Don’t Risk Radiation Exposure

The wastewater in some septic systems could be more dangerous than you realize

Don’t Risk Radiation Exposure
Aside from the regular, everyday hazards you face when in close proximity to wastewater, there are times when your risk for exposure to disease or harmful chemicals is significantly higher – and you probably won’t know it.

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Wearing the proper personal protective equipment when doing any maintenance or repair on a septic system should be nonnegotiable. Aside from the regular, everyday hazards you face when in close proximity to wastewater, there are times when your risk for exposure to disease or harmful chemicals is significantly higher — and you probably won’t know it.

One such unknown risk is when the homeowner is being treated for cancer or some other serious illness. The person’s body is releasing toxic drugs and chemicals, and the last thing on his mind will be how this could affect the septic system and that he should warn you about his treatment. It’s up to you to protect yourself.

One example of a harmful chemical is radioactive iodine, which is used primarily in the medical field for diagnostics, imaging tests, treating thyroid problems, nuclear medicine, cancer treatment and immunotherapy. The most common source that can impact residential septic systems is when someone in the home undergoes radionuclide therapy to treat thyrotoxicosis. This is a general term for diseases of the thyroid gland in which too much thyroid hormone is produced. Treatment with radioiodine is one of three main options to treat thyrotoxicosis.

After undergoing radionuclide therapy, patients generate wastewater with a considerable amount of radioactivity, which can reach levels of as much as 90 percent of the administered dose. During the first two days of treatment, 98 percent of the radioactive iodine will be excreted, so ideally the patient would stay in the hospital for two days to avoid contact with other people, but also to reduce discharge to their septic system. That will prevent a large amount of radioiodine being deposited into the septic tank.   

The Environmental Protection Agency says the average U.S. household water use is about 400 gallons per day. Radioactive iodine has a half-life of eight days, so it will lose about 8.3 percent of its activity each day. If you take into account the daily dilution of those with a septic tank and decay factors, it will take 87 days to get concentrations below the Nuclear Regulatory Commission limits for unrestricted access, but since septic systems disperse the effluent below grade, access is restricted. Those performing maintenance or repair work on a system should avoid contact with this wastewater during this time (or even longer if water usage is low) and take extreme caution using the appropriate PPE.

There is not enough radioactivity in typical doses of radioactive iodine to kill the bacteria in a septic tank. The Environmental Protection Agency website lists the doses to kill bacteria for food irradiation. They range from 1 to 30 kilogray. That is an exceedingly high radiation dose. According to the Health Physic Society, an organization that specializes in radiation, based on a typical dose there would be 0.15 microgray in the contents from the excreted patient, which is well under the amount to kill bacteria. 

About the Author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to


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