Possible Health Effects of Exposure to Residential Electric and Magnetic Fields (1997) / Chapter Skim
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Appendix B: Exposure Assessment in Residential Studies
Appendix B: Exposure Assessment in Residential Studies
Pages 285-301
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Appendix B Exposure Assessment in Residential Studies
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, and low current configuration was divided into ordinary low current configuration (OLCC) and end pole (VLCC)
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288 HEALTH EFFECTS OF RESIDENTIAL ELECTRIC ED MAGNETIC FIELDS Wiring Class ~ , ~ .
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Magnetic fields also are not shielded by the walls of houses, so they are more likely to reflect residents' actual exposure to power lines than would electric fields, which are easily shielded. Wertheimer and Leeper (1979)
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In principle, the 24-hr measurements should yield a more accurate depiction of long-term exposures of interest and possibly yield a more accurate indication of historical exposures of etiologic interest. WIRE CODES AND RESIDENTIAL MAGNETIC FIELDS Prediction of Mean Fields and Variance in Fields A few studies have examined the degree to which wire codes are associated with magnetic-field measurements.
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Measurements in the first four studies listed in Table B-1 were generally made near the centers of rooms, away from obvious local magneticfield sources. Because people spend some time near such local sources, somewhat higher values might be expected for personal exposure measurements, such as the EMDEX data in Table B-1, than for spot measurements (EPRI 1993c)
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The local sources within the home appear to add a random component that is more evident in low background fields (EPRI 1993cJ. The 1,000-homes study mathematically separated the power-line fields from other fields measured within the home, producing a clearer relationship between wire codes and power-line-derived magnetic fields, for underground, VLCC, OLCC, OHCC and VHCC, the median power-line 24-hr average field calculations were 0.03, 0.02, 0.04, 0.06, and 0.10 AT, respectively.
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In spite of the modest predictiveness of wire codes for fields overall, they found that 80% of the subjects with personal exposures above 0.2 AT were correctly classified based on their living in HCC homes, and 76% of the subjects with personal exposures below 0.2 AT were correctly classified based on living in LCC homes. A similar result was found in the Geomet study in which it was reported that wire codes correctly classified children's average exposure (above or below 0.2 ~T)
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Data from the 1,000-homes study permits the same type of calculation for average spot measurements at the centers of bedrooms instead of personal exposure (Table B-4~. An HCC house is 2.3 times more likely than art LCC house to have a bedroom field over 0.2 AT (a 70% chance)
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The replicated association of wire codes with residential magnetic fields extends to an association between wire codes and personal time-integrated exposure. As shown in the last section, use of wire codes to classify personal exposure into broad magnetic field categories (e.g., greater than 0.2 AT and less than 0.2 ~T)
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. Overall, the evidence is fairly consistent that wire codes are weakly predictive of magnetic fields measured in the home.
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However, if magnetic fields are the causal exposure, it is the historical, prediagnosis fields that are of interest, not the contemporary levels. Therefore, contemporary fields are of interest only to the extent that they are predictive of historical fields.
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. The correlation between the first-visit and the second-visit personal-exposure levels was "weak to nonexistent." The authors state, "The fact that personal exposures measured months apart in this study were weakly correlated, at best, raises questions as to the precision with which any set of contemporary measurements can be used to assess historical exposures to power-frequency magnetic-fields" (EPRI 1992b)
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At present, the overall impression is that present-day magnetic fields are somewhat predictive of historical fields, with correlations possibly as high as 0.5-0.8. Variability in the Wire-Code and Magnetic-Field Association Examination of the current literature on wire codes and residential magneticfield strengths reveals several anomalous results.
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, but found that none was more strongly correlated with the wire-code category than it was with the arithmetic mean of the 24-hr bedroom magnetic field. The EMDEX residential project found no relation between wire-code categories and maximum field strengths, nor was it found between wire-code categories and first measured differences in personal exposures (an estimate of field variance)
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. On the other hand, current flows are generally more variable at points in the power-delivery system that serve fewer customers; therefore, the most uniform power flows should be found on the multiple or thick three-phase primary lines and high-tension lines near high current configuration (HCC)
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