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Health Physics 563-613B

Health Physics 563-613B. Risk and Dose Limits Michael Evans Lecture 3 (Sep. 24, 2004). RISK. Hazard: anything that can cause harm ( e.g., chemicals, electricity, working from ladders, etc.. Risk: is the chance, high or low, that somebody will be harmed by the hazard American Cancer Society

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Health Physics 563-613B

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  1. Health Physics563-613B Risk and Dose LimitsMichael EvansLecture 3 (Sep. 24, 2004)

  2. RISK Hazard: anything that can cause harm ( e.g., chemicals, electricity, working from ladders, etc.. Risk: is the chance, high or low, that somebody will be harmed by the hazard American Cancer Society www.cancer.org

  3. Cause-consequenceAnalysis Cause-consequence analysis revolves around decision trees and the assumption that truly independent variables contribute to occurrences and outcomes. That is, what independent things must conspire together to bring about an event, and having occurred, what are the possible outcomes.

  4. RISKFault Tree A "fault" tree is effectively a statement of what events have to conspire together to bring about an undesired outcome. Traditionally these have been drawn top-down and therefore the undesired event known as the "top event". Because of the logical hierarchy of the items, it can be sometimes seen as a form of time sequence going from the bottom towards the top of the page.

  5. RISK : Differing Perceptions

  6. Nuclear Industry Advertisement @ 1970s Many different types of radiation may have different effects on cancer rates. Experiments with humans cannot be set up so effects are not well known. Different age and sex groups vary in sensitivity. A single big dose has a greater net effect over time than the same dose spread out over small increments. Extrapolation from high doses: we don't know how to extrapolate to low doses.

  7. RISK: Perception Risk is the probability of consequence. The perception of risk is determined by how the individual views its probability and its severity

  8. RISK: Perceptionand Decision Tables

  9. RISK PERCEPTION Smoking( 45,000 deaths per year - Canada) Smoke Alarms(368,000 home fires US, 16,975 injuries and $5.5 billion in property damage) Fire Extinguishers(85% of fire-related deaths occur in home fires) Radon(from 7,000 to 30,000 deaths per year -USEPA) Driving(Nearly 3,000 people died and 217,614 were injured Canada)

  10. RISK • Understanding Risk (Fischoff et al.) • Assessing risk involves decisions. • No approach is comprehensive - there are always biases. • Personal beliefs affect our interpretation of fact. • determining factors in risk decisions relate to the definition of the problem ( consequences, uncertainties…). • Decisions may be influenced through subtleties in the wording of the problem or the question.

  11. Data Interpretation:Radon

  12. Radon- Canadian recommendations Although provincial and territorial governments have jurisdiction over the health effects of background radiation, Health Canada has recommended that the guideline for exposure to radon gas should be 800 becquerels per cubic metre as the annual average concentration in a normal living area. The guideline is an upper limit, and Health Canada recommends taking action to reduce radon levels in your home if they exceed the limit. Because there is some risk at any level of radon exposure, homeowners may want to reduce their exposure to radon, regardless of levels.

  13. Radon- US EPA recommendations Radon is a radioactive gas and has been identified as a leading cause of lung cancer, second only to cigarette smoking in the United States. EPA's most recent health risk assessment estimates that 21,000 lung cancer deaths each year are due to radon. Test your home for radon, and have it fixed if it is at or above EPA's Action Level of 4 picocuries per liter. You may want to take action if the levels are in the range of 2-4 picocuries per liter. Generally, levels can be brought below 2 pCi/l fairly simply.

  14. Health Effects : Radiation Possible health effects from exposure to radiation may include cancer, birth defects in future generations and cataracts. These effects have been observed in studies of medical radiologists, uranium miners, radium dial painters, radiotherapy patients and atomic bomb survivors Observations are based on doses that are much higher than workers who are occupationally exposed. The Linear-non-threshold theory has not been reasonably proven down to low doses, and there have been no strong cause-effect studies showing relationships between low doses and health effects. Still prudent to assume that there are some health effects at lower doses

  15. Health Effects : Radiation Genetic damage may be caused by low doses although no direct evidence of this exists. Acute radiation exposure may cause both prompt and delayed effects. Chronic exposure ( small doses delivered over long time periods) may cause delayed effects but not prompt effects.

  16. Health Effects : Radiation Prompt effects: observable after large doses in a short period of time. 450 cSv in one hour will cause vomiting and diarrhea in a few hours; loss of hair fever and weight loss within a few weeks: 50 % chance of death within 60 days without medical treatment (LD 50-60). Delayed effects such as cancer may occur years after exposure to radiation. Genetic effects can occur when there is damage to the genetic material. Effects may show up in future generations although to date there have been no clear observations indicating genetic effects caused by radiation in human populations.

  17. Cancer Induction • Radiation can damage cell chromosomes causing the cell to undergo uncontrolled growth (malignancy). • Radiation may suppress the bodies’ natural immune system causing a reduce resistance to existing viruses which can multiply and damage cells • Radiation may activate viruses dormant in the body which then attack cell control mechanisms causing rapid growth

  18. Effects Deterministic effects : describe organ killing or dysfunction that is thought to have a certain threshold dose above which the effect is almost certain to appear. Stochastic effects : describe random effects which can be predicted for,populations and not for individuals. They are usually applied to low dose & dose-rate situations. Stochastic effects may occur in the individual and are termed somatic ( cancer induction ). Stochastic effects which are passed on to future generations in the germinal cells are hereditary or genetic.

  19. Risks of RadiationDose(mSv) vs Possible Effects 10 000 instantaneous Immediate illness and possible death if untreated. 1 000 instantaneous Illness (nausea) but death unlikely. 1 000 chronic Long term cancer induction in one in 20 - 25 persons. • No evidence of consequences - NEW yearly limit CNSC. • Natural background level for some populations - India. 2.2 Average annual dose received by all Canadians. No observable effects. 1 Yearly limit to the public with respect to CNSC licensed facilities.

  20. Risks of Radiation Dose ( mSv) Possible Effects 5000 - 8000 to gonads Permanent sterility in males and females 200 to testes Temporary reduction in sperm count No effect on sexual function < 50 whole body No observed effect on fertility sexual function

  21. Population groups Japanese A-bomb - acute victims and survivors Patients exposed to radiation for medical reasons - TB using lung and thymus irradiation - ankylosing spondilitis - secondary cancers from radiation therapy (Hodgkins, contralateral breast…) - Thorotrast and Radium 224 studies Workers - uranium miners - radium dial painters - U.S. Nuclear Navy Shipyard (NNS) workers

  22. Population groups Japanese A-bomb -dosimetry is unsure -single ethnic population makes worldwide extrapolation difficult -high dose & dose-rate Patients exposed to radiation for medical reasons -data is often anecdotal -genetic effects are not reliably recorded Workers -other factors such as cigarette smoking, toxins in the work environment may confound results

  23. Population data from lab studies

  24. Linear No-Threshold Theory(LNT) Extrapolation from the high dose region down to the low dose region causes problems in estimating the effects of low-level radiation

  25. Model Fitting • No Threshold • Classic LNT (Linear No-Threshold Theory) • Parabolic limiting • Hormesis

  26. Central Requirements for Radiation Protection ICRP 26: • No practice shall be adopted unless its introduction produces a net benefit • All exposures shall be kept as low as reasonably achievable, economic and social factors being taken into account (ALARA) 3 The dose equivalent to individuals shall not exceed the limits recommended for the appropriate circumstances by the Commission Benefits Detriment

  27. Central Requirements for Radiation Protection ICRP 60 § 115 - 129 : Justification (net benefit) Optimization (ALARA) Dose Limits Potential exposures Benefits Detriment

  28. Basic Units • Exposure • Total charge of ions of one sign produced in air • C/kg, 1 R = 1 esu/cm3 air = 2.58 x 10-4 C/kg • Absorbed dose • Energy absorbed per unit mass • 1 Gy = 1 J/kg = 100 rad • 1 cGy ≈ 1 R (numerically)

  29. n Equivalent dose • Not all radiation produces the same endpoint • 1 mGy of photons or 1 mGy of neutrons • radiation weighting factors used to modify the absorbed dose to express the same levels of risk between radiations • units: 1 Sv = 1 J/kg = 100 rem

  30. Effective dose • Not all tissues have the same risks • 1 mSv on lungs or 1 mSv on the large intestine • tissue weighting factors used to modify the equivalent dose to express the same levels of risk between tissues • units: 1 Sv = 1 J/kg = 100 rem

  31. Estimates of Cancer Incidencefrom Low-Level Radiation Number of additional cancers estimated to occur in 1 million people after exposure of each to 10 mSv of radiation Source BEIR, 1980 160 - 450 ICRP, 1977 200 UNSCEAR, 1977 150 - 350 Average for this example - 300

  32. Estimates of Cancer Incidencefrom Low-Level Radiation American Cancer Society estimates that 25% of all adults in the 20 - 65 year group will develop cancer from all causes ( smoking, food, drugs, lifestyle …). A population of 10, 000 not occupationally exposed to radiation will express 2,500 cases of cancer. If all 10, 000 were exposed to 10 mSv we estimate 3 extra cases to develop. (Cancer rate has increased from 25% to 25.03 %). A lifetime occupational dose of 100 mSv might increase chances to 25.3% and 1000 mSv to 28%. (This assumes a simple linear model).

  33. Risks and Collective Dose Collective dose is the sum of all dose received by all workers in a defined population. If 100 workers out of a population of 200 each receive a dose of 2 mSv, the average dose per worker is 1 mSv and the collective dose is 200 person- mSv. The total additional risk is assumed to depend upon the collective dose.

  34. Risks and Collective Dose For radiation protection the risk is assumed to be proportional to the amount of dose - not the rate at which it is received. For a given collective dose the risk is assumed to be the same even if a large number of people share the dose. Therefore spreading out the dose on a large population may reduce the individual risk, but not that of the whole population.

  35. Estimates of Collective Dose Source Average individual dose (U.S.) mSv / year Natural background 1.0 Releases - mining, milling etc. 0.05 Medical 0.54 Nuclear fallout 0.08 Nuclear Energy 0.003 Radon Gas & Daughters 1.98 Total 3.652 The average individual in the population receives about 3.6 mSv from natural and man-made environment.

  36. Risk and dose limits The risk associated with 1 mSv/year is calculated to build up to 1 extra cancer death in 100,000 persons per year after sustaining many such years of exposure. (BEIR, AAPM 18) Canada 2004 New Cases of Cancer: 458/100,000 Cancer Deaths : 215 / 100,00 Or a 1/1 million chance per 0.1 mSv/year

  37. Comparing Health Risks Health Risk Days of life expectancy lost Smoking 20/day 2,370 (6.5 years) Overweight 20% 985 (2.7 years) All accidents 435 (1.2 years) Auto accidents 200 Alcohol 130 Home accidents 95 Drowning 41 Natural background radiation 8 Diagnostic X-rays 6 Catastrophes (earthquakes…) 3.5 10 mSv occupational 1

  38. Cancer death probabilityICRP 60 C.2 Simple additive model. The excess probability rate after a single dose, D, is assumed to be proportional to the dose, but first after a minimum latency period and over a ‘plateau” period of time.

  39. Cancer death probabilityICRP 60 C.2 Simple multiplicative model. The excess probability after a single dose, D, is also assumed to be proportional to the background rate of cancer death , B (u). Age (u)

  40. Mortality AttributesICRP 60 pp153 & C.8 Choice of dose limits: Lifetime death probability. Time lost due to death Reduction of life expectancy Annual distribution of death probability Increase in age specific mortality C.8 Change in the total conditional death probability rate after an exposure of 50 mSv per year from age 18 to age 65 years.

  41. ICRP 60 (§ 89)Probability Co-efficients for stochastic effects Exposed Population Detriment Adult Whole Workers Population Fatal cancer 4%/Sv 5%/Sv Non-fatal cancer 0.8%/Sv 1.0%/Sv Severe hereditary Effects 0.8%/Sv 1.3%/Sv Total 5.6%/sv 7.3%/Sv Values between populations differ due to differences in age distributions.

  42. Historical dose limits Maximum permissible doses have been applied to four population groups: Occupationally exposed persons Members of the public near radiation sources The whole population (average dose) The developing fetus

  43. Historical dose limits USA Occupational Maximum Permissible Dose 1934 200 mrem (2 mSv) / day ICRP 1936 100 mrem (1 mSv) / day NCRP 1948 300 mrem (3 mSv) / week NCRP 1957 * 100 mrem (1 mSv) / week NCRP * similar to current value

  44. ICRP 60 (§ 194)Recommended dose limits Application Occupational Public Effective dose 20 mSv/y 1 mSv/y averaged over defined periods of 5 years (not to exceed 50 mSv in any single year). Annual equivalent dose Lens of eye 150 mSv 15 mSv Skin 500 mSv 50 mSv Hands and feet 500 mSv Effective dose (E): sum of weighted equivalent doses in all the tissues and organs of the body.. Equivalent dose (H): average dose in tissue or organ multiplied by the radiation weighting factor.

  45. atomic radiation worker mSv/quarter mSv/year public mSv/year whole body gonads, bone marrow 30 50 * 5 ‡ 150 bone, skin, thyroid 300 30 extremities (hands, feet) 380 750 30 15 lungs, eyes & single organs 80 150 pregnant ARW 10 mSv to abdomen for balance 0.6 mSv per two weeks * effective dose ‡ equivalent dose “Old” Canadian Dose Limits Atomic Radiation Control Act Amended SOR/85–355

  46. NRC Dose Limits

  47. Workers Limits • Why are workers limits 20x higher? • monitored by Health Canada • most do not actually receive this limit • public includes children • occupational hazard • benefit of working • radiation workers are a small fraction of the total population and the additional health burden on society is acceptable

  48. Occupational exposures This can all seem a bit abstract but…

  49. Occupational exposures Radiation is invisible to our senses of sight, feel, hear, taste, smell.

  50. ALARA / ALARP As Low As Reasonably Achievable / Practicable ( economic and social factors being taken into account) The key word is “reasonable”. In radiation safety there are three basic principles. Distance : 1/d 2 Time : 1/t Shielding : e -mx

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