Epidemiology

To study the influence of the factor “electric field” and/or “magnetic field” on a disease, the epidemiologist carries out investigations on relevant populations. Two main types of epidemiological studies are usually carried out : analytical and experimental (trials) epidemiology.

1. Ecological studies

Ecological studies focus on the comparison of groups, rather than individuals: they study the association (correlation) between exposure variables and health, when researchers do not have any individual data.

Ecological studies do not reflect the exposure or the health of each individual in the group but the average level of exposure and the health of populations. Researchers can study the same population at different times (temporal variations) or more populations of different geographical areas during the same period (geographic variation).

For example, this kind of survey can be used to study the relationship between the concentrations of air pollutants (CO2, ozone,…) and mortality collected in the following days from hospital data and death certificates.

Even if the results are not precise at individual level, ecological studies are interesting because they are quickly set up and quite inexpensive since based on already existing data. It can provide the base to build other studies as case control or cohort studies. They can generate hypotheses.

2. Case control studies

One selects a group of subjects that have the studied disease (cases), and a group of subjects without this disease (controls). For each subject of the investigation, one will search for information concerning exposure to the risk factors in their relevant past. For this reason, case control studies are qualified retrospectives, since the studied disease has already occurred when one searches for the earlier exposure to the risk factor. One then compares the exposure to the risk factor in both cases and controls. The advantage of this protocol is that it is inexpensive and feasible within a short time. Its principal disadvantage comes from the difficulty of rebuilding the story of cases and controls in a comparable way and without bias. The measurement of the association is called the odds-ratio (OR). Case-control studies are a good type of study for rare diseases.

3. Cohort studies (exposed versus non exposed)

During a given period, one supervises a group of people exposed to a risk factor as well as a group similar to the first, but not exposed to the studied factor. The appearance frequencies of the disease in the two groups are compared.

• Retrospective cohort studies
  In retrospective studies, researchers are looking in the past for the existence of a disease in two groups of people as alike as possible excepted for their exposure to EMF. In retrospective cohort studies, the measurement of association is called the relative risk (RR).
• Prospective cohort studies :
  Because researchers are waiting for the appearance of the disease with the passage of time, this type of study is called a prospective study.
  The advantage of prospective cohort studies is that it allows a better control of bias. Disadvantages are the high cost and difficulty of carrying out this type of study when the disease is rare or occurs after a long latency period. The measurement of association is called the relative risk (RR).

The term “Experimental” means that, contrarily to cohort studies, researchers control the exposure conditions of the subjects. Groups exposed and unexposed are monitored and compared with respect to the impact of the event studied. The assignment of a subject to a group or the other is randomized.

When properly conducted, these studies represent the ideal model to study the relationship between exposure to an agent and the occurrence of a disease, since the groups being compared differ only by one characteristic: the exposure. However, this approach is not always possible, often for ethical reasons if the exposure to which a group of subjects should be submitted is potentially harmful. Trials are mostly used to control the effectiveness of interventions (e.g. drugs).

Information bias concerns the estimation and measurement of parameters that influence the living organism. After 30 years of research, scientists have not managed to establish factor(s) of exposure to be studied in order to understand biological effects:

• What should be measured or calculated? electric field, magnetic field, electrical consumption, wiring code…
• Which parameters should be measured? peak, average, median, an accumulated dose…
• How long is it necessary to measure? specific measurement, 24h, 1 week…
• Where is it necessary to measure? inside the house, in front of the house, in the bedroom, at the workplace …
• When is it necessary to measure? during the day, during leisure time, at night
• Is the continuous or variable character of our exposure significant ?
• Which threshold should be chosen? 0.2 µT? 0.3 µT? 0.4 µT or higher?

Many studies are carried out with a threshold of 0.2 µT. Dr David Savitz first chose this threshold of 0.2 µT to establish a distinction between exposed people and non exposed people (Savitz, 1988). The goal was not to define a level of security but to establish a threshold for the study (Lynch C, 1997). The studies that followed were carried out with this threshold or other thresholds: 0.3 µT, 0.4 µT.

This relates to :

• the under-representation of subjects of under-priviledged socio-economic levels when the choice of controls is carried out by drawing names from a telephone list,
• for certain studies, there is a need for a given stability of housing for the controls: this involves lesser mobility of the controls than of the cases,
• refusal to answer a questionnaire or to authorize the measurement of fields inside the residence: non respondents can then be different from those who agree to take part in a study.

For domestic exposure, this bias primarily refers to the studies concerning the evaluation of fields radiating from surrounding power lines. High tension power lines are not laid out randomly in cities: they are often located in places where traffic congestion is considerable, air pollution is significant, and socio-economic status is low. Potential confounding factors (e.g. physical, chemical, genetic, nutritional, etc.) are numerous.

In the occupational environment, potential confounding factors frequently occur. In addition to the usual factors, such as socio-demographic characteristics, smoking, alcohol consumption, or general employment conditions, few studies consider factors such as organic solvents, poly-chlorinated biphenyls, welding fumes, or ionising radiation, which often characterize jobs exposed to electromagnetic fields (Knave B, 1988 and Gallagher RP, 1990).

Epidemiological studies suggesting an association are generally published in the scientific literature. On the other hand, epidemiological studies indicating a lack of association are not consistently published.