Environmental Radiation Risk Assessment

Transcription

1 Environmental Radiation Risk Assessment Jerome Puskin, PhD Center for Science & Risk Assessment Radiation Protection Division Office of Radiation and Indoor Air (ORIA)

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3 Outline 1. Ionizing radiation definitions, exposure 2. Scientific basis for radiogenic cancer induction 3. Estimating radiogenic cancers risks 4. Examples of EPA s use of radiation risk estimates 3

4 Definitions Ionizing radiation: Radiation with sufficient energy to remove electrons from atoms in a medium through which it passes LET (Linear Energy Transfer): energy lost per unit path length for a charged particle Radionuclide: Atom with an unstable nucleus that decays with a characteristic half-life, producing radioactive emissions of particular types and energies 4

5 Types of Radioactive Emissions Alpha-particles: 4 He nuclei, high-let, very shortrange Beta-particles: electrons, low-let, short-range Gamma-rays: photons, low-let, penetrating 5

6 Radiation Exposure 6

7 Radiation Dose Absorbed dose (D) = energy deposited per unit mass [SI unit, Gy ; 1 Gy =100 rad] Dose equivalent: H= w R x D Units: rem, Sv For photons & electrons, w R =1 For α-particles, w R =20 Effective dose = w T H T 7

8 Average Radiation Dose from Different Sources of Exposures Exposure Category Average Annual Effective Dose (mrem) Radon and thoron 228 Other ubiquitous background (cosmic rays, terrestrial radiation, ingested radionuclides) Medical (CT, conventional radiography, nuclear medicine, interventional fluoroscopy) Consumer (tobacco, building materials, air travel) Other (occupational, industrial, security, nuclear power) Total 627 8

9 Scientific basis for radiogenic cancer induction 9

10 Mechanistic Basis Radiation DNA damage Mutation in a (single) somatic cell Cancer Recent data have called this picture into question (non-targeted effects) 10

11 Single tracks of low - LET or high- LET radiation can produce Complex Clustered Damage in DNA Two examples of Complex Clustered Damage in DNA Parts of: Electron track Alpha-particle track [ Goodhead, IJRB 65, 7 (1994) ] DTG

12 Laboratory Studies Radiation has been found to induce In cells: mutations, chromosome damage, cell transformation, cell killing In animals: cancer in irradiated tissues 12

13 Dose Response 13

14 High-LET Effects Generally, same end-points as low-let High-LET more effective: Relative Biological Effectiveness (RBE) usually taken to be 20 for low doses of α-particles No reduction in effectiveness at low doses and dose rates 14

15 Epidemiological Studies 15

16 Lifespan Study (LSS) of Japanese A-Bomb Survivors Large population (120,000 survivors) Both genders, all ages ATB All organs irradiated Wide range of doses (instantaneous, mostly γ-rays) Good dose estimates Long, detailed medical follow-up 16

17 A-Bomb Dosimetry Dose estimate for each survivor by organ Based on location at the time of bombing Takes into account shielding by buildings and by the body 17

18 Leukemia in Bomb Survivors First cancers to be observed Short latency Highly radiogenic, low background rate Most of the risk disappears after years 18

19 Leukemia in A-Bomb Survivors 19

20 Solid Cancer Risks in A-Bomb Survivors 20

21 Solid Cancers: Variation in Risk with Age 21

22 Summary of Results on Solid Cancers in the Japanese A-Bomb Survivors 1. Risk nearly proportional to dose with no apparent threshold 2. Significant risk at doses > rad 3. Excess cancers seen for numerous sites 4. Risks decrease with age at exposure 5. Risks increase with attained age 22

23 Information from Other Epidemiological Studies Ancillary information on: Fractionated and chronic exposures α-particles Specific organ risks Western populations 23

24 Cohorts Exposed Medically 1. Fluoroscopy for TB (fractionated x-ray doses) breast cancer* 2. Treatment for chronic mastitis breast cancer* 3. Thymus irradiation, tinea capitis thyroid cancer* 4. Ankylosing spondylitis leukemia et al. 5. Peptic ulcers stomach cancer Ra injections for bone disease (α) bone cancer* 7. Thorotrast patients (α) leukemia*, bone*, liver* 8. Prenatal x-rays childhood cancer* 24

25 Occupational Cohorts Chronic or episodic exposures 1. Nuclear workers (N. American & European) leukemia 2. Radium dial painters bone 3. Radiologists and radiological technicians leukemia, breast 4. Underground miners (radon) lung* 5. Mayak workers (α,γ) lung*, liver*, bone, leukemia 6. Chernobyl clean-up workers leukemia 25

26 Environmentally Exposed Cohorts Chronic exposures, require dose reconstruction 1. Marshall Islanders thyroid effects from radioiodines 2. Techa River cohort leukemia, solid cancers ; data > risks per unit dose similar to LSS 3. Chernobyl population childhood thyroid cancer 4. Taiwanese residents of apartments built with 60 Co-contaminated steel leukemia, breast 26

27 Human Evidence for Radiation-Induced Cancers at Low Doses Excess cancers in A-bomb survivors demonstrated at (acute) doses of rad Evidence for cancer risk at 1-3 rad seen in studies of: Children exposed to prenatal x-rays Patients exposed to highly fractionated diagnostic doses of x-rays Occupationally and environmentally exposed cohorts indicate that chronic radiation is also carcinogenic 27

28 Estimating Radiogenic Cancer Risk in the U.S. Population 28

29 Calculation of Cancer Risk Estimates for U.S. Population 1. Mathematical models for each cancer site based on best fit to human epidemiology a. Incorporate dependence on age, time, and gender. b. Assume 5-yr minimum latency for solid cancers 2. For extrapolation to low doses and dose rates, assume LNT with DDREF 1.5 for low-let 3. Average two different approaches for extrapolating risk from A-bomb survivors to U.S. population 4. Life table corrections for competing risks 29

30 EPA Revised Blue Book 1. EPA Radiogenic Cancer Risk Models and Projections for the U.S. Population 2. Based on National Academy of Sciences report (BEIR VII) 3. Available on EPA web-site: 4. Cancer (average) incidence risk 1x10-3 per rad for uniform, whole-body, low-let radiation a. ~ 10-3 per rem (unit of effective dose) b. Higher for childhood exposures 30

31 Cancer Risk Coefficients for Radionuclides 1) Radionuclide-specific cancer risk coefficients (risk per unit intake for ingestion or inhalation) 2) Risk int = (intake) x (risk/unit intake) 3) To calculate risk coefficients for internal exposure: 31

32 Federal Guidance Report No.13 (EPA 1999) Provides risk coefficients for 800 radionuclides for ingestion, inhalation, and external exposures Age- and gender-dependent intakes, doses, and risk models Available electronically at 32

33 Radiation vs. Chemical Cancer Risk Assessments External exposure pathways for radiation Risk models more detailed dependent on cancer site, age at exposure, attained age Rrisk estimates more reliable Based on extensive human data More theoretical and empirical support for LNT However, low dose extrapolation still a major controversy 33

34 Sources of Uncertainties in Cancer Risk Coefficients 1) Biokinetic and dosimetric models (radionuclide-specific) 2) Dosimetric uncertainties in epidemiology studies 3) Sampling errors in epidemiological data 4) Age and temporal response 5) Extrapolating risks across populations 6) Response at low doses and dose rates (DDREF) 8) RBE of α-particles? 34

35 Examples of EPA s use of radiation risk estimates 35

36 Radionuclides in Drinking Water I. Beta/Photon Emitters Regulation set in 1976; updated in 2000 MCL: maximum 4 mrem/y to any organ, assuming 2L/d intake Whole-body dose of 4 mrem/y for 70 y ~3x10-4 lifetime cancer risk 36

37 Radionuclides in Drinking Water II. Alpha Emitters Biggest concern usually radium ( 226 Ra, 228 Ra) Ra is a Ca analog Incorporates into bone, irradiating bone and bone marrow for many years However, most of the estimated risk is from irradiation of soft tissues MCL of 5 pci/l estimated lifetime risk of 5x10-5 ( 226 Ra), 2x10-4 ( 228 Ra) 37

38 Radon in Homes Radon ( 222 Rn): inert gas from decay of 226 Ra Primary source of indoor Rn is soil gas Main risk: lung cancer caused by inhaled radon decay products, which deposit in the lung and irradiate airways with α-particles The average level in homes is 1.25 pci/l About 6% of all homes exceed the EPA action level of 4 pci/l 38

39 Radon Risk Risk estimates derived from an analysis of 11 miner cohort studies (NAS: BEIR VI) Strong synergism with smoking Estimates supported by case-control studies of lung cancer correlated with radon levels in homes Estimated average risk from lifetime exposure at 4 pci/l is 2%, but depends on smoking 20,000 projected radon-related lung cancers annually in the U.S. 39

40 Contact information: (202)