WHO S AFRAID OF RF RADIATION? Does Wireless Energy Transport Pose Health Risks?

Transcription

1 A imec / Holst Centre White Paper P.O. Box KN Eindhoven The Netherlands WHO S AFRAID OF RF RADIATION? Does Wireless Energy Transport Pose Health Risks? By Hubregt J. Visser, PhD November 2011

2 Contents Introduction 2 RF Radiation 2 Heating Effects 4 Long-Term Effects 4 Implementing Wireless Energy Transport 5 Summary 6 References 6 Introduction For the general public - and even for people with a science education - knowing to be subject to Radio Frequency (RF) radiation may arouse a feeling of uneasiness or even fear. Those feelings are instigated by the fact that you cannot see, hear, smell or feel RF radiation. Furthermore, the term radiation in itself has become very sensitive from the moment that the hazardous health effects of excessive radioactive decay (ionizing radiation) have become apparent. Media announcements of mostly unconfirmed scientific studies add to the feeling of uneasiness. So, naturally, whenever a new Global System of Mobile communications (GSM) base station is installed in an urban environment, the people living in the neighborhood may get worried. The same applies when we propose to use RF radiation for powering wireless sensors in office and home spaces [1]. In this paper we will outline the scientific facts regarding RF radiation, the known interactions with human tissue and discuss past and present epidemiologic studies. Finally, we will give recommendations on how to implement wireless energy transport systems. RF Radiation Whenever an electric current is changing 1, it will induce a changing magnetic field around this current. This changing magnetic field will create a changing electric field around the magnetic field lines. The changing electric field will create again a changing magnetic field around the electric field lines and so on. What we describe here November Or on a microscopic level whenever a charged particle accelerates or decelerates.

3 is the propagation of electromagnetic fields or radiation 2. Electromagnetic radiation may be ordered according to an increasing frequency f or a decreasing wavelengthλ 3. Going up in frequency we encounter successively: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X- rays and gamma rays. So, we see that RF radiation, visible light and radioactive decay are all forms of the same physical phenomenon. In Table 1 we have ordered the spectrum (roughly) in terms of frequency and wavelength. Table 1: Electromagnetic spectrum Type of f (Hz) λ (m) radiation RF and e12 >1.0e-3 microwave Infrared 0.3e12 4.0e14 7.5e-7 1.0e-3 visible 4.0e14 7.5e14 4.0e-7 7.5e-7 ultraviolet 7.5e14 3.0e16 1.0e-8 4.0e-7 X-rays >3.0e16 <1.0e-8 gamma >1.0e20 <3.0e-12 Another way of ordering the electromagnetic spectrum is based on photon energy E. In doing so, we make use of the particle 2 The keyword here is changing. A static electric current will not produce radiating fields. 3 Wavelength, λ, is related to frequency, f, through λf=c, where c is the velocity of light. characteristics of electromagnetic radiation. The photon energy is related to frequency f through E=hf, where h is Planck s constant. h= e-34js 4. The ordering by photon energy is given in Table 2. Table 2: Electromagnetic spectrum, energy Type of f (Hz) E (J) E (ev) radiation RF and microwave e e e-3 Infrared 0.3e12 4.0e14 2.0e e e visible 4.0e14 7.5e14 2.7e e ultraviolet 7.5e14 3.0e16 5.0e e X-rays >3.0e16 >2.0e-17 >124 gamma >1.0e20 >6.6e-14 >4.1e5 When the quantum energy exceeds the level of a few electronvolts (ev) - the threshold lying somewhere in the ultraviolet region of the electromagnetic spectrum - the energy is high enough to eject an electron from an atom, leaving behind a positively charged particle; i.e. an ion. This ionizing radiation is capable of mutating DeoxyriboNuclear Acid (DNA) and may thus cause cancer. We see that RF and microwave radiation cannot directly be carcinogenic, no matter the intensity of the radiation 5. With respect to indirect effects, current scientific evidence has not conclusively linked RF and microwave radiation to the development of cancer. We will discuss past and present epidemiologic studies, after discussing a known effect of RF and microwave radiation on human tissue: heating. 4 h= e-15ev 5 A higher intensity of non-ionizing electromagnetic radiation means that more photons are emitted, but the quantum energy of every photon remains the same.

4 Heating Effects The effect of heating of tissue by RF and microwave radiation is known since the 1930 s and has become well known since World War II with the introduction of radar. Microwave heating is caused by dipole rotation. Molecules having an electrical dipole moment (like water) will align themselves in an electromagnetic field. When this field alternates, the molecules will reverse direction and in this process distribute energy to adjacent molecules in the form of heat. This principle is used in microwave ovens. The first guidelines were already developed by the US military in the 1950 s and have evolved in today s limits. These limits are based on the fact that the human body can handle heat up to 4W/kg without risking permanent damage 6. The 4W/kg is for the whole body. For limbs or parts of the body, the limit may rise up to 80W/kg due to the cooling effect of blood circulation. The average time is 6 minutes. For the eyes and male s testes the limit is the same as for the whole body due to the limited blood flow in these organs. Based on these limits and applying a safety factor of 10, the International Commission on Non-Ionizing 6 The metabolic rate of a sleeping person is about 1W/kg, increasing to about 2.5W/kg during moderate exercise. Radiation Protection (ICNIRP) has issued guidelines for limiting to timevarying electric, magnetic and electromagnetic fields up to 300GHz [2]. The ICNIRP limits for frequencies relevant for wireless power transmission are stated in Table 3. Table 3: Exposure limits Frequency Plane Wave Power Density, Wm -2 Range MHz Occupational 10 General public Occupational f/40 f: 2000MHz frequency in 2-300GHz General public Occupational General public MHz f/200 f: frequency in MHz When transmitting in the license-free Industry Science and Medicine (ISM) frequency bands, respecting the international and national legal transmit power restrictions, these limits will not be exceeded [1]. So, respecting (inter)national legal power restrictions will prevent adverse heating effects of human tissue by RF and microwave radiation. Furthermore, RF and microwave radiation is non-ionizing and thus cannot directly cause cancer. What remains to be discussed are possible longterm adverse health effects of RF and microwave radiation. Long-Term Effects With long-term adverse health effects we mean an increased risk of developing cancer other than by ionizing radiation

5 With the widespread use of cell phones, a growing public concern about possible long-term adverse health effects has grown and several (epidemiologic) studies have been undertaken. Studies thus far have not shown a consistent link between cell phone use and cancers of the brain, nerves, or other tissues of the head or neck. Considering the very low levels and research results collected to date there is no convincing scientific evidence that RF and microwave radiation cause adverse health effects. We do know that the human body absorbs about five time more of the signals from FM radio and television broadcasting stations than it does from a GSM base station. Broadcast stations have been in use for over 50 years now. Up to date no adverse health effects originating from these broadcasting stations have been established. The largest, most prominent and scientifically sound epidemiologic study to date, i.e. the Interphone Study [3], concluded that Biases and errors limit the strength of the conclusions that can be drawn from these analyses and prevent a causal interpretation. A problem in the study was that data on mobile phone use was obtained through questioning the users. This lead to inaccurate reporting, recall bias, participation bias and so on. Notwithstanding these conclusions, the International Agency for Research on Cancer (IARC), part of the World Health Organization (WHO), recently classified radiofrequency electromagnetic fields as possibly carcinogenic to humans 7. This classification paves the way for further research. A large study of cell phone use and its possible long-term effects was launched in Europe in March The COSMOS study will enroll about cell phone users aged 18 years or older and will follow them amongst others through their mobile phone records - for 20 to 30 years [5]. Implementing Wireless Energy Transport Considering the above, we see that using RF and microwave radiation do not pose an immediate or mid-term risk to health. The long-term effects are not known. Regardless of personal views and interpretations of conducted (epidemiologic) studies concerning these long-term effects, it would be wise to apply the As Low As Technically Achievable (ALATA) principle concerning transmitting powers. This ALATA principle needs to be applied on top of the (inter) national legal transmit power limits. For wireless energy transport systems it will be beneficial to place the RF base station 7 The Classification is Group 2B. The different classifications are: Group1: The agent is carcinogenic to humans. Group 2A: The agent is probably carcinogenic to humans. Group 2B: The agent is possibly carcinogenic to humans. Group3: The agent is not classifiable as to its carcinogenicity to humans. Group 4: The agent is probably not carcinogenic to humans.

6 and the wireless sensor nodes on the ceilings of office and house spaces and direct the electromagnetic radiation along the ceiling. Thus the energy transmission will not be blocked by humans and humans will not be subject to (low intensity, nonionizing) electromagnetic radiation. Summary A general public concern does exist about health effects of RF and microwave radiation. A large group of people perceive risks of RF and microwave as likely and even possible severe. Through explaining the physics of electromagnetic radiation and ordering the electromagnetic spectrum with respect to photon or quantum energy, it is shown that ionizing radiation a cause of cancer cannot be associated with RF and microwave radiation. These types of radiation cannot be directly carcinogenic, no matter the intensity of the radiation. Furthermore, it is shown that RF heating caused by molecule dipole rotation in an electromagnetic field is well controlled by (inter)national power restrictions and limit recommendations. We see that using RF and microwave radiation do not pose an immediate or mid-term risk to health. The longterm effects are not known and research concerning possible effects in ongoing. Awaiting these results we propose to use the As Low As Technically Achievable (ALATA) principle concerning transmit powers. For wireless energy transport we furthermore propose to place RF base station and wireless sensor nodes on the ceiling of office and home spaces and direct the electromagnetic radiation along the ceiling. The benefit of this configuration is two-fold: Humans are not subject to electromagnetic radiation and energy transport is not blocked by humans. References [1] Hubregt J. Visser, No More Dead Batteries, Opening the Gateway to Smart Building Integration through RF Wireless Battery Charging, imec / Holst Centre White Paper, October Available at: [2] International Commission on Non- Ionizing Radiation Protection (ICNIRP), Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields (up to 300GHz), Health Physics, Vol. 74, No. 4, April 1998, pp [3] International Agency for Research on Cancer, Interphone Study Reports on Mobile Phone Use and Brain Cancer Risk, World Health Organization, Press Release No. 200, 17 May [4] International Agency for Research on Cancer, IARC Classifies Radiofrequency Electromagnetic Fields as Possibly Carcinogenic to Humans, World Health Organization, Press Release No. 208, 31 May [5] Available at: