This executive summary describes the final report for this project.
The EMF Research and Public Information Dissemination Program (RAPID) includes several engineering research in the area of exposure assessment and source characterization. RAPID engineering project #3: Environmental Field Surveys was performed to obtain information on the levels and characteristics of different environments, for which only limited data were available, especially in comparison to magnetic field data for the residential environment and for electric utility facilities, such as power lines and substations. This project was also to provide information on the contribution of various field sources in the surveyed environments.
Magnetic field surveys were performed at four sites for each of five environments: schools, hospitals, office buildings, machine shops, and grocery stores. Of the twenty sites surveyed, 11 were located in the San Francisco Bay Area and 9 in Massachusetts. The surveys used a protocol based on magnetic field measurements and observation of activity patterns, designed to provide estimates of magnetic field exposure by type of people and by type of sources. The magnetic field surveys conducted by this project produced a large amount of data which will form a part of the EMF measurement database.
Field and exposure data were obtained separately for area exposure and at exposure points. An exposure point is a location where persons engage in fixed, site specific activities near a local source that creates a significant increase in the area field. The area field is produced by area sources , whose location and field distribution is in general not related to the location of the people in the area.
The area field distribution of a site, calculated weighing each area s field distribution by the time (person minute) spent in the area, was not significantly different from the area field distribution obtained weighting each area by its surface area or giving to each area the same weight. Differences between weighting methods were less significant than differences between sites and between environments. Time Weighted Average Area (TWAA) field was found highest in grocery stores (1.93 mG average for the four sites), then in machine shops (1.42 mG), hospitals (1.27 mG), schools (0.83 mG), and office buildings (0.72 mG). The same ranking was obtained for the top 5th percentile of the area field distribution: 7.5 mG for grocery stores, 4.2 mG for machine shops, 3.7 mG for hospitals, 2.8 mG for schools, and 2.5 mG for office buildings. Differences between TWAA area fields obtained at different sites of the source environment were significant in comparison to differences between environments. In fact, the highest TWAA area field found in an office building (i.e. in the environment with the lowest average TWAA area field) was higher than the lowest average area field found in a grocery store ( i.e. in the environment with the highest average TWAA area field).
Exposure at exposure points was characterized by defining a zone of activity (range of distances from a source), an exposure duration (person minutes), and by determining, through measurements, the field at different distances from the source.
Time Weighted Average (TWA) fields, caused by the combination of area and point fields showed only a modest increase over TWAA fields for schools (from 0.8 to 0.9 mG), for hospitals (from 1.3 to 1.4 mG), and for office buildings (from 0.7 to 1.0 mG) and showed a strong increase for grocery stores (from 1.9 to 2.7 mG) and especially for machine shops (from 1.4 to 3.9 mG), indicating a large effect of local sources for these two environments.
Time weighted average field and time weighted field distribution was calculated separately for each group of people. In office buildings, secretarial and support staff had a greater exposure than professionals. In schools, teacher were slightly more exposed than students. Custodians and administrative staff were significantly more exposed than teachers and students. In hospitals, the medical staff and the maintenance staff were significantly more exposed than patients. Visitors were the least exposed. In machine shops welders had by far the largest exposure. In grocery stores clerks, cashiers, and butchers had much greater exposure than office staff and customers. Comparing groups of people across environments, it was found that welders in machine shops had the highest exposure, both in terms of TWA and of top 5th percentile were (TWA = 5.2 mG, top 5th percentile = 24.6 mG), followed by butchers in grocery stores (TWA = 4.1 mG, top 5th percentile = 12.8 mG) and clerks / cashiers in grocery stores (TWA = 4.0 mG, top 5th percentile = 11.9 mG). The groups with the lowest exposure were visitors in hospitals (TWA = 0.8 mG, top 5th percentile = 2.4 mG), students in schools (TWA = 0.9 mG, top 5th percentile = 2.9 mG), and professionals in office buildings (TWA = 0.9 mG, top 5th percentile = 2.6 mG). These data refer to the average person of a given type. Individual people may have higher or lower exposures and their exposure may vary significantly from day to day.
The relative contribution of each area source to area exposure was calculated. Net currents were the main contributor to area fields in schools, grocery stores, and hospitals. Office equipment was the main contributor to area fields in office buildings. Milling and welding equipment was the main contributor to area field in machine shops. Fluorescent lights were important contributions to area fields in grocery stores, office buildings, and hospitals. Electrical panels were an important contributor to area fields in machine shops. Power lines were, generally, a minor source of magnetic field exposure. A significant percentage of exposure, however, was caused by sources that could not be identified.
The percentage of total time spent above a field threshold was calculated for each type of persons, for different field thresholds (2, 5, 10, 20, and 40 mG). It was found, for instance, that clerks / cashiers in grocery stores had the highest percentage of time (56%) spent above 2 mG and that professional in office buildings had the lowest (14%).
Enertech Consultants, Inc. has posted their own version of the executive summary, complete with a table summarizing time-weighted average exposures in different environments.