7.1 Development and refinement of dosimetric models for exposure analysis and assessment

Project management: Gernot Schmid, Seibersdorf Labor GmbH
Start: 1 November 2021
End: 31 October 2024

Background

A central component of the internationally recommended protection concept against health effects of low-frequency electric and magnetic fields are the reference levels defined by ICNIRP ^[1] and IEEE^[2] and the associated basic restrictions. The latter are based on proven biological effects of intra-body electric field strengths and magnetic flux densities at frequencies below 10 MHz. Whilst the magnetic permeability for most tissue types does not differ from air and thus the internal magnetic flux density, which corresponds to the external value, the internal electric field strength is difficult to access and furthermore strongly dependent on the type and structure of the tissue. The dielectric properties of tissue types have been theoretically modelled ^{[3, 4, 5]} and partly measured in numerous studies (e.g. on pigs ^[6]). However, uncertainties – specifically in relation to the finer structures in human tissue – in hives remain. The IEEE therefore lists both the measurement of tissue conductances and the modelling of skin, muscle, and nerve tissue as the most urgent research gaps to be addressed (Chapters 2.1 and 2.2 ^[7]).

Objective

Based on the current state of data and simulation techniques, this research project aims to determine the dielectric properties of tissues of the peripheral nervous system using tissue samples. The focus is on skin and muscle tissue; these are especially exposed because of their location in the body. Based on the improved data basis, body part area models (induction models) which are currently used will be refined and used for the numerical simulation of field configurations. Of particular interest is the simultaneous effect of electric and magnetic fields such as those found in the vicinity of overhead power lines. Statements on the conservatism of the reference values (in air) derived from the baseline values (in the body) are expected.

Implementation

The research project is based on researching the current data on electrical conductivity, permittivity, mass density, specific heat, and thermal conductivity of human and mammalian tissues and their measurement methods. On the other hand, currently used numerical calculation methods for dosimetric modelling in the low-frequency range are reviewed and their advantages and disadvantages evaluated, especially with regard to numerical artefacts.

Based on these findings, the electrical conductivity and permittivity of skin, muscle, and fat tissue are determined in the frequency range up to 1 MHz using optimised measurement methods. In the case of skin tissue, its fine structure in epidermis, dermis, and subcutis is considered in separate measurements. In the case of muscle tissue, particular attention is paid to the anisotropy of conductivity and permittivity. In parallel to measurements on human tissue samples, measurements of electrical conductivity in vivo based on impedance measurements and MRI scans on volunteers are carried out. This new approach should provide further insight into the question of possible discrepancies between in vitro measurement data on tissue samples and the in vivo properties of the tissues.

Based on the original data obtained, available body part area models will finally be improved so that the correlation between externally acting field strengths and the field magnitudes induced in the body tissues can be determined with considerably less uncertainty than before. Special attention is paid to the analysis of artefacts caused by the discretisation of the computational space and by anatomical inaccuracies of the tissue models. In addition, the effect of tissue anisotropy on model accuracy is taken into account as well as “critical” tissue structures such as boundary layers or thin layers. With the optimised models and methods obtained in this way, extensive numerical calculations will be carried out in order to check the conservativeness of the reference values. In particular, exposure situations with simultaneous exposure to electric and magnetic fields are also considered.

References

1) International Commission on Non-Ionizing Radiation Protection, 2010. Guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz to 100 kHz). Health physics, 99(6), pp.818-836.

2) Laakso, I. and Hirata, A., IEEE Standard for Safety Levels with Respect to Human Exposure to Electromagnetic Fields, 0-3 kHz, C95. 6-2002 IEEE Standard for Safety Levels with Respect to Human Exposure to Electromagnetic Fields, 0-3 kHz, C95. 6-2002, 2002.

3) Foster KR, Schwan HP. Dielectric properties of tissues and biological materials: a critical review. Crit Rev Biomed Eng. 1989;17(1):25-104. PMID: 2651001.

4) Gabriel, S., Lau, R.W. and Gabriel, C., 1996. The dielectric properties of biological tissues: III. Parametric models for the dielectric spectrum of tissues. Physics in medicine & biology, 41(11), p.2271.

5) IT'IS Foundation, Dielectric properties

6) Gabriel, C., Peyman, A. and Grant, E.H., 2009. Electrical conductivity of tissue at frequencies below 1 MHz. Physics in medicine & biology, 54(16), p.4863.

7) Reilly, J.P. and Hirata, A., 2016. Low-frequency electrical dosimetry: research agenda of the IEEE International Committee on Electromagnetic Safety. Physics in Medicine & Biology, 61(12), p.R138.