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Radio waves in human tissue

Ilkka Laakso defended his doctoral dissertation at Aalto University on the best ways to simulate human exposure to radio waves. Radio waves penetrate human tissues and cause warming. Dosimetry is used to determine how much. It can also be used to monitor excessive exposures by determining limit values for radiation.

Ilkka Laakso is investigating human exposure to radio frequency (RF) electromagnetic radiation, in other words, radio waves. To put it more precisely, he is studying how such exposure can best be simulated through numerical computation.

“The results help to make smoother and more reliable computational assessments of radio-wave exposure. The research has been carried out purely by means of scientific computation. Basically, it is electromagnetics research, but it also includes health and numerical aspects”, says Laakso.

The first time when radio waves became part of the human environment in Finland was when national radio broadcasting started. Today there are a large number of various wireless applications, so the exposure rate to different frequencies has continuously increased.

Are radio waves harmful to human health?

Radio waves are absorbed by lossy materials such as human tissues, and this causes heating in the materials.

“Regarding radio waves, the human body serves as a lossy dielectric object. Radio waves penetrate the human body and any other corresponding material. This means that when radio waves meet the interface between air and human skin, part of the radiation is reflected and part penetrates through the interface, slowly damping due to energy absorption”, Laakso explains.

Warming is the only scientifically established adverse health effect of radio waves.

“An extremely large temperature rise is obviously harmful, the kind occurring inside a microwave oven. In principle, all devices emitting radio waves cause similar warming, only much less. Additionally, exposure to smaller rises in temperature over a longer period of time may lead to adverse effects.”

To prevent adverse health effects, limit values have been determined based on the specific absorption rate (SAR). A temperature rise within the SAR limits is considered to be safe. For example, manufacturers of consumer electronic devices must make sure that the SAR values are not exceeded. At workplaces with strong RF radiation levels the employer is required to ensure that the workers are not exposed to excessive radiation.

Computational dosimetry

Dosimetry refers to calculation of the amount and site of the absorbed dose. The term was originally used for ionizing radiation, for which the term radiation dose is used. In radio wave dosimetry the dose metrics is based on the electromagnetic power absorbed into tissues, expressed as the SAR values.

“The term ‘computational dosimetry’ is used when dosimetry is conducted using only a computer, without measurements. Living people cannot be directly investigated, since we cannot measure field strengths inside the body. Compared with measuring, simulations are also faster and more straightforward, and it is easier to take into account various exposure situations.”

The computing resources provided by CSC facilitated the simulations needed, and the process was handled conveniently and quickly.

Picture: Temperature distribution at the surface of the head, in the brain and in the eyes when exposed to plane-wave irradiation. © Ilkka Laakso
Laakso päävisualisointi

Better limit values

Laakso's research has practical applications that deal mainly with standardization, for example, in determining safety limits. The results help to create better exposure limit values.

“Changes made to safety limits have possible effects on devices' research and development and on occupational safety and health. Additionally, the results help to make computational dosimetry more accurate and reliable, which might reduce the need for measurements and speed up exposure assessments”, reflects Laakso.

Laakso carried out his research at Aalto University's Department of Radio Science and Engineering as a member of a small research group. He recently moved to Japan.

“Since the beginning of April, I have been working as a post-doc researcher at the Nagoya Institute of Technology. The topic of my research is the same as in my doctoral thesis. During the years of my doctoral studies I participated in a few conferences in this part of the world, and I managed to make contacts here. The contacts helped me to get the post-doc position”, says Laakso.

Uncertainties affect the accuracy of dosimetric simulations

The results of Ilkka Laakso's doctoral thesis help to classify and identify error and uncertainty factors, as well as various modelling choices that affect the accuracy of dosimetric simulations. He classified three types of uncertainty factor that are similar in any problem of numerical computation.

1. The first concerns an error in the result calculated with purely computational algorithms vs. the accurate solution. The error can be reduced by using more accurate methods.

2. The second uncertainty type depends on how accurately the simulated situation corresponds to the real world situation. For example, a spherical model for the human body can be accurately calculated, which makes the first error type negligible. But the spherical model has little to do with the real world human body, so the second error type is a major one. This error type includes uncertainties relating to the human body model and, for example, errors in the electrical parameters of different tissues and their source models.

3. The third uncertainty class describes variation in the results when the modelled physical situation is changed when, for example, simulating RF exposure of different people in different positions.

Päivi Brink