[ RadSafe ] LNT and the WHO guidance on red meat

[jjshonka at shonka.com]

All.  




I appreciate the humor, but my question was serious.  


The EPA sets guidelines for all federal agencies, including the NRC.  EPA published Guidelines for Carcinogen Risk Assessment in 2005 ( http://www2.epa.gov/risk/guidelines-carcinogen-risk-assessment ).  I am trying to understand when regulators should use linear extrapolation from POD (point of departure, where you have data and uncertainty bounds) and when non-linear can be used.  I think EPA makes a conservative assumption that carcinogens that directly affect DNA can cause cancer regardless of the dose.  The non-linear approach (to extrapolation from known data to lower levels) is only used when the agent is non-mutagenic ( my reading of Section 3.3.1 of that report).  For EPA, linearity is considered health protective.  


As an aside, if one limits the consumption of farm raised fish for PCB concerns, and consumers substitute red meat as a protein source, cardiovascular disease (instead of cancer) from the red meat may cause more deaths than are saved by avoiding fish with PCBs.  Thus, linearity might, at times, be non-protective.  Hormesis aside, radiation does not seem to fall into that category since there would be no substitution if a source of radiation exposure is avoided.  (use of surgery instead of x-ray imaging is not a realistic option).




There seems to be a complication for radiation.  If EPA accepts that non-linearity has been observed for radiation, a reference dose based on a threshold, a daily exposure that would not cause harm, might be established using the 2005 approach.  That is what some are advocating.  Unlike potential exposure pathways such as airborne contamination of carcinogens, radiation has many exposure pathways and it would be difficult to prevent a sum of exposures exceeding any reference dose. 


If one assumes that a reference dose for radiation is 1 mSv/a, there are many sources whose sum exceed that reference dose.  The sum (and variability) of three sources alone: natural background at ground level, medical exposure, exposure from flight, (as well as what seems to be trivial exposures from most licensed uses of radiation such as nuclear power) can easily exceed 10 mSv/a for some individuals (not the average).  How does one control that sum and which element contributing to the sum has to be subject to further controls?  In my view, a reference dose of at least 100 mSv would be needed to avoid conflict between various low level contributors to exposure, but that value (100 mSv) is unlikely to be accepted.  Or is the reference dose only applied to the average exposure, with outlier individuals not provided that protection?    ​




Joe Shonka