Inhalation dose
Introduction
For many sites the more likely atmospheric releases release a range of isotopes for which the short term inhalation dose dominates (thyroid dose from inhalation of radio-iodine and whole body dose from a range of nuclides) and it is this dose route that the early protective actions concentrates on reducing.
When we breath in (either through our mouth or our nose) we pull air into our lungs. Along with that air comes the dust and the other gases floating around our head at the time. Our body has several ways to reduce the amount of dust reaching the lungs (the hairs in our nose, for example) and ways of cleaning out the dust that does reach there (it can be transported out of the lungs and swallowed, or absorbed through the lung lining into the body, for example). When we breath out we exhale some of the dust that we breathed in and most of the gases. However, some the dusts and gases may stay in the body for a while. If they are radioactive they will give us a continuing radiation dose until they are removed from the body or decay away naturally.
Dose per Unit Intake Factors
When radioactive material (dust or gas) is breathed in its fate depends partly on its physical and chemical properties. Some of the radioactive material (a large fraction in the case of gases) is not retained and is immediately breathed out. Depending, mainly on the particle size, but also on the chemistry, particles may be deposited on surfaces at various depths into the mouth/nose, throat and lungs. The body has a number of ways of coping with such deposits. They can be exhaled, coughed out, cleared from the lungs and swallowed, absorbed into the blood stream (either through the lungs or GI tract) or remain in the lungs indefinitely. Once in the blood, some chemical forms are excreted, some deposit reasonably uniformly around the body and some are deposited more in one place than another. This is quite complicated.
Sophisticated mathematical models, supported by experiment data, have been developed to model the uptake and retention of radioisotopes and the resulting dose to the body over different periods of time (unlike external radiation you cannot walk away from internal radiation but take it with you until you excrete it or it decays). These models are used to generate tables of dose per unit intake factors for use in dose modelling.
Several organisations publish tables of dose per unit intake factors for inhalation. This includes the ICRP Publication 119 . This gives a quick review of the modelling and then dose per unit intake values (Sv/Bq) for inhalation of a wide range of radionuclides with different chemical properties (based on how long the isotope stays in the lung and the fractional absorption in the gastrointestinal tract).
Inhalation dose is related to the concentration of radioisotopes in air, the time exposed, the breathing rate and the dose per unit intake thus:
DI,A = Σ TIACN x BRA x DPUIN,A
Where:
DI,A is the Inhalation dose for the age group A (Sv)
TIACN is the Time Integrated Air Concentration (Bq.s.m-3) for isotope N
BRA is the Breathing Rate for the age group A (m3.s-1)
DPUIN,A is the dose per unit intake factor for isotope N and age group A (Sv/Bq)
The summation is over all isotopes present
Reducing Inhalation dose
To reduce inhalation dose you must reduce the amount of radioactivity retained in the lungs of people exposed and the easiest way to do this is to prevent inhalation.
Evacation. The most effective protective action in terms of fraction of dose averted is to evacuate the person from the area before the radioactivity arrives. Evacuation may still be appropriate after the plume has arrived if the release is expected to last for long enough after the evacuation would be complete for a suitable dose to be averted. Care is needed when implementing an evacuation when a plume is overhead to ensure that people don’t get a higher dose than they would have had staying in shelter for the duration of the release. The problem with evacuation is that it is difficult to organise and very disruptive of the lives of those involved and therefore stressful
Sheltering is a commonly used protective action which can reduce inhalation dose in the short term. According to the NRPB “In this context sheltering refers to staying indoors, with doors and windows closed and ventilation systems turned off. It provides protection from external irradiation from radioactive material in the air and that deposited on the ground, and from inhalation of radioactive material”. This is easier in a built up area than out on moors or hills.
A useful public information film on shelter-in-place “Protective Actions for Radiation Emergencies - Get Inside, Stay Inside, Stay Tuned” from US Centers for Disease Control and Prevention (CDC) can be found on the CDC website.
It is claimed that a solidly built and reasonably airtight UK house will reduce inhalation of particles by a factor of three. However, it is thought to be relatively ineffective for vapours and the protection only lasts for a few hours. Air exchange between the house and the outside, driven by temperature differences and wind, allows the internal air to become contaminated in time.
Respiratory Protection In the UK it has not been policy to recommend respiratory protection (masks) to the general public as a protective action against inhalation dose in the event of a radiation accident. This should be more widely discussed.
Radiation dose from resuspension
During the original plume transit some of the radioactive material may deposit on the ground and onto other surfaces. This may later be washed into drains and gutters by rainfall, may migrate into the soil profile, may be moved by the activities of animals, people or vehicles, it may enter foodchains by contaminating the surfaces of vegetables and fruit or by being taken up by roots of foliar surfaces of plants.
Some of the deposited material will become airborne again when disturbed by wind or human activities such as walking or vehicle movement. This process is known as resuspension.
Following a release of radioactivity people in the area may continue to be exposed to an inhalation dose and an external radiation dose from radioactivity that has been resuspended from surfaces. Unsurprisingly, we call these doses resuspension doses.
You can learn more about the science of resuspension from PHE-CRCE-047 (Estimation of radiation doses from inhalation of resuspended materials in emergency situations . This gives a useful table showing the time integrated air concentration (TIAC) in Bq.s.m-3 / Bq.m-2 for a range of integration times and isotopes.
We can use this is show that resuspension is small compared to the transit concentrations thus:
The TIAC from resuspension over the first year following a unit deposition for Cs-137 is 0.74 Bq.s.m-3 / Bq.m-2. (From table B2).
If the deposition velocity of Cs-137 is taken to be 0.02 m.s-1 then the original airborne concentration to achieve a unit deposition ( 1 Bq.m-2 ) is 50 Bq.s.m-3.
The original TIAC is therefore 50 / 0.74 = 67.6 times the resuspension over the first year. The "first pass" inhalation dose is over 50 times the "one-year resuspension" inhalation dose.