The Hong Kong Observatory detects minute amounts of Cs-137 in air for the first time in HK

From TheStandard.Com.HK

"The observatory said yesterday that a "very minute amount" of cesium-137 was detected at King's Park during a 24-hour period ending Saturday.  The concentration was 0.000067 becquerels per cubic meter, only 1/37-millionth of the level that requires protective action."

If accurate, Cs-137 activity at those levels are not a concern.  Their stated safety threshold in this article is a different story:

"The danger level is 2,480 becquerels per cubic meter, according to the observatory."

Wow. That's more Cs-137 than is currently being detected in air outside the Fukushima plant.  This must be another typo, either that or these guys are extremely generous with allowable exposures for the public.

Let's assume that through the course of a day, a person ingests (via inhalation, consumption, etc.) a quantity of Cs-137 particulate that yields 2480 Bq.  For an adult, the average exchange of air in one day from breathing comes out to about 7200 liters of air - a lot of volume. Through normal breathing over the course of the day, it is conceivable that a fair amount of radioactive aerosol could find its way into our bodies, but how would this translate into an actual absorbed dose? 

Source: Wikipedia

If you notice the diagram above, there are a couple of steps that this isotope takes before it reaches a stable state.  The first action comes by way of an ejected electron (beta ray), where an electron with an energy of 0.5120 million electron volts is discharged 94.6% of the time, and an electron with an energy of 1.174 million electron volts is discharged just 5.4% of the time.  Let's calculate absorbed doses for the beta component of these decays first, taking both electron energies into account:

• The beta contribution of 2480 Bq of Cs-137 at a distance of 1cm (or internally at 0.5cm dmax) will yield 0.00049965 Sieverts per hour. 
• Converted to rem, that comes out to 49.965 milli rem per hour.  That amounts to 1199 milli rem (1.199 rem, 11.99 milli sieverts) per day. 

Why so high?  2480 Bq is a considerable amount of activity.  It literally amounts to 2480 electrons at  energies listed above blasting within your body per second, and this will continue on until it is excreted from the body.  Cs-137 has a biological half-life of 70 days, so in about a years time, less than 5% should remain in our system.

The second calculation is less significant in dose, but we must also add it to the numbers above to get the whole picture.  This dose is derived from the gamma aspect of the Cs-137 decay.  We are primarily dealing with a photon with an energy of 0.6617 million electron volts which has no charge, and can travel longer distances through air unimpeded before they strike us and deposit dose.

• The gamma contribution of 2480 Bq of Cs-137 at a distance of 1cm will yield 1.894889E-006 sieverts per hour. 
• Converted to rem, this comes out to 0.1894889 milli rem per hour, or 4.5477 milli rem per day.

In sum, a total of 1.203 rem per day, a ridiculously high dose.