Answer to Question #12570 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
Can radium equivalent activity and internal hazard index be applied to plant samples? I understand that this could be applied to building materials, but I recently saw that some researchers have calculated this for plant material. I am particularly interested in a medicinal plant that grows in Sri Lanka and its radium-226 (226Ra), thorium-232 (232Th), and potassium-40 (40K) content.
This is an interesting question. I was previously unaware of the concept of radium equivalent activity, and therefore the related concept of hazard index, being used for plant samples. As you suggested, we did review the article you referenced in this regard, "Measurement of natural radioactivity in selected samples of medical plants in Iraq" (Kareem et al. 2016).
Although these authors did apply the concepts of radium equivalent activity and internal hazard index to the concentrations they measured of uranium, thorium, and potassium in numerous plant species, I do not believe these concepts were ever intended to be applied to circumstances involving ingestion-related pathways of exposure. These concepts were derived based on external gamma exposure and radon progeny inhalation pathways, and that is how these concepts have been traditionally used.
As you are probably aware, radium equivalent activity is often used as a relative measure of the gamma-ray exposure rates (and therefore external exposure risk) associated with 226Ra vs. 232Th vs. 40K. For example:
Raeq = ARa + 1.43ATh + 0.077AK
where Raeq is the radium equivalent, ARa is the activity of 226Ra, ATh is the activity of 232Th, and AK is the activity of 40K, all in becquerels per kilogram (Bq kg-1).
The above equation is developed on the assumption that based on an annual exposure limit of 1 millisievert per year (mSv y-1), 370 Bq kg-1 of 226Ra, 259 Bq kg-1 of 232Th, and 4,810 Bq kg-1 of 40K all produce the same gamma-ray dose rate. The radium equivalent is related to both the external gamma dose and the internal alpha dose from radon and its progeny from decay of the radium (e.g., as may be found in building materials) resulting in respiratory tract doses to the inhabitants of the structure (Beretka and Matthew 1985).
Also, the related concept of "internal hazard index" has been used for approximating relative risk (radium vs. thorium vs. potassium) from inhalation of alpha particles emitted from the short-lived radionuclides of radon and applied to the safe use of certain building materials in the construction of dwellings (Righi and Bruzzi 2006).
Another good journal article that provides useful insights on application of the concepts of radium equivalent activity, hazard index, and internal hazard index is by Darwish et al. (2015).
In conclusion, from my perspective these concepts do not appear to be relevant to ingestion-related pathways, such as concentrations of radium, thorium, or potassium in food products including plant materials. The basis of relative dose or risk that is fundamental to the concepts of radium equivalent activity and the related concept of (internal) hazard index results from the relative external-exposure dose rates and/or the potential for radon/radon progeny inhalation doses associated with each of the three nuclides. These pathways do not appear to me to be directly relevant to doses that could be received from radium vs. thorium vs. potassium associated with ingestion, particularly given that ingestion dose is also a function of (1) aspects of the decay schemes of these three nuclides other than their contributions to gamma exposure and radon progeny inhalation from radium decay (for example, thorium is a high-yield alpha emitter and although this may be irrelevant to its contribution to external dose, it is very relevant to the internal dose it delivers through the ingestion pathway) and (2) the metabolic behavior (solubility, organ-specific uptake, absorption and retention, etc.) of the chemical species (compounds) ingested, not just those specific physical properties associated with the radionuclide decay schemes, as are the circumstances with radium equivalent activity and hazard index.
Steven H. Brown, CHP
Beretka J, Matthew PJ. Natural radioactivity of Australian building materials, industrial wastes and by-products. Health Physics 48:87–95; 1985.
Darwish DAE, Abul-Nasr KTM, El-Khayatt AM. The assessment of natural radioactivity and its associated radiological hazards and dose parameters in granite samples from South Sinai, Egypt. Journal of Radiation Research and Applied Sciences 8:17–25; 2015.
Kareem AA, Hady HN, Abojassim AA. Measurement of natural radioactivity in selected samples of medical plants in Iraq. International Journal of Physical Sciences 11:178–182; 2016.
Righi S, Bruzzi L. Natural radioactivity and radon exhalation in building materials used in Italian dwellings. Journal of Environmental Radioactivity 88:158–170; 2006.