Human Exposure Limits

Transcription

1 Human Exposure Limits Session 3 0 Version December 2014

2 Learning objectives In this session we will: Learn about the international exposure limits for workers and the public Learn about methods for assessing compliance Learn about important considerations when choosing measurement equipment Learn about how to present measurement results 1

3 Increasing frequency Established mechanisms for RF energy 6. Surface heating of the body 5. Microwave hearing effect 4. Localised heating in the head and torso 3. Localised heating in the limbs 2. Whole body heating 1. Electro-stimulation of nerve and muscle ICNIRP 1998 EMF Guidelines 2

4 2. Whole body heating Core body temperature varies naturally about 1ºC Core body temperature rises of more than 1-2 ºC over prolonged periods can have adverse health effects such as heat exhaustion (headache, nausea & dizziness) and heat stroke SAR = specific absorption rate (units of W/kg) The rate at which RF energy is absorbed in body tissues Normal loads from activity vary from about 1 to 10 W/kg A WBA SAR of 1 W/kg for one hour, and up to 4 W/kg for short periods, will result in a rise of less than 1ºC in core body temperature in resting subjects in moderate thermal environments. Worker limit of 0.4 W/kg accounts for other thermal loads. 3

5 Formula for SAR SAR = σ ρ E2 (unit of W/kg) Where: σ = dielectric conductivity (Siemens/m S/m) ρ = tissue density (kg/m 3 ); E = internal electric field strength (V/m) It can also be calculated from the rate of temperature rise. 4

6 Factors affecting whole body SAR Intensity of the RF field Frequency of the RF field Polarisation of the RF field Size of the person Grounding of the person 5

7 Normalised SAR (W/kg per W/m 2 ) RF energy absorption versus frequency Section 5.4 in WHO EHC 137,

8 4. Localised heating in the head and trunk The brain and vital organs are more sensitive to heating than the limbs. For the public: Head/trunk 2 W/kg. Limbs 4 W/kg Localised SAR is averaged over 10 grams. Point SAR distribution 3dB contours 7

9 Measurement of handset SAR International recommended limit: 2 W/kg in mass of 10 g brain metabolic rate of 8-10 W/kg Maximum temperature rises during phone use: eye ~0.02 C brain <0.10 C much less than normal 1 C daily variation. International measurement standards. 8

10 SAR testing of mobile devices 9

11 6. Surface heating At frequencies above 10 GHz the skin depth of absorption is very small. Skin depth penetration at 10 GHz = 3.8 mm Skin depth penetration at 100 GHz = 0.36 mm RF power at high frequencies is absorbed as surface heating, similar to heating from an infrared lamp. The exposure limit is based on incident power density. 10

12 Indirect effects: shocks and burns Can result from contact with a live RF conductor. Can also result from point contact with a passively charged, large metallic ungrounded conductor. Point contact results in higher current densities than grasping contact. RF burns tend to damage deeper layers of the skin. They can be very painful and may take a long time to heal. 11

13 International Commission on Non-Ionizing Radiation Protection Partner of the World Health Organization ICNIRP is an independent group of international experts responsible for providing scientific advice on possible health effects and for assisting in the development of protection policies. ICNIRP Recommendations: Based on evaluation of all scientific evidence Include substantial safety factors Protective of all persons (young, old, sick) from all established hazards 12

14 WHO and ITU recommend ICNIRP limits...endorses the guidelines of the International Commission on Non- Ionizing Radiation Protection (ICNIRP) and encourages Member States to adopt these international guidelines...' If such limits do not exist, or if they do not cover the frequencies of interest, then ICNIRP limits (Appendix I) should be used' WHO Standards and Guidelines 13

15 WHO support for ICNIRP following reviews of all the peer-reviewed scientific literature, including thermal and non-thermal effects. The standards are based on evaluations of biological effects that have been established to have health consequences. The main conclusion from the WHO reviews is that EMF exposures below the limits recommended in the ICNIRP international guidelines do not appear to have any known consequence on health. WHO Standards and Guidelines 14

16 ICNIRP EMF Limits provide protection against known adverse health effects. An adverse health effect causes detectable impairment of the health of the exposed individual or of his or her offspring; a biological effect, on the other hand, may or may not result in an adverse health effect.' the scientific literature published since the 1998 guidelines has provided no evidence of any adverse effects below the basic restrictions and does not necessitate an immediate revision of its guidance on limiting exposure to high frequency electromagnetic fields'

17 Radio Frequency Exposure Developing conservative limit values Adverse effects threshold (100% = 4 W/kg) Worker level: 10% = 0.4 W/kg Public level: 2% = 0.08 W/kg Note: Diagram not to scale. 16

18 ICNIRP basic restrictions for RF Basic restrictions: Based directly on established health effects. Physical quantities are SAR and power density (S). Only power density in air can be readily measured. ICNIRP 1998 EMF Guidelines 17

19 ICNIRP basic restrictions SAR Type of exposure Occupational General public Frequency range Whole-body average SAR (W/kg) Localized SAR (head and trunk) (W/kg) Localized SAR (limbs) (W/kg) 100 khz-10 MHz MHz-10 GHz khz-10 MHz MHz-10 GHz NOTE 1 f is the frequency in Hertz. NOTE 3 All SAR values are to be averaged over any 6-minute period. NOTE 4 The localized SAR averaging mass is any 10 g of contiguous tissue; the maximum SAR so obtained should be the value used for the estimation of exposure. ICNIRP 1998 EMF Guidelines 18

20 ICNIRP reference levels Reference levels: Provided for practical exposure assessment purposes. Derived quantities E, H and S. Compliance with the reference level intended to ensure compliance with the relevant basic restriction. If the value exceeds the reference level, it does not necessarily follow that the basic restriction will be exceeded. If a reference level is exceeded, it is necessary to test compliance with the relevant basic restriction. ICNIRP 1998 EMF Guidelines 19

21 ICNIRP reference levels for RF fields Type of exposure Occupational exposure General public Frequency range Electric field strength (V/m) Magnetic field strength (A/m) Equivalent plane wave power density S eq (W/m 2 ) 1-10 MHz 610/f 1.6/f MHz MHz 3f 1/ f 1/2 f/ GHz MHz 87/f 1/2 0.73/f MHz MHz 1.375f 1/ f 1/2 f/ GHz NOTE 1 f is as indicated in the frequency range column. NOTE 2 For frequencies between 100 khz and 10 GHz, the averaging time is 6 minutes. NOTE 5 For frequencies exceeding 10 GHz, the averaging time is 68/f 1.05 minutes (f in GHz). ICNIRP 1998 EMF Guidelines 20

22 Power Density (W/m 2 ) Compliance limits for antennas RF Workers f/40 Public f/200 Worker Public Frequency (MHz) ICNIRP 1998 EMF Guidelines 21

23 ICNIRP limit values may be reviewed ICNIRP (2009): Some published studies showed that in the frequency ranges of body resonance (~100 MHz) and from 1 to 4 GHz for bodies shorter than 1.3 m in height (corresponding approximately to children aged 8 y or younger) at the recommended reference level the induced SARs could be up to 40% higher than the current basic restriction under worstcase conditions. However, this is negligible compared with the large reduction factor of 50 (5,000%) for the general public. Possibility of future adjustment to reference levels. Post WHO Environmental Health Criteria process (2015?). ICNIRP has not identified evidence pointing to a need to change the basic restrictions related to SAR. ICNIRP 2009 Statement 22

24 Compliance for antenna sites Version December 2014

25 Antenna exposure zones Exceedance Zone (No Access) Occupational Zone (Workers) Compliance Zone (Public) 24

26 General principles for RF exposure evaluations Identify appropriate compliance limits Determine if RF exposure assessment is needed If needed: Calculations Measurement If exposure limits may be exceeded, apply mitigation. 25

27 Compliance versus environmental Compliance relatively high fields: Calculation Broadband measurements Narrowband measurements SAR assessments Environmental low level fields: Calculation Broadband measurements Narrowband measurements Communication 26

28 Antenna field regions Reactive Near-Field Region λ/2π 2D 2 /λ or 0.5λ Radiating Far-Field Region Antenna Radiating Near-Field Region 27

29 Near-field and far-field In the near-field: E and H are not in phase The field impedance varies The antenna pattern is still being formed Simple formulas tend to over-estimate In the far-field: The E and H vectors are in phase The impedance is 377 Ohms (in free space) The antenna pattern is fully formed Simple formulas may be used 28

30 Large and small antennas Electrical size is not physical size. Large antenna dimensions >>λ. Three field regions: Near field reactive out to λ/2π Near field radiating at distances out to 2D 2 /λ Far field at distances > 2D 2 /λ Small antenna dimensions < λ Two field regions: Near field reactive out to λ/2π Far field at distances > 2D 2 /λ 29

31 Non-uniform exposure when close to source 30

32 Compliance: broadband measurements Measure reference levels Establish compliance zones Practical field measurements Includes all other sources in frequency range Near-field E and H Averaging Other hazards 31

33 Basic broadband RF survey instrument Metering Instrumentation Sensor 3.14 Leads 32

34 Units of measurement Meters will either measure: Electric field via dipole antennas Magnetic field via loop antennas However, some display mw/cm²: Internal conversion based on E/H = Z Care must be taken that correct probe is used In near field, may need to use both E and H probe. 33

35 Desirable meter characteristics Meter: Responds to modulated fields Good accuracy Good sensitivity Wide dynamic range Portability Not sensitive to temperature changes Peak and average value options Lightweight Probe: Responds to E or H only Small size and non-perturbing Broad and flat response Isotropic and linear response Sums multiple signals correctly Low noise Robust 34

36 Calibration factors Survey instruments do not have a flat response with frequency, so calibration factors are used. True field = Measured field x Calibration Factor Use the calibration factor closest in frequency to the service being measured. Calibration accuracy is typically ±1dB. 35

37 RF measurement procedures (1) Before conducting a survey: Identify frequencies and powers present Estimate likely hazard zone distances Choose E, H or both probes, as appropriate Use calibration factor to determine meter indication for relevant exposure limit Note other environmental hazards, e.g., working at height Check meter is operating correctly before and after the survey 36

38 RF measurement procedures (2) During a survey: Observe the maximum exposure limit! Start in low field and avoid body shielding Scan over body height to locate the peak Scan metallic members, but beware of probe coupling effects Rotate probe to peak reading, but hold still for measurement Take enough measurements, applying calibration factor 37

39 Measurements close to antennas 38

40 Complex sites Contributions from many sources. 39

41 Environmental radio signals FM GSM900 GSM1800 3G LTE T-DAB TETRA 40

42 Environmental RF evaluations Why? What? Where? When? How? 41

43 Environmental: narrowband measurements Calibrated receiver and antenna Good sensitivity Measures individual sources Polarisation of antenna Standardised positioning Effects of radio traffic Time consuming Antenna Spectrum analyser Study on the Feasibility of Epidemiological Studies on Health Effects of Mobile Telephone Base Stations Final Report, Neubauer et al., ARC-IT 0124, March

44 Environmental RF measurements 43

45 Environmental levels from mobile network antennas Median levels more than 10,000 times below the limit values. Joyner, Van Wyk and Rowley,

46 Average contributions of environmental RF sources Joseph et al,

47 Compliance for multiple sources Calculate the fractions of the limit for each source. Add the fractions together. If the total 1 then the location is compliant. If the total > 1 then the location is non-compliant. Shared sites. ICNIRP 1998 EMF Guidelines 46

48 Measurement uncertainty Instrument errors Inaccuracies in the response of the meter and incorrect use. May be limited to ±1dB if: Meter is properly zeroed Calibration factor correctly applied Probe rotated to remove isotropic error Environmental errors Due to objects which alter the field being measured. May be limited to ±2dB if: Foreign objects (e.g. cars, etc) are removed A large number of measurements are made to account for reflections 47

49 Compliance: calculations Assessment without measurement Requires information about antenna, transmitter and so on Based on conservative assumptions Basis for compliance declarations Multiple sources? 2 m 48

50 Far-field estimation of field strengths S = P G 4 d² W/m² P = Power to antenna (W) G = Linear isotropic gain d = Distance from antenna (m) 49

51 ITU EMF Estimator (ITU-T K.70) 50

52 Estimation of compliance distances d = P G 4 S (m) S = (W/m²) P = Power to antenna (W) G = Linear isotropic gain d = Distance from antenna (m) 51

53 Activity Individual exercise: Calculating exposure levels and compliance distances 52

54 Calculating exposure levels Power into the antenna = 43 dbm Antenna gain = 16 dbi Distance from the antenna = 10 m 53

55 Calculating exposure levels Power into the antenna = 43 dbm = 20 W Antenna gain = 16 dbi = 40 Distance from the antenna = 10 m S = P G 4 d² W/m² S = 20 x ² = 0.64 W/m² 54

56 Calculating compliance distances Power into the antenna = 43 dbm Antenna gain = 16 dbi Limit at 900 MHz for the public = 900/200 = 4.5 W/m 2 55

57 Calculating compliance distances Power into the antenna = 43 dbm = 20 W Antenna gain = 16 dbi = 40 Limit at 900 MHz for the public = 900/200 = 4.5 W/m 2 d = P G 4 S (m) d = 20 x = 3.8 m 56

58 Reporting Results: Public Communication Who is the audience? Provide the actual reading and value relative to the exposure limit: Fraction of the exposure limit or times below the limit. Compare based on power density Provide an estimate of uncertainty Use pictures, charts and simple tables where possible 57

59 Example: UK Mobile Phone Base Station Audit Operators declare compliance Regulator measures sample of sites: On-going program 2001 to 2012 Now only on request Emphasis on schools and hospitals 58

60 Human exposure limit summary In this session we have: Learned about the international exposure limits for workers and the public Learned about methods for assessing compliance Learned about important considerations when choosing measurement equipment Learned about how to present measurement results 59