[ RadSafe ] lethal amount of Po-210

[Rainer.Facius at dlr.de]

All:

Given the large size of the ballpark of current "guesstimates" (micro-
to several milligram), I think we can rather confidently narrow down to
a factor of 10 the uncertainty about the amount of Po-210 necessary to
kill within an exposure time of 20 days. In doing so I ignore a possible
additional chemo-toxicity since that would only decrease that amount.

To begin with, effective dose is irrelevant as it is based on radiation-
and tissue weighting factors for late cancer mortality, the former
relating to (large) RBE-max values, i.e., estimates of RBE for
malignancy at vanishing doses.

For killing of mammalian cells we have enough data to estimate the RBE
of alpha particles. At large doses this is rarely significantly larger
than 2. On the other hand the protraction of exposure over 20 days might
lead to a DREF (dose rate effectiveness factor) of about two so that in
effect the equivalent lethal dose would be equal to the (physical)
absorbed dose. 

With an LD-50,30 of about 3.3 Gy for hematopoetic, 8 Gy for pulmonary
and 15 Gy for gastrointestinal failure, 8 Gy whole body exposure is
within a factor of 2 a safe guess as for the lethal absorbed dose.

To deliver this 8 Gy evenly to a 75 kg body (600 J) we need in this body
6.93*10^14 decays at 5.407 MeV per decay. If the biological retention
time of Po were infinite, then an initial load of 2.53 microgram Po-210
(=7.27*10^15 atoms) would suffice to produce that many decays in 20
days. The largest uncertainty for me is this biological retention time.

For a retention half-time of 
01 d: 33.7 microgram (=9.65*10^16 atoms), 
03 d: 11.5 microgram (=3.29*10^16 atoms), 
09 d: 4.91 microgram (=1.41*10^16 atoms), 
27 d: 3.23 microgram (=9.25*10^15 atoms), 

as an initial load would produce again the 6.93*10^14 decays within 20
days. This reflects the uncertainty of a factor of ten mentioned above. 

With 10^14 cells per human body these numbers correspond to some 100 to
1000 atoms per cell if evenly distributed within the body.

Unless we have reasons to postulate that Po selectively accumulates and
is retained in tissue whose destruction is immaterial for the survival
of the individual, deviations from an even distribution in one organ
would be balanced by the opposite deviation in another organ.

On the microscopic scale the volume irradiated by an alpha particle of
5.4 MeV would at most be that of a "decay-"sphere of about 42 micrometer
radius, i.e., 3.1*10^-10 l. The 6.93*10^14 decays result in a
microscopic distribution where on average 2900 alpha decays occurred in
this decay-sphere - i.e., a rather dense and even spatial distribution
on a microscopic scale too.

Regarding chemo-toxicity I found (DOI: 10.1542/peds.2005-0172) that a 20
month old boy of 11.2 kg recovered from a severe acute Tellurium
toxication (the chemically next of kin of Polonium) which 8 hours
postingestion yielded a large blood concentration of 200 microgram/l.
Given that, it is hardly conceivable that the above minute Po
concentrations (0.1 microgram/kg) would seriously affect a healthy adult
by its chemistry. 

Let's see how my guess looks in the light of data coming up during the
autopsy tomorrow.

Regards, Rainer

Dr. Rainer Facius
German Aerospace Center
Institute of Aerospace Medicine
Linder Hoehe
51147 Koeln
GERMANY
Voice: +49 2203 601 3147 or 3150
FAX:   +49 2203 61970