Contact currents and childhood leukaemia?

Depuis 1979 et la première étude épidémiologique faisant un lien entre les champs magnétiques (CM) et la leucémie infantile, beaucoup d’études ont été menées mais n’ont pas encore permis de conclure. En 2002, le Centre International de Recherche sur le Cancer (CIRC ou IARC en anglais) a classé les CM 50 Hz en catégorie 2B, c’est à dire “peut-être cancérigènes pour l’homme”. Ce classement est basé sur les résultats des études épidémiologiques et sur l’absence de démonstration d’un mécanisme d’action par les études en laboratoire (sur des animaux exposés toute leur vie ou sur des cellules). A ce jour, cette association pose toujours question.

Pendant plusieurs années, les chercheurs ont travaillé sur une hypothèse intéressante basée sur les courants de contact.  C’était l’objectif d’un projet mené au sein du BBEMG entre 2005 et 2011. Si une telle hypothèse était vérifiée, des mesures simples de protection, basées uniquement sur des recommandations par rapport aux installations électriques pourraient suffire.

Voir des informations sur l’origine des courants de contact dans les pages techniques.

Why studying contact currents?

Epidemiological studies and meta-analysis highlighted a small but increased risk of acute leukaemia for children exposed to average magnetic fields in excess of 0.3-0.4 µT. Years after years, researchers replicated studies, sought bias and confounding factors without reaching any final conclusion.

External magnetic fields could lead to effects on the human body through induced currents which give rise to low internal electric fields (rough estimate of 0.2 mV/m for a magnetic induction of 20 µT, see figure 1).

According to the World Health Organization (2007), 1 mV/m (*) (internal electric field) would be needed to potentially induce biological effects (not necessarily pathogenic). Therefore, external magnetic field could not alone explain the relation with childhood leukaemia.

Courant de contact

Figure 1

(*) Based on current evidence threshold values around 10-100 mV/m seem more likely (WHO, 2007)

Should it be another factor related to magnetic fields that could explain the increased risk put in advance in epidemiological studies?

The missing link could be contact currents. This hypothesis has been studied in the USA for 15 years.

Why focus on contact currents?

L’hypothèse de base et la suivante: la relation relevée par les études épidémiologiques sans permettre de conclure pourrait être due à un facteur intermédiaire. Les courants de contact sont des candidats plausibles pour expliquer le lien entre la leucémie infantile et les champs magnétiques résidentiels comme montré par une étude de l’EPRI (Electric Power Research Institute): il existe une relation statistiquement significative entre les tensions de contact et le champ magnétique près des lignes à haute tension: des tensions de contact plus élevée ont été mesurées dans des maisons soumises à des champs magnétiques plus élevés.

Ces courants de contact pourraient induire un champ électrique de quelques mV/m dans la moelle osseuse des enfants (**) (pour un courant de contact réaliste de quelques microampères µA), sans être ressenti par l’enfant.

De plus, avant le lancement de l’étude sur la relation entre les champs magnétiques externes et les courants de contact, les chercheurs américains ont analysé la fréquence avec laquelle les enfants sont en contact avec le robinet lors du bain (figure 1) (voir Courants de contact pour une explication technique). L’analyse a montré que les enfants jouaient souvent avec le robinet.

Cette hypothèse des courants de contact a été testée par une équipe du BBEMG.

(**) It is hypothesized that acute lymphoid leukaemia could find its origin in bone marrow, the place where haematopoiesis (i.e. the formation of blood cellular components) is occurring during childhood.

Contacts currents in Belgian residential housing

The BBEMG research programme on contact currents was carried out in two phases. First, researchers performed a measurement campaign in Belgian residences to measure local ambient magnetic fields and evaluate contact current levels. Second, using the obtained results, possible correlation between the contact currents and the ambient magnetic field was analysed.

In Belgium, is there a correlation between the proximity of overhead powerlines and contact current values in houses?

Ambient magnetic fields and contact currents were measured during a five year period in about 150 houses in Belgium, 10 % of which had an ambient magnetic field higher than 0.4 µT (the houses were specifically chosen as close as possible to electrical power lines). The median value of the magnetic field measurements is 0.02 µT.

Contact currents were measured in the bathtub, in the shower and near electrical appliances such as the washing machine.

The level of contact currents measured in the bathroom ranges from 1 to 1000 µA. The median contact current in the frequency graph is 8 µA. Twenty percents of houses have contact currents higher than 10µA and 5% of have contact currents higher than 100 µA.

Contact currents were also measured near electrical appliances. In this case less than 50% of the houses present contact currents higher than 20 µA and 15% have contact currents greater than 100 µA.

No correlation was found between ambient magnetic fields in residence and contact current values (figure 2). In some houses, even with very weak ambient magnetic field (0.01 µT), significant contact currents are measured.

Figure 2 – Contact current versus ambient magnetic field.
The dots indicate measured values in the sample of Belgian houses.
The solid line represents the simulated value of contact currents
due only to the ambient magnetic field (US hypothesis).

The solid line in Figure 2 can be considered as the part of contact currents that could be induced by the external magnetic field only. The ambient magnetic field alone can thus not justify the measured values of contact currents in the sample of Belgian houses.

These results are not in accordance with the US study. After analysing many parameters Belgian researchers hypothesised that the most probable cause of contact currents in Belgian residences could be leakage capacitive current.

Could leakage capacitive current be a satisfactory explanation of contact currents in Belgian residences?

Nous avons déjà introduit les courants de fuite dans l’exemple de la machine à laver. Dans cet exemple, il s’agit de courants de fuites résistifs.

Les courants de fuite capacitifs existent en permanence car ils proviennent d’un effet capacitif entre un câble et une zone de référence. Comme la plupart des câbles électriques n’ont pas de gaine métallique, des courants capacitifs pourraient être induits dans le fil de terre ainsi que dans les conduites métalliques comme celles d’eau et de gaz.

Pour étudier les courants capacitifs, il est important de comprendre que la maçonnerie dans laquelle le câble est enfoui peut être considérée comme un conducteur, capable de transporter les courants vers d’autres câbles ou conduites.

Les courants de fuite capacitifs pour les câbles utilisés en Belgique à 230 V (entre la phase et le neutre) sont compris entre 3 et 13 µA/m. Comme les maisons contiennent des centaines de mètres de câbles, ces courants de fuite peuvent entraîner des différences de tension significatives entre les conduites métalliques à proximité.

Les chercheurs ont fait de nombreuses mesures pour tester cette hypothèse. Ils concluent qu’en Belgique, les courants de contact proviennent d’un effet capacitif plutôt que par un champ magnétique ambiant.

How to understand differences between the origin of contact currents in Belgian and in the US residential housing?

Two technical factors can explain this difference: on one hand, the characteristics of leakage capacitive currents, on the other hand the influence of the grounding system.

On one hand, the main voltage in residential housing in the USA is 110 V (at 60 Hz), while it is 230 V (at 50 Hz) in Belgium. Since capacitive leakage currents increase with increasing voltage, higher capacitive leakage currents are expected in Belgium, leading to potentially higher contact currents.

On the other hand, the grounding system in US is more prone to the apparition of contact voltages induced in pipes and wires by the 60 Hz magnetic fields from the electrical network.

Conclusions

The contact current hypothesis as the missing link was interesting in the USA. However, in Belgium, it does not allow us to connect external magnetic fields and childhood leukaemia.

Note that in 2011 US researchers published the results of an epidemiological study investigating the association between contact voltage exposure and childhood leukaemia in California. The authors concluded that in this population, there was no evidence of an association between childhood leukaemia and exposure to contact voltage or magnetic fields and a weak correlation between measurements of contact current and magnetic fields. Contact currents should not be the missing link in the USA either!

That does not prevent us from taking contact currents into account. As currents that cross our body correspond to quite high internal electric fields, it makes sense to check our electrical installation. BBEMG engineers noticed that a lot of private electrical wirings do not meet legislation standards and could correspond to high contact currents. It is recommended to check the conformity of the house wiring, with particular attention being paid to the quality of the earth and the presence of equipotential bonding between metallic pipes (water and gas), radiators and the grounding, and the distance between wires and pipes.

Contact currents depending on the body impedance, for the same potential difference, they would be larger through a naked wet person than through a dry person wearing shoes. Thus it is of importance to particularly insist on the rooms with water (bathroom as a priority) with the installation of appropriate differentials.

It should be noted that in well-designed house wiring in good maintenance, contact current  intensities are generally very weak. Moreover, given the modernisation of the residential equipment, in particular the use of PVC water pipes, contact voltage is expected to drop. Eventually, as the name suggests, contact currents could occur in case of a contact with a conductive frame of an electrical equipment meaning that people are not continuously submitted to these currents.

References

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