[ RadSafe ] Indoor Radon

[Scott, Bobby]

Reply for Issue: Radsafe Digest Vol.18, Issue 3

 

To: Philippe Duport

 

Dear Philippe:

 

You state in your e-mail to John Johnson the following:

 

"...LUNG doses from radon decay products, gamma radiation and airborne
ore dust are of comparable magnitude and that, in low grade U mines,
radon decay products contribute less than half the total radiation dose
- a systematic bias in epid studies (see P Duport, Radiation Protection
Dosimetry Vol. 98, no. 3. pp. 329-338, 2002)."

 

I am quite interested in the gamma-ray contribution to the dose as it
appears not to be usually mentioned when discussion radon issues.  My
interest stems from the fact that we have been studying for several
years induced protective effects associated with exposure to low-dose
gamma and other forms of low-LET radiation.  Our research findings
indicate the following with respect to the induction of stochastic
radiobiological effects by low-dose and low-dose-rate gamma irradiation:

 

(1)        Brief exposure at a high rate to low doses of gamma rays can
suppress stochastic radiobiological effects (e.g., genomic instability,
mutations, neoplastic transformation, and cancer).  This is discussed in
two recent papers (Scott 2004, Scott 2005a).

 

(2)        Chronic exposure at low rates to gamma radiation could
greatly suppress the occurrence of stochastic radiobiological effects as
compared to brief exposure at a high rate (Scott 2004, 2005a, 2005b).
For brief exposure at a high rate, suppression in biological effects
could occur for doses up to around 100 mGy.  However after chronic
exposure at low rates, the zone of suppression may be extended to > 400
mGy (Scott 2005b).

 

(3)        The suppression is considered to involve selective removal of
genomically compromised cells via a novel protective apoptosis mediated
(PAM) process (Scott BR 2004).

 

(4)        For gamma-ray doses on the order of 1 mGy, excess cancer risk
extrapolated from high to low doses based on the linear-no-threshold
(LNT) model are likely phantom risks (Scott 2004, 2005a).  Our research
allows evaluating the ratio P_decrease/P_increase (Scott 2005b) at
different low doses. Here the underline "_" is used to indicated a
subscript that follows. P_decrease is the probability for a suppression
(decrease) in the cancer risk and P_increase is the probability for an
increase as would be expected based on using the LNT model to
extrapolate from high-dose data down to the low dose of interest. For
gamma-ray induced cancer after 1 mGy, the central estimate of the
indicated ratio is 9,740,000 (Scott 2005b).  After 0.1 mGy the ratio
increases ten-fold.  However, these are just rough approximations.
However, it can be concluded that the odds for a decrease in cancer risk
after 1 mGy of gamma rays is more than 5 orders of magnitude greater
than for an increase in risk as would be predicted using the LNT model.
Thus, for doses on the order of 1 mGy and lower one should be looking
for evidence for a decrease in cancer risk when conducting
epidemiological studies.

 

(5) Chronic exposure at low rates to gamma rays could suppress cancer
induction by alpha radiation as well as by other carcinogens (Scott
2005a). The usual assumption of additivity of gamma-ray and alpha
radiation risks (based on the LNT model) at very low doses is probably
invalid (Scott 2005a).

 

Based on the above considerations, the inverse dose rate effect
demonstrated for cancer induction by radon may be related to the
gamma-ray component of the dose.  The gamma-ray efficiency for
activating the PAM process would be expected to increase as the
gamma-ray dose rate decreases.  The PAM process would not be expected to
be activated by alpha radiation alone (Scott 2004).

 

 

References:

Scott BR (2004). A biological-based model that links genomic
instability, bystander effects, and adaptive response.  Fundamental and
Molecular Mutagenesis, Mutation Research 568:129-143.

 

Scott RB (2005a). Low dose radiation-induced protective process and
implications for risk assessment, cancer prevention, and cancer therapy.
Nonlinearity in Biology, Toxicology and Medicine (in press).

 

Scott BR (2005b). Low-dose radiation risk extrapolation fallacy
associated with the linear-no-threshold model.  Invited paper, BELLE
Newsletter (submitted).  

 

Sincerely,

Bobby R. Scott

Lovelace Respiratory Research Institute

Albuquerque, NM USA