A imec / Holst Centre White Paper
P.O. Box 8550
5605 KN Eindhoven
The Netherlands
www.imec-nl.nl
www.holstcentre.com
WHO’S AFRAID OF RF RADIATION?
Does Wireless Energy Transport Pose Health Risks?
By Hubregt J. Visser, PhD
November 2011
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 changing1, 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
1
Or – on a microscopic level – whenever a charged
particle accelerates or decelerates.
November 2011
is the propagation of electromagnetic
fields or radiation2.
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, Xrays and gamma rays.
characteristics of electromagnetic radiation.
The photon energy is related to frequency f
through E=hf, where h is Planck’s constant.
h=6.62606896e-34Js 4. The ordering by
photon energy is given in Table 2.
Table 2: Electromagnetic spectrum, energy
Type of
radiation
RF and
microwave
Infrared
Table 1: Electromagnetic spectrum
Type of
f (Hz)
λ (m)
radiation
RF and
microwave
Infrared
0.0 - 0.3e12
>1.0e-3
X-rays
0.3e12 –
4.0e14
4.0e14 –
7.5e14
7.5e14 –
3.0e16
>3.0e16
7.5e-7 –
1.0e-3
4.0e-7 –
7.5e-7
1.0e-8 –
4.0e-7
<1.0e-8
gamma
>1.0e20
<3.0e-12
visible
ultraviolet
Another way of ordering the
electromagnetic spectrum is based on
photon energy E. In doing so, we
make use of the particle
E (J)
E (eV)
0.0 - 0.3e12
X-rays
0.3e12 –
4.0e14
4.0e14 –
7.5e14
7.5e14 –
3.0e16
>3.0e16
0.0 –
2.0e-22
2.0e-22 –
2.7e-19
2.7e-19 –
5.0e-19
5.0e-19 –
2.0e-17
>2.0e-17
0.0 –
1.2e-3
1.2e-3 –
1.7
1.7 –
3.1
3.1 124
>124
gamma
>1.0e20
>6.6e-14
>4.1e5
visible
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.
f (Hz)
ultraviolet
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 radiation5. 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.
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.
4
h=4.13566733e-15eV
A higher intensity of non-ionizing electromagnetic
radiation means that more photons are emitted, but
the quantum energy of every photon remains the
same.
5
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.
Radiation Protection (ICNIRP) has issued
guidelines for limiting exposure to timevarying electric, magnetic and
electromagnetic fields up to 300GHz [2].
The ICNIRP exposure limits for frequencies
relevant for wireless power transmission are
stated in Table 3.
Table 3: Exposure limits
Frequency
Range
10-400MHz
4002000MHz
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.
Occupational
exposure
General public
exposure
Occupational
exposure
General public
exposure
2-300GHz
The first exposure guidelines were
already developed by the US military
in the 1950’s and have evolved in
today’s exposure limits. These limits
are based on the fact that the human
body can handle heat up to 4W/kg
without risking permanent damage6.
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 exposure time is 6
minutes. For the eyes and male’s
testes the exposure limit is the same
as for the whole body due to the
limited blood flow in these organs.
Plane Wave Power Density, Wm-2
Occupational
exposure
General public
exposure
10
2
f/40 f:
frequency in
MHz
f/200 f:
frequency in
MHz
50
10
When transmitting in the license-free
Industry Science and Medicine (ISM)
frequency bands, respecting the international
and national legal transmit power
restrictions, these exposure 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.
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
exposure 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 humans7.
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 2010. The COSMOS study
will enroll about 250000 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.
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 exposure 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 exposure 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
2011. Available at:
http://www.iwweth.org/downloads
[2] International Commission on NonIonizing 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. 494-522.
[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 2010.
[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 2011.
[5] Available at:
http://www.ukcosmos.org