The radiation hazards of 125i and 131I




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THE RADIATION HAZARDS OF 125I AND 131I

Radioiodine as 125I, or less frequently as 131I, is in use in many departments for the labelling of proteins etc. The purpose of this note is to identify the hazards associated with this work and to recommend safe working practices.


Radioiodine is a specially dangerous radiochemical as:
(i) Solutions of the iodide can give rise to a release of free iodine unless the pH of the solution is carefully controlled. Volatilisation of this free iodine poses an inhalation and skin absorption hazard.
(ii) Solutions of the iodide are skin permeable and are initially used at very high concentrations (4 MBq/microlitre). The hazard of skin absorption of iodide must be guarded against.
(iii) In the event of accidental ingestion or inhalation of radioiodine, at least 30% of the uptake will be concentrated in the thyroid gland giving rise to a large radiation dose even where the activities involved are small, eg a thyroid uptake of 37 kBq (1 Ci) of 125I commits the thyroid to a radiation dose of 50mSv corresponding to an “effective dose” of 2.5mSv.
Nuclear Properties of 125I
125I decays by electron capture with a half-life of 60 days. In this process, an orbital electron is captured by the nucleus and a neutrino is emitted from the nucleus and carries away most of the nuclear disintegration energy. The remainder the disintegration energy appears in the form of gamma rays of energy 35 KeV and conversion electrons and characteristic X-rays produced by processes involving the orbital electrons.
Summarising, we find that every 100 125I disintegrations leads to the emission of:
(i) 100 neutrinos (undetectable)

(ii) 7 gamma ray photons of 35 KeV

(iii) 93 conversion electrons (range 1cm in air)

(iv) 140 characteristic X-ray photons of 28 KeV

(v) 27 characteristic X-ray photons of 4 KeV.

External Radiation Hazard 125I
If we consider the external radiation hazard posed by a glass vial containing 125I, we find that the only radiations of significance are the 28KeV and 35 KeV photon radiations. This radiation is quite penetrating in tissue; 50% of the energy being dissipated at depths 2.5cm.
The dose rate from this radiation at a distance of 1m from a vial containing 37MBq (1mCi) of 125I is 1.25Svh-1. Using the inverse square law, we see that the hand dose at a distance of 0.1m will be 125Svh-1 and the contact dose rate on the surface of the vial very high indeed.
Fortunately, thin lead foil of thickness 0.5mm attenuates the radiation by a factor of at least 1000 so experiments and solutions are easily made safe using this shielding material.
It should be noted that most Amersham 125I solutions are supplied in a mini-vial in a special plastic coated lead shield and it is unnecessary to remove the vial from its shielding when dispensing. A protective lead apron should be worn when dispensing 125I in activities 200 Mbq.
A crude estimate of the radiation dose rate in any particular situation can be obtained using the standard issue Geiger Muller tube minimonitor. At the energy of the 125I radiation (30KeV) the dose rate response of the Minimonitor is approximately twice that at higher photon energies, so typically 50cps would correspond to a dose rate of 10Svh-1 when the radiation is incident on the end window of the tube.
Internal Radiation Hazard 125I
As noted previously, an uptake of radioiodine concentrates in the thyroid gland. For a radiation worker, the maximum permissible annual intake of 125I is 1MBq. Ingestion of this activity results in a radiation dose to the thyroid of 400 mSv, resulting in an “effective dose” of 20 mSv.
To prevent an uptake of 125I, we must observe the usual precautions for handling unsealed radioactive material and we must pay particular attention to the inhalation and skin absorption hazards.
Dispensing of radioiodine - It is common to find that 1% of the contents of a vial of radioiodine is present as free iodine vapour above the solution in the vial, it is therefore essential that all dispensing of these solutions is carried out in a well ventilated fume cupboard in a radioisotope dispensary. A buttoned-up laboratory coat and disposable gloves should be worn.
There is some evidence that the “Triflex” type of disposable glove may be permeable in some instances to iodide solution, so where high activities are involved, it is recommended that two gloves be worn on each hand. Experience elsewhere suggests that, occasionally, skin absorption through the forearms and front of the chest may be a factor in iodine uptake.

If a perspex beta shield is placed in the front of the fume cupboard and the sash is lowered to about a one foot opening, the airflow over the forearms will be enhanced and the perspex screen will guard against iodine absorption over the front of the body. This arrangement is recommended where large activities of iodine are dispensed.


Unsealed radioactive materials must be kept in storage ventilated to the outside atmosphere to prevent any build-up of radioiodine in the laboratory atmosphere. Solid and liquid waste are particularly important in this respect - used chromatography columns and the like have caused some problems in other establishments. Care should be exercised when disposing of liquid waste. If a weak solution of radioiodine is left in the trap or U-bend of a disposal sink, subsequent disposals of acid solutions etc may result in the evolution of free iodine.
Contamination monitoring of 125I is rendered difficult by the fact that only the 28/35 KeV X-radiation is available for detection by the monitor. Whereas every beta particle penetrating the end window of a GM Minimonitor is detected, only 1 in 150 of these X-ray photons will give rise to a count in the standard minimonitor. In consequence, the GM Minimonitor is virtually useless for contamination monitoring; 1cps above background corresponds to a contamination level of 150Bq cm-2 ie 50 x the formerly accepted maximum surface contamination level of 3 Bq cm-2.
For contamination monitoring, a Minimonitor X-ray scintillation counter probe must be used. This is fitted with a 1mm thick, 23mm diameter sodium iodide crystal with a beryllium window. At a contamination level of 3 Bq cm-2 this probe has a counting rate of 2 cps in addition to its background count rate (2.5 cps) when the probe is held at a distance of 1cm from the contaminated surface.
Thyroid monitoring for 125I content is also possible using the X-ray probe. Placing the probe centrally on the front of the lower neck, a count rate of approximately 1 cps (in addition to background) is obtained for a thyroid burden of 100 Bq 125I. It is evident that the probe is more than amply sensitive for the detection of radioiodine uptakes at levels well below the maximum allowed.
As well as monitoring the apparatus, laboratory protective clothing and his person, a worker using radioiodine should also monitor his thyroid with the X-ray probe at the conclusion of an iodination.
Routinely, radioiodine workers will have a thyroid check by the departmental Radiation Protection Supervisor at three monthly intervals and an entry will be made in a log book kept for this purpose. In the event of a significant uptake, the working methods should be reviewed. Facilities exist within the University for more accurate evaluation of any radioiodine uptake.
Nuclear Properties of 131I
131I has a half life of 8 days and decays with the emission of beta particles of maximum energy 0.81 MeV and gamma rays of various energies, the most important of which are photons of 0.36 MeV (79% of disintegrations).
External Radiation Hazard 131I
Glass of thickness 1mm will stop virtually all the beta radiation emitted by 131I, so the hazard of skin irradiation from this radiation is easily guarded against.
The gamma radiation, however, is exceedingly penetrating and 3.2cm of lead is required to attenuate the radiation by a factor of x 100. Thin lead foil is worthless in this respect and activities 10MBq of 131I should be stored behind 1 inch thick lead bricks.
For the 131I gamma radiation we find:-
Dose rate at 1m from 1 Mbq of 131I = 0.06 Sv/hr

ie, Dose rate at 1m from 1 mCi (37 Mbq) = 2.20 Sv/hr
As previously noted, the standard GM Minimonitor may be used to estimate the gamma radiation dose rate; 1Svh-1 will produce a count rate of 2.5cps above background.
Internal Radiation Hazard 131I
The maximum permitted annual intake of 131I for a radiation worker is 0.8 Mbq; this produces an uptake of 240 kBq in the thyroid committing the thyroid to a radiation dose of 400 mSv and giving an “effective dose” of 20mSv.
All the precautions listed for the prevention of an 125I uptake are necessary when working with 131I. Contamination monitoring however is simplified in that the beta radiation emitted by 131I is readily detected by the standard GM minimonitor.
At a contamination level of 1 Bq cm-2, this monitor will register approximately 1 cps above background when the probe is held close to the surface.
The minimonitor 5.42 X-ray probe may be used to obtain a rough estimate of the 131I thyroid burden. 1kBq of 131I in the thyroid produces a count rate of 1 cps above background at the “Adam’s apple”.
The departmental radiation supervisor will check the thyroid count of workers using 131I at three monthly intervals and keep a log of the results. Count rates exceeding 10 cps above background will be reported to the University Radiation Protection Service since a 10 cps count rate implies a thyroid dose of 17mSv and an “effective dose” of 0.8mSv.
Accidents with Radioiodine
As with any other radiochemical accident, help should be sought at the earliest opportunity from the Departmental Radiation Protection Supervisor and the University RPA.
Skin contaminations of the fingers respond well to a brief immersion of the affected area in a strong solution of potassium permanganate.
When a spillage involving unreacted radioiodine occurs, the spilled material should be treated with excess of sodium thyosulphate (hypo) solution to render it chemically stable prior to beginning the decontamination operation.
100 mgm of potassium iodide or iodate taken orally immediately after an accidental intake of radioiodine will reduce uptake of radioiodine by the thyroid, but as this material may cause mild discomfort in some individuals, the University Medical Officer should be contacted for advice before resorting to this remedy.

J M Gray


University RPA August 2005



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