Half life and isotope decay of Radioactive Iodine ( I131 )

A lot is said about the short half life of I131, the type of radioactive iodine used for thyroid treatment.  Here are the facts about half life with regard to I131.

I131 has a half life of slightly more than 8 days.  For simplicities’ sake, we’ll round it to 8 days.  If I131 has a half life of 8 days, then it should disappear in 16 (2 half lives), right?  Not exactly.  Every half life the amount of radioactive iodine is reduced by 50%.  The other 50% is no longer I131, but is now stable (non-radioactive Xenon 131.  Here’s an example:

Let’s start with 100 millicuries –

Day #1 – 100 millicuries remains

Day #8 – 50 millicuries remains

Day #16 – 25 millicuries remains

Day #24 – 12.5 millicuries remains, etc.

For anyone who has told you that I131 disappears in 16 days, they are VERY mistaken and giving you very wrong advice.

Let’s look at this in terms of contamination now –

Let’s say you receive a dose of 100 millicuries.  The majority of the dosage is being eliminated from your body through urine, feces, sweat, saliva and breath.  If we say that a mere 0.5% of that dose results in contamination in your home, what does this really mean in terms of exposure to others?

As we discussed in another post, the radioactive beta particles being released upon decay are what causes cell damage.  Just how many beta particles are being given off by this contamination?

0.5% of 100 millicuries is emitting approximately 987,900,000 beta particles EVERY MINUTE.

So how long is a contaminated area still “hot”?

After 11 half lives (88 days), the number of beta particles being emitted by that contamination is approximately 481,601 radioactive beta particles every minute!

Now think about the person receiving second hand exposure to that contamination and getting internal uptake to their thyroid.  The above example illustrates so simply why it is important to avoid exposing others to contamination.

Although an 8 days half  life seems like a short period, the total time it will take for I131 contamination to reach safe background levels may be a long time. The best course of actions is to remove any potential radioactive iodine contamination with Bind-It™.

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