van der Woord M P, Kromhout H, Barregård L, Jonsson P
Department of Environmental Sciences, Wageningen University, The Netherlands.
Am Ind Hyg Assoc J. 1999 Nov-Dec;60(6):713-9. doi: 10.1080/00028899908984493.
Occupational exposure to 50-Hz magnetic fields was surveyed among electric utility workers to investigate (1) components of exposure variability, (2) patterns of autocorrelation between short-term measurements, and (3) imprecision and misclassification due to short-term measurements. Spot measurements every 10 seconds during 81 working days were analyzed for 42 electric utility workers from 10 occupational subgroups and during 8 working days for 4 office workers from the same company. For the 8-hour time-weighted average (TWA) magnetic fields, the variability was partitioned into its components: within workers, between workers, and between groups. For spot measurements of magnetic fields, the within-day variance component also was examined. Autocorrelation functions were determined and numbers of short-term measurements necessary for reliable estimates of 8-hour TWA magnetic fields were assessed. Spot measurements of magnetic fields, as well as 8-hour TWA magnetic fields, were approximately log normally distributed among workers. The mean exposure to magnetic fields was 0.47 microT (n = 81 days) in electric utility workers and 0.12 microT (n = 8 days) in office workers. A large fraction, 76% of the spot measurements total variance, could be attributed to variability within days. For the 8-hour TWA magnetic fields, between-group variability was small and of the same magnitude as between-worker variability. Significant autocorrelations between short-term averages of 7.5, 15, and 30 minutes were present, when taken within periods of 30 minutes. One-hour averages showed no autocorrelation. Simulations showed that, due to high within-day variability and autocorrelation, a limited number of short-term measurements of magnetic fields in electric utility workers are likely to result in imprecise estimates of 8-hour TWA magnetic fields. Measurement strategies relying on short-term (spot) measurements are therefore likely to result in misclassification of exposure and consequently absent or spurious exposure-response relations.
对电力公司工人的职业性50赫兹磁场暴露情况进行了调查，以研究：（1）暴露变异性的组成部分；（2）短期测量之间的自相关模式；（3）短期测量导致的不精确性和错误分类。对来自10个职业亚组的42名电力公司工人在81个工作日内每10秒进行的现场测量进行了分析，对同一家公司的4名办公室工作人员在8个工作日内的测量也进行了分析。对于8小时时间加权平均（TWA）磁场，将变异性分解为其组成部分：工人内部、工人之间和组间。对于磁场的现场测量，还检查了日内方差分量。确定了自相关函数，并评估了可靠估计8小时TWA磁场所需的短期测量次数。磁场的现场测量以及8小时TWA磁场在工人中大致呈对数正态分布。电力公司工人的磁场平均暴露量为0.47微特斯拉（n = 81天），办公室工作人员为0.12微特斯拉（n = 8天）。现场测量总方差的很大一部分（76%）可归因于日内变异性。对于8小时TWA磁场，组间变异性较小，与工人之间的变异性大小相同。在30分钟内进行的7.5、15和30分钟的短期平均值之间存在显著的自相关。1小时平均值没有自相关。模拟表明，由于日内变异性和自相关较高，电力公司工人有限数量的磁场短期测量可能会导致对8小时TWA磁场的估计不准确。因此，依赖短期（现场）测量的测量策略可能会导致暴露的错误分类，从而导致不存在或虚假的暴露-反应关系。
