A spot of Real DOOMWATCH today from Joshua Priest, who is a chemistry student and it's definitely Food for Thought...
"The release of Iodine into the atmosphere (probably from purposeful venting of the reactor core as the pressure built up due to Zircalloy breakdown to form hydrogen gas) was substantial in terms of activity however its dispersion, as the volume released would not be very great I would assume, mainly as venting like this is instantly measurable and normally attracts scrutiny and criticism (and rightfully so considering some of the safety failings, such as problems with backup generators delivered to the sites which would have averted any major issues to health) and thus when diluted in the atmosphere the dose on the ground would be very low and difficult to measure with accuracy, a meaningful result would thus require long background and research count times in a specific area as natural radon levels, which are the main natural source of radiation to humans varies from house to house in some areas depending on underground geology and vent sites. The risk to people therefore comes not from added soil radioactivity, practically zero, but from bioaccumulation over time, e.g. from soil grazing animals such as cows, Iodine is soluble in water and thus milk and is transferable via ingestion, where thyroid accumulation significantly increases cancer risks.
Very important is the fact of the short half life of this material (8 days for 131, 50ish off the top of my head for 125) causes it to be highly radioactive (decays and releases radiation quickly) but also to decay extremely quickly, problematic considering both the long count time runs and detection limits (bad for Geiger muller tubes even when the correct energy of radioactive particle is selected), also beta decay is even less efficient for Geiger muller measurement with lower counting efficiencies than for gamma.
Iodine 129 has a very long half life (and is produced inside nuclear reactor cores), and although this means it is less radioactive in terms of dose per second it will stay in the environment and ingestion will affect health, this low dose is fine outside you body, however in the cells themselves it will increase thyroidal cancer rates however its effects will be much less pronounced than the more radioactive forms, the low activity will make this very difficult to measure also.
Finally as an interesting fact, the amount of radiation (in total) aspirated into the atmosphere by a coal fired power station each year is higher than the amount of radioactivity produced by a nuclear reactor generating the same amount of electrical energy, also this radioactivity (classified Naturally Occurring Radioactive Material NORM) is not required to be altered in any way, and other fossil fuel such as oil, which again is extracted with significant radioactivity can be dumped over the side into the sea, and in the case of coal ash, is placed in cements and is not disposed of in the way nuclear waste of the same activity (classed as intermediate level waste) must be."
With thanks to Joshua Priest