Answer to Question #12945 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
I had a 3.7 GBq dose of radioactive iodine (131I) for papillary thyroid cancer. After spending 14 days in isolation and confined to a recreational vehicle (RV), I have not felt comfortable going back in to do a thorough cleansing. I also have a fear of letting grandchildren in there due to my not wearing socks constantly. I am worried the carpet and mattress is contaminated with radioactivity. I don't want to replace carpet or mattress if not needed. I am being told that there would not be ANYTHING radioactive left as my treatment was almost five months ago. They act like I am extreme for even worrying after the length of time. If I understand correctly, the half-life of 131I is eight days and the radioactive iodine would all be "decayed" after 88 days, even if I had not been careful. I have both two- and five-year-old grandsons and DO NOT want to risk any harm to them. Help me understand what I need to do. We need to deal with this and let me have PEACE about it.
At this point it is perfectly safe to go in and clean the RV and let your grandsons play there. Let me go through the math and physics for you.
As you mentioned, radioactive 131I has an eight-day half-life. After one half-life half of the 131I is gone. After two half-lives half of that is gone (and only a quarter of the original radioactivity is left). Let's look at this graphically, assuming we have 8,000 Bq at the start. After one half-life, we have 50% of the original amount left, or 4,000 Bq.
If someone were to take a sensitive radiation detector into the RV, they would not be able to detect your footprints or any other contamination. Any radioactivity remaining would be indistinguishable from naturally occurring radioactivity in the environment. Indeed, if you put your entire treatment of 131I in a spot on the mattress, it would be less than the amount of naturally occurring potassium-40 (40K) in your body.
There is no radiation risk reason for replacing mattresses or carpets. You may want to scrub the showers for sanitary reasons, but there is no need to for the purpose of reducing radioactivity. The bottom line is that there is no radiation risk to your grandchildren. You should take them to your RV and have fun with no regrets.
Kent Lambert, CHP, FHPS