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Low Dose Linearity and Hormesis for Effects of Radiation and Chemicals

Low Dose Linearity and Hormesis for Effects of Radiation and Chemicals. Lecture at UC Berkeley 2:00 p.m. Wednesday February 14th, 2001 by Richard Wilson Mallinckrodt Research Professor of Physics Harvard University. LINEARITY AT LOW DOSES IS USUAL!!. Walking blindfold across

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Low Dose Linearity and Hormesis for Effects of Radiation and Chemicals

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  1. Low Dose Linearity and Hormesis for Effects of Radiation and Chemicals Lecture at UC Berkeley 2:00 p.m. Wednesday February 14th, 2001 by Richard Wilson Mallinckrodt Research Professor of Physics Harvard University

  2. LINEARITYAT LOW DOSESIS USUAL!! Walking blindfold across University Avenue is safe: (Risk (R) = 0) IF THERE ARE NO CARS! The risk (R) increases roughly in proportion to the number of cars.

  3. I will argue that:---- • Low dose linearity is common in societal risks • Contrast Acute and Chronic Effects • Cancers caused by pollution • look like other cancers • 30% of people get cancer These are enough to set LINEARITY as the DEFAULT

  4. Acute Effects Characteristics • One dose or dose accumulated in a short time KILLS • 1/10 the dose repeated 10 times DOES NOT KILL

  5. CHRONIC EFFECTS including CANCER Characteristics A dose just sub-acute can give effects if repeated. Usually not all people affected - dose response is flatter

  6. Typically an accumulated Chronic Dose = Acute LD50 gives CANCER to 10% of the population. E.g. LD50 for radiation is about 350 Rems. At 350 Rems about 10% of exposed get cancer. (more or less depending on rate of exposure)

  7. Development of a Model PROPOSITION: ANY ESTIMATE OF RISK implies A MODEL (simplest: next year will be like last year)

  8. Early Optimism Based on Poisons There is a threshold below which nothing happens ______BUT:____ J.G. Crowther 1924 For radiation cancers: Probability of Ionizing a Cell Linear with Dose

  9. Repair Mechanisms • Thousands of CELLS are MODIFIED each SECOND • NOT ALL LEAD to CANCER THEREFORE : REPAIR OR REJECTION MECHANISMS MUST EXIST

  10. Repair Mechanisms BUT Does the Mechanism Reject/Repair: ALL DAMAGED CELLS UP TO XXXX? (implying a threshold) OR 99.999% of CELLS INDEPENDENT OF DOSE WE DON’T KNOW

  11. SINCE 1970 ATTEMPTS to REDUCE RISKS TO ONE IN A MILLION PER LIFE ________________ THIS LEADS TO A BATTLE

  12. THE BATTLE LINEAR Physicists Academics Environmentalists THRESHOLD Industry Biologists Toxicologists REGULATORS IN THE MIDDLE

  13. CHEMICAL INDUSTRY SPENT • MILLIONS OF DOLLARS to prove that • ORGAN DOSE/APPLIED DOSE • ----> 0 • as • APPLIED DOSE • ----> 0

  14. But look at DNA adducts AN ADDUCT DOES NOT LEAD TO CANCER BUT DOES PROVE THAT THE CHEMICAL REACHED THE CELL

  15. DNA adducts are (nearly) linear with applied dose over 5 orders of magnitude! (FOR SOME SITES AND CANCERS)

  16. GENERAL MODELS ARMITAGE & DOLL, 1954/7 dN/N = p1p2.....pk-1pk =a1t a2t.....ak-1t ak(dt) CANCER RATE = dN/Ndt = A tk-1 Ln Rate = ln A + k-1 lnt

  17. ARMITAGE & DOLLexplained AGE DISTRIBUTION OF CANCERS LATENCY SYNERGISM AT HIGH DOSES

  18. The POLLUTANT IS PRESUMED TO ACT TO INCREASE PROBABILITYAT ONE STAGE (OTHER STAGES CAUSED by NATURAL BACKGROUND)

  19. When we add a chemical or radiationIS THE FORMULA:

  20. CRITICAL ISSUES FOR LINEARITY • The POLLUTANT ACTS • in the same way as • WHATEVER ELSEINFLUENCES THE • CANCER RATE • CANCERS CAUSED BY • THE POLLUTANT • ARE INDISTINGUISHABLE FROM OTHER CANCERS

  21. CANCER RATE is ANCHORED at HIGH DOSESso that the slope of dose-response(RISK) may be greater than the simple line to zero

  22. NOTHING SAID ABOVE SAYS WHETHER THE SLOPE IS + POSITIVE OR  NEGATIVE

  23. THE POLLUTANTgives CANCER AT HIGH DOSES So: DEFAULT SLOPE is POSITIVE BUT… IF ANTICANCER EFFECTS CAN BE DEFINITIVELY SHOWN AT LOWER DOSE DEFAULT SLOPE SWITCH TO NEGATIVE

  24. e.g ARSENIC THE ARGUMENTS APPLY TO ANY CARCINOGEN

  25. Arsenic risk • Skin lesions may be unique • There may be a threshold at • 50 -150 ppb • (Data from Taiwan and also from Inner Mongolia) • BUT • Internal cancers may be different

  26. Arsenic risk • For internal cancers • At 500 ppb Measured Risk • (Chile) is 10% • If linear, • risk is one in a million • at 5 parts per trillion!! • “background” is about • 2 parts per billion

  27. MANY TOXICOLOGISTS insist that: Animal (rodent) experiments show that there must be a threshold. BUT WHERE? (Toxicologists failed to predict human cancers for 100 years!) If at 1 ppb it might as well be ZERO.

  28. NOTHING SAID ABOVE SAYS THAT THE MEDICAL OUTCOME IS CANCER it applies to all chronic effects Reduction in lung function caused by air pollution

  29. RISK due toLIFETIME exposure toAIR POLLUTIONis 3 to 5%in the USA!

  30. It is not only mutagens that show linearity Linearity may be usual Consider Non-carcinogens and carcinogens SIMILARLY!!!!

  31. MY CONCLUSION(REPEAT OF 20 YEARS AGO) IT IS NOT POSSIBLE TO REGULATE A ONE IN A MILLION LIFETIME RISK CONSISTENTLY • ATTEMPTS TO DO SO • ARE • ARBITRARY • and • CAPRICIOUS

  32. Many Legislators still want< 1 in a million! Where does this leave regulators??? IN THE MIDDLE (as usual) BUT scientists should keep telling the legislatures the facts of life!

  33. USE COST-BENEFIT ANALYSIS Reduce exposures if it costs less than: Nuclear Regulatory Commission $2,000 per Man Rem US EPA $5.1 million per statistical life

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