Post on 07-May-2015
transcript
Are mobile phones safe?
Protons for Breakfast
Week 5
In the event of an alarm sounding…
How it all fits together…
Electromagnetic waves
Atoms
Heat
Electricity
What this evening is about…
1. What do we mean by safe?
2. How do mobile phones work?– What is the hazard?
3. How do electromagnetic waves interact with matter?
4. SAR
5. Microwave Ovens SAR
6. Mobile Phone SAR
7. Are mobile phones safe?
Warning…The word ‘radiation’
• The word ‘radiation’ means– Anything which ‘radiates’ on ray-like paths
• Could be – Sound– Radio– Light– Infra-red light– Particles or waves emitted from the nuclei of atoms
• Nuclear Radiation
Safe?
1. What do we mean by ‘safe’?
Picture Credit: Lessemf http://www.lessemf.com/mw-oven.html
Mobile Phones
2. How do mobile phones work?
Mobile Phones (1) Magic?
Any sufficiently advanced technology is indistinguishable from magic
Arthur C. Clarke
Mobile Phones (2)Not magic
• Mobile Phones are ‘radio’ phones
• Operating frequency is in the microwave region of the spectrum
• Handset power is kept to a minimum by having a network of local transmitters and receivers
• 20,000 base stations in UK
Roughly
hexagonal cells
Mast
• Each colour represents a base station operating at a slightly different frequency
Mobile Phones (3)Recognise the masts?
• Each cell has a transmitter and receiver mast • Notice the typical three way structure
Mobile Phones (4)New 3G Masts
Photo Credit Brighton and Hove Green Party
Mobile Phones (4)Linking to base stations
• Masts broadcast with a power of between 60 watts to 120 watts
• Handsets broadcast with a power of about 1 watt
• Handsets send signals every few minutes to establish which is the nearest mast
• A central computer keeps track of – all the telephones that are
switched on – Remembers which cell they are
in
Mobile Phones (5)Making a mobile-to-mobile call
Its complicated!
Network Control
Knows in which cell every telephone is
Originating Telephone
Receiving Telephone
Mobile Phones (6)Digital encoding
Hello! How are you?
Sound
180 pulses of encoded sound every second No sound: No signal
Encoding to Radio Signal
Hello! How are you?
Mobile phone Safety
• The safety issues surrounding mobile phones concern…
• the interactions of the microwaves emitted by – mobile phones – base stations
with human tissue
6 degrees
50 m to 200 m
• ‘WiFi’ Wireless Networking• Bluetooth devices• Wireless keyboards and mice• DECT cordless phones• Baby Monitors• ‘Walkie Talkie’
And its not just mobile phones!
• All involve electromagnetic waves in the radio and microwave part of the spectrum
Mobile Phones
3. How do electromagnetic waves interact with matter?
?
Electromagnetic spectrum
1 101 102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022
Radio & TVInfra Red
Microwaves
Gamma-Rays
X-Rays
Ultra Violet
Microwaves0.8 GHz
to 1000 GHz
Non-ionising Radiation
(generally not so bad)
Ionising Radiation
(generally bad)
Frequency (Hertz)
What ‘absorbs’ the energy?‘Natural’ FrequencyResonance
• If electrons (very light) jiggle – natural frequency is in the visible or ultra violet range
• If atoms (10000 times heavier) jiggle– natural frequency is in the infra-red/microwave range
• If whole molecules (Many times heavier) jiggle– natural frequency is in the RF/Microwave range
Waves & Matter (6)Summary
• Electromagnetic waves interacting with matter can be– Reflected– Absorbed– Transmitted
• Exactly what happens depends on
• the frequency of the electric field
• the natural frequencies of the atoms and molecules
• Microwaves emitted by mobile phone systems
• Are absorbed by human tissue
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Abs
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Frequency
Atoms, Molecules & Materials‘Natural’ FrequencyResonance
• Well below the natural frequency
• Not much absorption
• Well above the natural frequency
• Not much absorption
• Near the natural frequency
• Absorption
Waves & Matter (3)
When electromagnetic waves are ‘absorbed’ by
matter …
Atoms jiggle faster:
i.e. increase their temperature.
Heat
Electricity
Electromagnetic waves
Atoms
• When an electromagnetic wave interacts with matter…– Some absorption – Some reflection– Some transmission
Waves & Matter (5)
How much energy gets absorbed?
S.A.R.Specific Energy Absorption Rate
4. How much energy is absorbed?
Terminology (6)Summary
• Power– watts
• Intensity– watts per square metre
• Specific energy Absorption Rate (SAR)– watts per kilogram
Powerwatts
• Power is measure of…– the total amount of energy
emitted per second
Intensity watts per square metre
• Intensity is measure of – the amount of energy
flowing across per unit area per second
– It is like brightness• A light looks less bright as
one moves further away from it, because the same power is spread over a larger area
Specific energy Absorption Rate
(SAR) watts per kilogram
• Measures energy absorbed per kilogram per second
• A high SAR means – a substance heats up
quickly
• A low SAR means – a substance heats up
slowly
• SAR is a property of – an emitting device– in a particular position with
respect to – an absorbing substance
Specific energy Absorption Rate
(SAR) watts per kilogram
• SAR is a property of – an emitting device– in a particular position
with respect to – an absorbing
substance
Terminology (5)Specific energy Absorption Rate (SAR) watts per kilogram
• 10 cm from 20 W source 2 W absorbed in hand
Intensity 200 watts per square metre
SAR 20 watts per kilogram
• 1 metre from 20 W source 0.02 W absorbed in hand
Intensity 5 watts per square metre
SAR 0.2 watts per kilogram
SAR:Example using light
rather than microwaves
• 1 cm from 20 W source 20 W absorbed in hand
Intensity 8000 watts per square metre
SAR 200 watts per kilogram
Terminology (6)Summary
• Power– watts
• Intensity– watts per square metre
• Specific energy Absorption Rate (SAR)– watts per kilogram
Microwave Hazard
The potential hazard from mobile phones and other wireless devices arises from
the absorption of microwave radiation
Mobile phones • only emit a watt or two• so little power it makes
experiments difficult
Microwave ovens• emit a few hundred watts• makes experiments easy
Microwave Ovens
5. Microwave ovens
Electromagnetic spectrum
1 101 102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022
Radio & TVInfra Red
Microwaves
Gamma-Rays
X-Rays
Ultra Violet
Microwaves
Conventional Oven
Microwave Oven
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Abs
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Frequency
Microwave Ovens The absorption of microwave energy by liquid water
• Well below the natural frequency
• Not much absorption
• Well above the natural frequency
• Not much absorption
• Near the natural frequency
• Absorption
2.4 GHz
Absorption (1)Too strong
• If power were absorbed too strongly,– Microwaves would only penetrate a short distance– Surface would be heated– Inside would remain uncooked
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Frequency (GHz)
Microwave ovens operate at 2.45 GHz
Absorption (2) Too weak
• If power were absorbed too weakly,– Microwaves would go right through – No cooking
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Frequency (GHz)
Microwave ovens operate at 2.45 GHz
Absorption (3)
• If power is absorbed just right,– Microwaves penetrate about 5 cm (2 inches) – Cooks the outer 5 cm of the food– Good enough for most cases
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Frequency (GHz)
Microwave ovens operate at 2.45 GHz
Microwave OvensSummary
• A microwave oven cooks food by heating it• The heating comes from
– intense waves at 2.45 GHz • rather than
– a wide spectrum of waves at infra red frequencies.– Frequency chosen because of absorption properties of
water molecules at that frequency.
Microwave GenerationCavity Magnetron Valve
Microwave Ovens
Magnetron
Microwave Ovens
Inside a microwave oven
Microwave Power
• Power– This is a 700 watt oven– Think of 7 x 100 watt light bulbs
MicrowaveElectric field
• 700 watts – Around 140 000 volts per metre– Look what happens to a CD
Microwave Intensity
• 700 watts – transmitted into an area of around 1/25th square metre– Between 104 to 105 watts per square metre– (Most intense sunlight around 103 watts per square metre)
– Very Dangerous– Could I have a stupid volunteer please?
A Cautionary Tale
Don’t try this at home!
August 14, 2002 I don't want to sound like I know everything in the world or even like I know quite a lot. But you had a question regarding “If a microwave oven door were to open while it was still on, what would happen? Could it hurt you?- JP” Well ..Having the thought process that I have, kinda how should I put it? ...Stupid? or inventive or even in-between. Well, my microwave door did happen to come off. Magic Chef 900-watt microwave. Well, I did my best to try to fix it but the hinge on one side did not attach properly, therefore having a gap between the door and the appliance. Being me (stupid) I wondered if it would burn fast or would it gradually warm up. I slid my finger between...You probably dying to hear what happened... But it didn't gradually warm up at all. It was instant heat! It didn't scar me or anything like that, but sure scared the H*** out of me to find out it got so hot so quick. I didn't get any blisters either. But it just burned like touching something hot on the tip of my finger being that is the only thing I put in. Well you know the old adage, "You learn from your mistakes", stands true. lol -
Microwave OvenSAR inside oven
• Inside – 700 watts:– Absorbed in 1 kg of water: SAR = 700 watts per kg– Your brain weighs about a kilogram!
• QUESTION: After 1 minute, what temperature rise results from an SAR of 700 watts per kg?
Microwave Ovens (13) SAR
SAR Wattsper kilogram
Temperature Rise in 1 kg of ‘Brain Fluid’
Microwave 700 8 ºC (ish)
Mobile Phone 1 Can’t be measured directly
Expect 1/700 of microwave temperature rise
The effects of blood flow reduce this further
Mobile Phones
Back to Mobile Phones
6. Mobile Phone SAR
Mobile Phones (6) Typical SAR with phone near the head
• SAR Averaged over 10 grams• By law must be less than 2 watts per kilogram
Power Intensity Maximum
SAR
Watts Watts per square metre
Wattsper kilogram
1 200 1
• Typical figures
Mobile Phones (7) Is this SAR very big?
• Microwave Oven heating– SAR about 700 W per 1 kg– 8 ºC rise after 1 minute?
Magnetron
• Heating by a mobile phone – SAR of 1 W per kg– Expect 1/700 of microwave
temperature rise 0.011 ºC
– Blood flow reduces this.
Photo Credit: Wikipedia
Mobile Phones (8) Your telephone
• Look in the small print!
Mobile Phones
7. Base Station SAR
Mobile Phones (11) Base Stations
• Power
6 degrees
50 m to 200 m
EIRO
Equivalent isotropic RadiatedOutput
Actual Pattern of Emitted Radiation
60 W
How this is specified
1000 W EiRO
Actual Pattern of Emitted Radiation
Mast near my children’s school Power• 30 dBW • 1000 W • Equivalent isotropic
Radiated Output• 60 W to 120 W in
actual power
0.3Watts per square metre
0.01Watts per square metre
SAR 0.001Watts per kilogram
Mobile Phones (13) Comparison of handsetsand base stations
• Base station SAR is extremely low– but you can’t choose whether you want it or not!
Power Intensity Maximum
SAR
Watts Watts per square metre
Wattsper kilogram
Handset 1 200 About 1
Base Station 60 0.01 About 0.001
WiFi (11) Wireless Networking
• WiFi– 802.11(b), 802.11(g), 802.11(n)
• Operates at 2.4 GHz
Power Intensity
(at 1 metre)
SAR
(at 1 metre)
Watts Watts per square metre
Wattsper kilogram
0.1 Less than 0.01 About 0.0001
Mobile Phones (13) Comparison of handsetsand base stations
Power Intensity Maximum
SAR
Watts Watts per square metre
Wattsper kilogram
Handset 1 200 About 1
Base Station 60 0.01 About 0.001
Wi Fi 0.1 < 0.01 About 0.0001
Mobile Phones
7. Are Mobile Phones Safe?
Mobile Phones (14) SAR and Safety
• NPL can estimate the SAR for a mobile phone in a standard position near a simulated head– e.g. 1.2 watts per
kilogram
• NPL can’t advise on whether that dose is safe
Mobile Phones (14) SAR and Safety
Health Protection Agency• Radiation Section
– formerly the National Radiological Protection Board
• Based on ICNIRP guidelines– International Commission on Non-Ionising Radiation Protection
– determines what we believe is safe
Is 2 watts per kilogram a safe value of SAR?
Is exposure to 10 watts per square metre safe?
Mobile Phones (15) What are the risks?
• Basic safety assumption is that the main effect of exposure to microwaves is thermal
• Are there ‘non-thermal’ effects?• If so are they harmful?
• Normal thermal vibrations are much larger than vibrations induced by microwaves emitted by mobile phones.
• Question of safety must be resolved by experiment– Experiments are very hard
Mobile Phones (17) Are mobile phones safe?
Stewart Report • A group of non-industry related experts
Gave rise to MTHR
Mobile Telecommunications and Health Research• ONGOING Research Program • Funded 50:50 government & industry
Mobile Phones (18) Are mobile phones safe? Stewart Report
• Mobile Phones are ‘pretty’ safe
• There may be ‘effects’ of Mobile Phones
• Precautionary Approach: – More research please
• Children
• Driving
Mobile Phones
What do we mean by ‘safe’?• Our knowledge is not complete,
– There is some risk (or potential risk) that mobile phone technology could be associated with adverse health effects.
– As long as we use mobile phones, the risk will never be zero
Mobile Phones (22) What could we do?
• Ban mobile phones ?– The ‘Precautionary principle’– How many deaths or cancers would be ‘acceptable’?– How many lives are saved by mobile phones?
• Reduce mobile phone power ?– Increase the number of mobile phone masts
• Reduce mast power ?– Increase the mobile phone handset power– Increase the number of masts
• Ban mobile phone use while driving ?– Done: has it been effective?
• Nothing ?
Mobile Phones (23) What should we do?
Go forth and DECIDE!
Is current research enough?
What else – if anything - should we do?
On-line Resources
• www.protonsforbreakfast.org –This PowerPoint ™ presentation.–Handouts as a pdf file
• blog.protonsforbreakfast.org –Me going on about things
• links.protonsforbreakfast.org –Links to other sites & resources
See you next week to discuss Nuclear Power
Goodnight
Risk is expressed mathematically,
but our judgement of risk is not mathematical
Perception of riskMore willing
to take risk
Less willing
to take risk
Example
Benefits Direct IndirectBase Stations/
Mobile Phones
Technology Familiar UnfamiliarCars/
Mobile Phones
Exposure Voluntary InvoluntaryBase Stations/
Mobile Phones
Exposure Fair UnfairPoverty/
Wealth
Exposure Personal ControlNo Personal Control
Driver/
Pedestrian
Consequence Not Dreaded DreadedCancer/
Broken bone