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 (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 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
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 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 910 10 11 10 12 10 13 10 14 10 15 10 16 10 17 10 18 10 19 10 20 10 21 10 22 Radio & TV Infra Red Microwaves Gamma- Rays X-Rays Ultra Violet Microwaves 0.8 GHz to 1000 GHz Non-ionising Radiation (generally not so bad) Ionising Radiation (generally bad) Frequency (Hertz)
What absorbs the energy? Natural Frequency Resonance 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
Atoms, Molecules & Materials Natural Frequency Resonance 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
Power watts 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 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
Absorption (2) Too weak If power were absorbed too weakly, –Microwaves would go right through –No cooking
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
Microwave Ovens Summary 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 Power Power –This is a 700 watt oven –Think of 7 x 100 watt light bulbs
Microwave Electric 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 1/25 th square metre –Between 10 4 to 10 5 watts per square metre –(Most intense sunlight around 10 3 watts per square metre) –Very Dangerous –Could I have a stupid volunteer please?
A Cautionary Tale Dont 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 Oven SAR 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 Watts per kilogram Temperature Rise in 1 kg of Brain Fluid Microwave700 8 ºC (ish) Mobile Phone1 Cant 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 PowerIntensityMaximum SAR WattsWatts per square metre Watts per kilogram 12001 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? 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 (12) http://www.sitefinder.ofcom.org.uk /
EIRO Equivalent isotropic Radiated Output Actual Pattern of Emitted Radiation 60 W How this is specified
Mast near my childrens school Power 30 dBW 1000 W Equivalent isotropic Radiated Output 60 W to 120 W in actual power 0.3 Watts per square metre 0.01 Watts per square metre SAR 0.001 Watts per kilogram
Mobile Phones (13) Comparison of handsets and base stations Base station SAR is extremely low –but you cant choose whether you want it or not! PowerIntensityMaximum SAR WattsWatts per square metre Watts per kilogram Handset1200About 1 Base Station600.01About 0.001
WiFi (11) Wireless Networking WiFi –802.11(b), 802.11(g), 802.11(n) Operates at 2.4 GHz PowerIntensity (at 1 metre) SAR (at 1 metre) WattsWatts per square metre Watts per kilogram 0.1Less than 0.01About 0.0001
Mobile Phones (13) Comparison of handsets and base stations PowerIntensityMaximum SAR WattsWatts per square metre Watts per kilogram Handset1200About 1 Base Station600.01About 0.001 Wi Fi0.1< 0.01About 0.0001
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 cant 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 risk More willing to take risk Less willing to take risk Example Benefits DirectIndirect Base Stations/ Mobile Phones Technology FamiliarUnfamiliar Cars/ Mobile Phones Exposure VoluntaryInvoluntary Base Stations/ Mobile Phones Exposure FairUnfair Poverty/ Wealth Exposure Personal Control No Personal Control Driver/ Pedestrian Consequence Not DreadedDreaded Cancer/ Broken bone