INDUCED RADIOACTIVITY FROM ELECTRON-BEAM IRRADIATION

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This analysis is based on a British document, the “Report on the Safety and Wholesomeness of Irradiated Foods” by the Advisory Committee on Irradiated and Novel Foods. It is directed to the Department of Health and Social Security, Ministry of Agriculture, Fisheries and Food, Scottish Home and Health Department, Welsh Office and Department of Health and Social Services, Northern Ireland. The relevant part is the addendum, “Report on the Radiological Implications of Irradiated Foods” by the National Radiological Protection Board, dated March 1985, which is quoted below.

Regarding the mechanism of induced radioactivity, the report states:

“Radioactivity may be induced in food by two main physical processes:

a. Photoneutron reactions (gamma, n). This process involves the absorption by a nucleus of a photon and the emission of a neutron. An incident radiation energy of at least 1.5 MeV is required for this process to occur. The loss of a neutron may render the nuclide radioactive; the emitted neutron may then interact with another nucleus and induce radioactivity.

b.Isomer activation. This process involves the absorption by a nucleus of a gamma photon and the prompt emission of a photon of lower energy, leaving a metastable nucleus which decays by emitting a further gamma photon to return to its original stable state.

A less important physical process which might contribute to the induction of radioactivity in food is the photoproton reaction (gamma, p). This process involves the absorption by the nucleus of a photon and the emission of a proton. At energies below 16 MeV, the Coulomb barrier tends to prevent the emission of a charged particle from the nucleus. However, experimental evidence suggests that (gamma, p) reactions occur below this threshold energy, possibly due to a direct interaction in which the energy of the photon is absorbed by a single proton which is then emitted before the energy can be shared by the other nucleons.”

The report also states

“At 10 MeV, x-rays are about 30 times more efficient than electrons at inducing radioactivity.”

The report gives dose estimates for photon energies of 5MeV and 10MeV, at 1-day and 10-days post-processing. The worst-case-scenario that is considered is 100 grams/day of irradiated food (about 4 ounces, out of a UK average per capita dietary intake of 3.5 pounds of food/day).

“Taking the maximum theoretical induced activity level of 3.5 x 10^-2 Bq/gram (10MeV photons) and pessimistically assuming all this to be ¹²⁶I produced by a photoneutron reaction in stable iodine, leads to an estimated annual committed effective dose equivalent of about 25 milliSv. This estimate assumes that all food is consumed 1 day after irradiation…”

If the food had been consumed 10 days after irradiation, the dose would be about 45% of the stated annual dose equivalent.

“…using more realistic isotope concentrations and assuming a delay of 10 days between irradiation and consumption…produces an estimated dose of less than 0.001 milliSv/year. Both these estimates should be compared with the dose limit for a member of the public, which is 5,000 milliSv/year in any single year, with exposure being limited on the basis of 1,000 milliSv/year averaged over an entire lifetime.”

The report states that

“all food is radioactive due to the presence of various naturally occurring radionucleides. For example, the Ministry of Agriculture, Fisheries and Food reports natural activity levels in fish and shellfish to range between 0.04 Bq/gram to 0.1 Bq/gram. The theoretical levels of induced activity in irradiated foods quoted in the table above are less than the natural activity levels reported in fish and shellfish.” 

The report recommends

“…on the treatment of small doses…practices which lead to exposures of less than 5 milliSv/year and result in collective effective dose equivalent commitments of less than 1 man Sv should not be of concern to regulatory authorities…consumer products that give rise to individual exposures of less than 5 milliSv/year or which have an activity content of less than 0.4 Bq/gram should not be of concern to regulatory authorities.”

CONCLUSIONS

Here are some conclusions one can draw about induced radioactivity in irradiated foods, based on the above document:

1.The report clearly states that radioactivity can be induced in food irradiated by gamma rays or high-speed electrons. 

2.The worst-case scenario makes assumptions about the kinds and scope of isotopic change that should not be dismissed, as the report does, as being unrealistic, without additional evidence. 

3.The report states "consumer products ...which have an activity content of less than 0.4 Bq/gram should not be of concern to regulatory authorities.” The maximum theoretical induced activity level in irradiated food is .035 Bq/gram, 4 to 9 times the level naturally occurring level in fish and shellfish (0.04 Bq/gram to 0.1 Bq/gram.). The report does not provide natural activity levels for other foods or explain why the .4 Bq/gram level of safety was chosen. Fish and shellfish are not the major components of most people's diets, and more representative foods should have been chosen.

4.The worst-case scenario is based on the assumption that foods are eaten one day after irradiation. *An Indian study found that four of five children who ate a diet of 60% freshly irradiated wheat developed chromosomal changes in their blood that are precursors to cancer. The scientists believe that wheat should be stored for 12 weeks before use in order to avoid this in the future. Given the British report’s stated differences in radioactivity levels after 1 and after 10 days of storage, it is possible that the chromosomal changes were due to the induced radioactivity in the food. Storage, then, seems prudent before irradiated foods are consumed. Should we be confident that irradiators, especially of fruits and vegetables, will observe the necessary precautions? Discards from irradiation facilities may be sold without adequate storage, and irradiation facilities being used to camouflage lower-quality food that is then sold to the poor in countries where it is produced.

5.The worst-case scenario is based on the assumption that an individual’s daily consumption of irradiated foods is limited to 100 grams, or 4 ounces. This assumption is naive, especially if warning labels are removed. First, the worst-case scenario does not take into account the likelihood that some foods will be irradiated twice (e.g. ground beef and then a prepared chili as a whole). Second, people eat more fruits and vegetables now than in 1985, and so the per capita dietary intake of 3.5 lbs./person/day of food used by the report is likely to be larger. (Vegetarians and children will be particularly affected). Third, even if we take the 3.5 lbs/day as a standard, with a diet that is 40% irradiated, a great deal more induced radioactivity is consumed than the report assumes:

Calculation:

3.5 lbs food/person/day = 1400 grams. 40% irradiated x 1400 grams = 560 grams of irradiated food/day= 5.5 times the amount of irradiated foods expected to be consumed according to the report.

SUMMARY

There is evidence for induced radioactivity in foods. According to the worst-case scenario presented, it seems to be negligible, although one may question the wisdom of allowing any increase in exposure to radioactivity. Low-level radiation is pervasive in our environment, including our bodies and our foods, ultimately from nuclear weapons testing and power plant leaks including permitted releases. The relationship between dose and effect is supralinear, not linear, and low-level radiation is more dangerous than originally thought by regulators.

The report does not state if the fish/shellfish levels of natural radioactivity (the proposed comparison food) are typical or atypical. If fish/shellfish have higher-than-average levels of natural radioactivity compared to other foods, using them as the comparison food has the effect of raising the permissible level of induced radioactivity that is considered 'normal.'

The report’s assumption that consumption of irradiated foods will be small is not supportable (it assumes 100 grams or approximately 4 oz/person/day, or 15% of the total weight of food consumed). In the US, with meat, fruit and vegetables approved for irradiation, and people increasing their intake of fruits and vegetables, this 15% estimate is unrealistic.

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Note

*A hotly disputed study conducted in India in the 1970s found significant numbers of intrauterine deaths and depression in immune response in rats and mice fed freshly irradiated wheat. There was also an increase in polyploid cells, a chromosomal abnormality, in the bone marrow of rats and mice and the blood of children and monkeys fed freshly irradiated wheat. Similar results were not obtained from non-irradiated wheat or irradiated wheat that had been stored for three months prior to feeding.

One of the researchers, Dr. Vijaylaxmi, a geneticist now at the University of Texas, "strongly suspects there are radiolytic products which are problematic in these studies." "In regard to polyploid cells, we do not know the significance of these cells so we expressed a note of caution, but scientific studies have shown that they are associated with cancer--whether polyploid cells cause cancer or cancer produces polyploid cells, we don't know," Dr. Vijaylaxmi says. "But we do know the significance of intrauterine death and depressed immune response and I would consider that a strong factor in whether one chooses to eat irradiated food. If the choice was given to me, I wouldn't eat irradiated food."