exposure estimates. There was a small positive association between all NHL and low-grade NHL and duration of employment in any magnetic field-exposed job, but only up to 20 years. Cumulative magnetic field exposure was associated with a rising, then falling, risk of NHL. Rate ratios for intermediate/high-grade lymphoma were increased for the highest levels of lifetime cumulative exposure (RR = 3.7 and RR = 2.3), and were most pronounced for those exposures occurring 10–20 years previously. Hodgkin's disease and multiple myeloma mortality did not appear to be associated with exposure. Associations were stronger for NHL subgroups, suggesting that further analysis by subgroup may be warranted, but the absence of dose-response gradients diminishes the likelihood that associations represent causal relationships. Am. J. Ind. Med. 32:392–402, 1997. © 1997 Wiley-Liss, Inc.     

Physical activity and magnetic field exposure in pregnancy

BACKGROUND: Peak magnetic field exposure was associated with increased risk of miscarriage in 2 recent studies. Reduced physical activity levels in healthy pregnancies may affect measured exposure and thus bias results. METHODS: We recruited 100 pregnant women to wear an Actigraph accelerometer and EMDEX magnetic field monitor for a 7-day period. We evaluated the association between physical activity and magnetic field exposure (peaks and time-weighted average) using generalized estimating equations and linear mixed models. RESULTS: We found a positive association between level of activity and likelihood of incurring elevated exposure in the person-day analysis, most strongly for cutpoints of 16 or 20 mG, for both working and nonworking women among whom odds ratios in the uppermost quartile ranged from 2.1 to 2.6. A positive association was found using person-minutes only among nonworking women. CONCLUSION: Physical activity may affect peak magnetic field exposure. If the early nausea and later cumbersomeness of healthy pregnancies leads to reduced physical activity, this could distort measured magnetic field-health outcome associations.     

Comparison of task-based exposure metrics for an epidemiologic study of isocyanate inhalation exposures among autobody shop workers

Because many occupational epidemiologic studies use exposure surrogates rather than quantitative exposure metrics, the UMass Lowell and Yale study of autobody shop workers provided an opportunity to evaluate the relative utility of surrogates and quantitative exposure metrics in an exposure response analysis of cross-week change in respiratory function. A task-based exposure assessment was used to develop several metrics of inhalation exposure to isocyanates. The metrics included the surrogates, job title, counts of spray painting events during the day, counts of spray and bystander exposure events, and a quantitative exposure metric that incorporated exposure determinant models based on task sampling and a personal workplace protection factor for respirator use, combined with a daily task checklist. The result of the quantitative exposure algorithm was an estimate of the daily time-weighted average respirator-corrected total NCO exposure (microg/m(3)). In general, these four metrics were found to be variable in agreement using measures such as weighted kappa and Spearman correlation. A logistic model for 10% drop in FEV(1) from Monday morning to Thursday morning was used to evaluate the utility of each exposure metric. The quantitative exposure metric was the most favorable, producing the best model fit, as well as the greatest strength and magnitude of association. This finding supports the reports of others that reducing exposure misclassification can improve risk estimates that otherwise would be biased toward the null. Although detailed and quantitative exposure assessment can be more time consuming and costly, it can improve exposure-disease evaluations and is more useful for risk assessment purposes. The task-based exposure modeling method successfully produced estimates of daily time-weighted average exposures in the complex and changing autobody shop work environment. The ambient TWA exposures of all of the office workers and technicians and 57% of the painters were found to be below the current U.K. Health and Safety Executive occupational exposure limit (OEL) for total NCO of 20 microg/m(3). When respirator use was incorporated, all personal daily exposures were below the U.K. OEL.     

Evaluation of potential confounders in planning a study of occupational magnetic field exposure and female breast cancer.

We examined potential confounding factors that, if unaccounted for, could possibly produce a spurious association in a study of breast cancer among women occupationally exposed to magnetic fields. For each risk factor, we estimated strength of association, prevalence in the general population, and prevalence of the risk factor in the exposed group required to explain completely hypothetical odds ratios between occupational exposure to magnetic fields and breast cancer. We performed similar analyses for two, three, four, and five confounding factors acting simultaneously. Factors numerically capable of substantial confounding included obesity, continent of birth, family history of breast cancer in a first-degree relative, densities on the mammogram, benign proliferative breast disease, history of cancer in one breast, and consumption of at least two alcoholic drinks per day. Nevertheless, only continent of birth, history of cancer, obesity, and consumption of alcohol could potentially be related to occupation. Uncontrolled confounders, either alone or in combination, could possibly account for odds ratios in the 1.2-1.3 range but were very unlikely to produce an odds ratio of more than 1.5. A spurious negative association between magnetic fields and breast cancer could occur if the exposed group included a large number of immigrants from Asia and Africa.     

Smoking exposure among school children in Brazil

The article describes the prevalence of tobacco exposure among adolescents at the National Adolescent School-based Health Survey (PeNSE) and investigates socio-demographic and behavioral factors associated with smoking. The profile of a current smoker was defined as reporting having smoked at least one cigarette in the previous 30 days. The socio-demographic characteristics studied were age, sex, race/skin color, mother education, household assets index and school (public or private). Risk and protective behaviors included were alcohol and drug experimentation, sexual intercourse, consumption of at least one glass of alcohol in the past 30 days and perform or be willing to perform physical activity on most days of the weeks. Prevalence of current smoker was 6.3% (95%CI:5,87-6,74) and was positively associated with older age, lower education of the mother, brown skin color, study in public school and presence of all the risk and protective behaviors studied. On the multivariate analysis, smoking remained associated with age and risk behaviors. To perform or be willing to perform physical activity were inversely related to smoking. The coexistence of risk behaviors is also present in adolescence, suggesting that health promotion policies to the adolescence might have a broader impact, including on tobacco use.     

Residential magnetic field exposure and childhood brain cancer: a meta-analysis

BACKGROUND: We conducted a meta-analysis of studies on magnetic field exposure and childhood brain tumors to evaluate homogeneity in the results, to examine reasons for heterogeneity, and to derive a summary effect estimate. Comparison of results from studies of childhood brain cancer and childhood leukemia may also help to assess the potential for selection bias in childhood leukemia studies. METHODS: We included results from 13 studies. Using an inverse variance-weighted method, summary effect estimates were calculated separately for distance, wire codes, and measured and calculated magnetic fields. Sensitivity analyses were conducted to assess the influence of individual studies, the potential for selection bias, and the possibility of publication bias. RESULTS: With the exception of wire-code studies, results were compatible with homogeneity across studies. The summary odds ratios (95% confidence intervals) were 0.88 (0.57-1.37) for distance <50 m and 1.14 (0.78-1.67) for calculated or measured magnetic fields above 0.2 microT. For measured or calculated exposures above 0.3 or 0.4 microT, the summary odds ratio was 1.68 (0.83-3.43), with no differences by method of exposure assessment. No single study had a substantial effect on the summary estimates. There was no indication of publication bias. CONCLUSIONS: With the exception of high cut-point analyses (0.3/0.4 microT), where the possibility of a moderate risk increase cannot be excluded, no increase in childhood brain cancer risk was evident for any of the exposure metrics.     

Effect of a nighttime magnetic field exposure on sleep patterns in young women

Since poor sleep quality is associated with multiple health problems, it is important to understand factors that may affect sleep patterns. The purpose of this study was to determine the effect of a continuous, 60-Hz, nighttime magnetic field exposure on sleep outcomes in young women sleeping at home. The study was a randomized crossover trial, comparing intervention (0.5-1.0 micro T above ambient levels) with ambient magnetic field levels, during two 5-night measurement periods. Subjects lived in the Seattle, Washington, area and were 20-40 years of age, had regular menstrual cycles, were not taking oral contraceptives, and had not breastfed or been pregnant during the previous year. The study was conducted between March and September of 2001. Sleep outcomes were measured via actigraphy. The range of magnetic field exposure was 0.001-0.50 micro T during the ambient period and 0.41-1.21 micro T during the intervention period. Sleep outcomes were not significantly different between the intervention and the ambient measurement periods. The intervention magnetic field had no effect on sleep patterns, suggesting that this exposure may not be an important factor in predicting sleep of young women who sleep at home.     

Environmental exposure to gasoline and leukemia in children and young adults–an ecology study

 Benzene is an established cause of leukemia in adults, especially acute non-lymphocytic leukemia (ANLL). A few studies have indicated that exposure to gasoline is a cause of childhood leukemia. The purpose of this study was to investigate if environmental exposure to benzene from gasoline and car exhaust was associated with leukemia in children and young adults. The exposure to gasoline and car exhaust was estimated by the number of cars per area. In this ecology study, data on the incidence of cancer in each municipality of Sweden during an 11-year period (1975–1985) were compared with the number of cars per area. Data on the incidence of cancer for persons aged 0–24 years at diagnosis were collected from the National Swedish Cancer Register. The following diagnoses were studied: non-Hodgkin’s lymphoma, acute lymphocytic leukemia (ALL), chronic myeloid leukemia (CML), and acute myeloid leukemia (AML). We found an association between AML and car density. In municipalities with more than 20 cars/km² the incidence of AML was 5.5 [95% confidence interval (CI) 4.4–6.8, n=89] as compared with 3.4 (95% CI 1.9–5.7, n=15) cases per 1 million person-years in municipalities with less than 5 cars/km² (P=0.05). No association was found for the other sites of cancer studied. The association between AML in young adults and car density might be attributable to exposure to benzene from gasoline vapors and exhaust gases, but further investigations are necessary before any definite conclusion can be drawn. Received: 31 July 1996 / Accepted: 29 November 1996     

Occupational magnetic field exposure and cardiovascular mortality in a cohort of electric utility workers

In electric utility workers, occupational exposure to magnetic fields has previously been associated with mortality from acute myocardial infarction (AMI) and arrhythmia but not from chronic coronary heart disease (CCHD) or atherosclerosis. To investigate these health endpoints further, the authors examined mortality from AMI (n = 407) and CCHD (n = 369) in a cohort of 35,391 male workers at the Southern California Edison Company between 1960 and 1992. Exposure was estimated according to duration of employment in occupations associated with high levels of magnetic field exposure and was calculated as cumulative exposure to magnetic fields expressed in micro-Tesla-years. Adjustment was made for age, calendar time, socioeconomic status, race, and worker status (active or inactive). The authors found that men working longer in high-exposure occupations or working as electricians, linemen, or power plant operators had no increased risk of dying from either AMI or CCHD compared with men who never worked in high-exposure occupations. For cumulative exposure, no association was observed with mortality from AMI (rate ratio per 1 micro T-year = 1.01, 95% confidence interval: 0.99, 1.02) or CCHD (rate ratio per 1 micro T-year = 1.00, 95% confidence interval: 0.99, 1.02). These results, indicating no exposure-related risk increase for AMI mortality, do not confirm previous results.     

某妇幼保健机构工作人员职业暴露监测

目的了解某妇幼保健院工作人员职业暴露情况,探讨职业暴露相关危险因素及预防对策。方法对该院2012年1-12月间55名工作人员发生职业暴露的监测数据进行统计分析。结果共发生工作职业暴露55例、57例次。其中暴露源不明确31例次（54.39％）,暴露源情况明确26例次（45.61％）。 57例次职业暴露人员中,保洁员和护士占82.46%（47例次）,其职业暴露发生率分别为9.76%和5.32%。职业暴露发生率居前3位的科室是门急诊、产科和保洁部,分别为29.51%、9.80%和9.76%;暴露人员的工龄≤3年者占87.72%（50例次）。暴露种类以锐器伤为主,占94.74%（54例次）,暴露物以头皮钢针为主（42.11％）,其次为注射器针头（19.30％）。 职业暴露主要环节为拔针（29.83％）,其次为医疗废物处置（28.07％）。结论保洁员、护士和低年资工作人员是职业暴露高危群体,应采取相关的防范措施,加强对其职业防护教育培训;加强对重点部门、环节的管理,降低医院工作人员职业暴露危害。     

急性白血病患儿睡眠紊乱与疲乏的相关性研究

目的探讨急性白血病患儿的睡眠紊乱与疲乏的相关性。方法采用便利抽样选取65名急性白血病儿童病例,对其主要照顾者进行问卷调查,内容包括儿童睡眠紊乱量表(Sleep Disturbance Scale for Children,SDSC)和PedsQL TMMFS儿童多维疲乏量表。结果急性白血病患儿的睡眠量表总分为(47.83±10.74)分,睡眠紊乱总发生率为6.15%,各维度中睡眠-觉醒转换障碍和睡眠呼吸障碍和觉醒障碍发生率最高。疲乏总分为(60.41±17.26)分,三个维度得分由低到高依次为一般疲乏、休息疲乏和认知疲乏。患儿的睡眠紊乱与疲乏呈负相关(r=-0.51,P0.01),即患儿睡眠紊乱越严重,疲乏程度越高。结论急性白血病患儿治疗期间最常见的睡眠紊乱为睡眠-觉醒转换障碍和睡眠呼吸障碍和觉醒障碍,疲乏主要表现为一般疲乏和休息疲乏。急性白血病患儿的睡眠障碍与疲乏状况呈负相关。     

Selection bias and its implications for case-control studies: a case study of magnetic field exposure and childhood leukaemia

Based on the epidemiological association between residential exposure to extremely low frequency-magnetic fields (ELF-MF) and childhood leukaemia, the International Agency for Research on Cancer classified ELF-MF as a possible human carcinogen. Since clear supportive laboratory evidence is lacking and biophysical plausibility of carcinogenicity of MFs is questioned, a causal relationship between childhood leukaemia and magnetic field exposure is not established. Among the alternative explanations, selection bias in epidemiological studies of MFs seems to be the most plausible hypothesis. In reviewing the epidemiological literature on ELF-MF exposure and childhood leukaemia, we found evidence both for and against the existence of selection bias. To evaluate the potential for selection bias, we examined the relationship of socioeconomic status to subject participation and exposure to MFs. We find that, often, reporting of selection processes in itself is biased and incomplete, making the interpretation and evaluation of a potential for bias difficult. However, if present, such a bias would have wide implications for case-control studies in general. We call for better reporting and for evaluation of the potential for selection bias in all case-control studies, as well as, for the development of novel methods in control selection and recruitment.