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Research methods for assessment of exposure to magnetic fields at the electric grid frequency in epidemiological studies

The majority of epidemiological studies that have investigated exposure to electromagnetic radiation from the electric grid have focused on the association between exposure to magnetic fields from high voltage lines in residential areas and the risk of developing childhood leukemia.The difficulty in evaluating exposure to magnetic fields is partly a consequence of their physical features and partly due to the wide variety of different sources of exposure that we encounter daily, as explained below.

 

Difficulties of evaluation of exposure to magnetic fields

  • Exposure prevalence – The entire human population is exposed to some degree to magnetic fields at the electrical grid frequency. Therefore it is not possible to compare population groups that have been exposed to these fields with groups that have never been exposed. The only comparison possible is between groups that have had different degrees of exposure to magnetic fields.
  • Inability to “identify” the exposure – as far as is known, the human body is unable to sense magnetic fields and therefore it is impossible to estimate the degree of past exposure using interviews and questionnaires.
  • The magnetic field changes all the time – both within a time range of seconds and depending on the time of day, the seasons and the years. In most studies, when estimation of a person’s past exposure is required, it is assumed that the exposure has not changed over time, unless there are findings indicating that the electrical facilities (transformer stations, high voltage lines) were different in the past.
  • The magnetic field changes over space – for example, there may be different intensity magnetic fields in different regions of a certain structure.
  • Exposure complexity – in the course of the day people are exposed to different levels of magnetic fields. For example in the home, in the workplace, in school, while travelling and while outside. Exposure sources are many and varied. Current knowledge regarding the contribution of each one of the magnetic field sources to the overall exposure is sparse. Exposure assessment in most available studies was limited to a single environment, for example home or workplace. In recent years several studies assessed the contribution of all environmental exposures of each individual.

The level of magnetic field exposure can be assessed by several methods. In some studies the estimation is quantitative and in some it is non quantitative.

 

Summary of the methods used for measurement and estimation of exposure to magnetic fields caused by the electric grid

 
Measurement method   Means of measurement   How the magnetic field is estimated   Disadvantages of the measurement method

Distance

   Distance estimation  

The distance from the magnetic field source (for example high voltage line, transformer station) is measured. In most cases the distance (in kilometers) is used as a marker for the level of exposure

   The estimate of magnetic field strength is crude, non- quantitative and not related to other sources of magnetic fields, just to the calculated reduction in magnetic field strength with increasing distance from the field source
             

Wire coding

   Residential classification  

Residences are categorized according to their distance from electrical facilities and according to characteristics of the electrical facilities. Five types of power lines are classified with this method: very high current flow level, regular high current flow, regular low current flow, very low current flow and below ground

 

  

The estimate of magnetic field strength is crude. Exposure to magnetic fields from sources inside the home is not addressed

This method was used in the past (the 1980s and 90s), mainly in studies in the United States

 

Historical estimation of magnetic fields

   Estimation or calculation  

Magnetic field strength is calculated or estimated at a specific time point in the past based on historical data from the electricity company or according to electrical facility characteristics (type of power line, the transformer or transformer station)

 

   The non-quantified nature of the assessment impairs its accuracy

Measurements in the field

   Device for measuring magnetic fields  

Isolated measurement of magnetic field intensity or measurement for unit of area at a certain point in time or during a defined time period (for example 24 hours, a week etc.) of all magnetic field sources contributing to the field at this point

It is possible to carry out one measurement or a number of measurements at different places in the house (for example at the front door, or in the children’s bedroom) in order to identify spatial differences in the field, or to carry out the measurement over the course of several days in order to observe changes over time

 

   The exposure takes place at a specific time point and does not apply to any past exposures of the population in question or to the overall exposures of the individual, whilst a valid epidemiological study requires information on past exposure (which is impossible to collect)

Personal measurements

   Device for measuring magnetic field carried on the user’s body   Measurement of all daily individual exposures to magnetic field sources during a defined time period using a wearable measurement device  

The measured exposure is personal and does not necessarily represent exposure experienced by the population at large.

The exposure occurs at a specific point in time whereas the performance of a valid epidemiological study requires information on past exposure (which is impossible to collect).

The cost of performing the measurements is high
 

List of sources

 
Updated on 20.4.2017