Answer to Question #8596 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Therapeutic Nuclear Medicine

The following question was answered by an expert in the appropriate field:


My son is going to receive 5550 MBq of radioactive 131I and is being treated as an outpatient. He will be staying at our house, and we are on a septic system. Will the radiation affect the flora of the septic system? Will it have any effect on how our septic system functions?


There is not enough radioactivity in 5550 MBq of 131I to kill the bacteria in your septic tank. The Environmental Protection Agency Web site lists the doses to kill bacteria for food irradiation. They range from 1 to 30 kilogray. That is an exceedingly high radiation dose. A colleague of mine did a calculation and came up with a potential dose of 0.15 mGy to the contents from the excreted 131I. Compared to 1,000,000 mGy, 0.15 mGy is tiny.

I think that you can feel safe that your septic tank will not be affected. Additionally, the 131I decays with a half-life of eight days so it decays to essentially nothing in two months.

Marcia Hartman, MS

Answer posted on 19 October 2009. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.