Safety Standards for EMF
Exposures
Lecture 9
January 29, 2016
Current Approach to Setting the
Standards
• 1. US approach
– A. Find the lowest level where damage is observe
reproducibly and set the level below this level by
some safety factor.
– B. Old USSR approach set the level below the
lowest observed effects.
– C. Implications of the different approaches
Electric Shock
Humans and animals are most susceptible to frequencies at 50 to 60 hertz because the internal frequency of the nerve
signals controlling the heart is approximately 60 hertz. (Electric Shock Precautions) Source: (Nave & Nave)
The National Electrical Code (NEC) in the U.S. considers 5 mA (to be a safe upper limit for children and
adults; hence the 5 mA Ground Fault Interrupter (GFI) circuit breaker requirement for wet locations
. (The Physical Effects of Electricity) The values in Table 1 should be used as a guide instead of absolute data points.
For instance, 99% of the female populations have a “let go” limit above 6 mA with an average of 10.5 mA. 99% of the
male populations have a “let go” above 9 mA, with an average of 15.5 mA. (The Physical Effects of Electricity
Table 1. Physiological Effects of Shock
Electric Current (1sec.
Contact)
Physiological Effect
Voltage required to produce the current with
assumed body resistance:
100,000 ohms
1,000 ohms
1 mA
Threshold of feeling, tingling sensation.
100 V
1V
5 mA
Accepted as maximum harmless current.
500 V
5V
Beginning of sustained muscular
contraction ("Can't let go” current).
1000 V
10 V
Ventricular fibrillation, fatal if continued.
Respiratory function continues.
10,000 V
100 V
Sustained ventricular contraction
followed by normal heart rhythm
(defibrillation). Temporary respiratory
paralysis and possibly burns.
600,000 V
6000 V
10-20 mA
100-300 mA
6A
Body Resistance
Table 2. Typical Human Body Resistance to Electrical Current
Body Area
Resistance (ohms)
Dry Skin
100,000 to 600,000
Wet Skin
1,000
Internal body (hand to foot)
Ear to Ear
400 to 600
~100
Source: (Nave & Nave)
Table 2 shows some of the typical human body resistances to electrical current. Barring broken skin, body-circuit resistance,
even in contact with liquid, will probably be not less than 500 ohms. However, the current flow at this resistance
and 120 volts is 240 mA—over twice what is required to cause death. (Biological Effects of Electric Shock)
Some History with Respect to
RF and ELF Safety Standards
• 1. Debate on Thermal vs. Non-thermal Health
effects from radar goes back to at least
1960s.
• 2. Wertheimer, N., and E. Leeper.1979.
• 3. Cordless phones, cell phone, WiFi
• 4. In all cases there are problems in going
from the physics through the chemistry to
the biology to possible health effects.
Basis for Setting RF Exposure Limits.
• 1. Based on extensive literature review and
epidemiological studies.
• 2. Heating above normal tissue values by
approximately 1C over about 6minutes
• 3. Do not consider other mechanisms such as
changes in radical concentrations or the
potential for long term effects or on sub
populations such as children, sick people,
or the elderly.
• 4. Does not include feedback and repair
processes in the body.
On Average, My Temperature Is Just
Right!
• 1
Current FCC Guidelines at RF
Table 1. FCC Limits for Maximum Permissible Exposure (MPE)
Source: OET Bulletin 56, 4th ed., 08/1999, FCC
(A) Limits for Occupational/Controlled ExposureC
Frequency
Electric Field
Magnetic Field
Power Density
Averaging Time
Range
Strength (H)
Strength (H)
(S)
│H │2 ∙ │H │2 or S
(MHz)
(V/m)
(A/m)
(mW/cm2)
(minutes)
0.3-.3.0
614
1.63
(100)*
6
3.0-30
1842/f
4.89/f
(900/f2)*
6
30-300
61.4
0.163
1
6
300-1500
f/300
6
1500-100,000
5
6
(B) Limits for General Population/Uncontrolled Exposure
Frequency
Electric Field
Magnetic Field
Power Density
Averaging Time
Range
Strength (H)
Strength (H)
(S)
│H │2 ∙ │H │2 or S
(MHz)
(V/m)
(A/m)
(mW/cm2)
(minutes)
0.3-1.34
614
1.63
(100)*
30
1.34-30
824/f
2.19/f
(180/f2)*
30
30-300
27.5
0.073
0.2
30
300-1500
f/1500
30
1500-100.000
1
30
f = Frequency in MHz
*Plane-wave equivalent power density
Electric Field Exposure Limits.
Table 2. IEEE C95.6 Environmental Electric Field Maximum Permissible
Exposures (MPEs), Whole Body Exposure
General Public
Frequency range
(Hz)
1-368
c
Controlled Environment
E – rms* (V/m)
5000
Frequency range
(Hz)
a,d
1-272
E –rms*
(V/m)
20,000
b,e
a
Within power line rights-of-way, the MPE for the general public is 10 kV/m under normal load
conditions.
b
Painful discharges are readily encountered at 20 kV/m and are possible at 5-10 kV/m without
protective measures.
c
Limits below 1 Hz are not less than those specified at 1 Hz.
d
At 5 kV/m induced spark discharges will be painful to approximately 70% of adults (wellinsulated individual touching ground).
e
The limit of 20,000 V/m may be exceeded in the controlled environment when a worker is not
within reach of a grounded conducting object. A specific limit is not provided in this standard.
*rms = root mean square
The International Commission on Non Ionizing Radiation Protection, ICNIRP, sets electric field
exposure limits at 50 Hz to 5 kV/m and magnetic flux density limits at100 µT.
Limited Summary of Current Standards
• 1. At low frequencies the limits are set on the
basis of electric fields large enough to fire
a nerve cell. E ≈ 5kV/m for f< 368Hz
• 2 At RF exposure limits are set on the basis of
heating or an specific absorption rate,
SAR = 1.6W/kg over 1gram at 900MHz
3. Far Field exposures of P= 1mW/cm2 for
•
6 minutes in the frequency range 30MHz
to 300MHz
Statement of the Problem for the
Standards Bodies.
• 1. The need to have something you can
manufacture to, or conditions that allow you to
install transmitters.
• 2. Second you know that over time the science is
going to get better and you are going to see
biological effects at lower levels and for
exposures over longer periods of time.
• 3. The problem is where do you set the numbers
and how do you adjust them at a later date.
Standards Setting
• 4. If you change the standards
– A. What are the economic implications and health
implications?
– B. What you do about the imbedded equipment?
5. What are risks to you take and at what cost by
setting the standards at any given level?
Loss of life expectancy and total number of lost days
Cause
Days
Cause
Days
Being unmarried-male / female
3,500 / 1,600
Average job accident
74
Cigarette smoking-male / female
2,250 / 800
Drowning
41
Heart disease
2,100
Falls
39
Being overweight 30% / 20%
1,300 / 900
Fire-burns
27
Being a coal miner
1,100
Generation of electricity
24
Cancer
980
Suffocation
13
Low Socio-economic status
700
Fire arms accidents
11
Living in unfavorable state
500
Natural radiation / From nuclear industry
8 / 0.02
Army in Viet Nam
400
Medical x-rays
6
Smoking Cigar / Pipe
330 / 220
Poisonous gases
7
Job Dangerous / with RF exposure / safe
300 / 40 / 30
Coffee
6
Accidents Motor vehicle / to pedestrians
207 / 37
Oral contraceptives
5
Pneumonia, influenza / Diabetes
141 / 95
Reactor accidents-UCS / RCS
2 / 0.002
Alcohol (US average)
130
PAP exams
-4
Accidents in home
95
Smoke detector in home
-10
Suicide
95
Air bags in car
-50