"We’ve created a mushroom sandwich index”

2 April 2019

Cecilia Gustavsson at the Division of Applied Nuclear Physics is one of the researchers behind last autumn’s mass experiment Radiant Earth. The results are now in and reported to the upper secondary school classes across Sweden who collected mushrooms and samples of soil and droppings to be analysed by researchers measuring radioactive caesium levels after 1986’s Chernobyl accident:

How did you analyse the samples?

“We’ve used a so called germanium detector that measures gamma radiation or photons from the decay of caesium-137 and other isotopes. The germanium detector provides a very good energy resolution and depicts the gamma energies in histograms or bar graphs. If the caesium-137-bar is filled up by many hits, then there’s a lot of caesium-137 in the sample.”

 Are you done with the analyses now?

“Both yes and no. So far, we have only analysed the 250 mushroom samples from different parts of Sweden. They are edible mushrooms, mostly funnel chanterelles or Cantharellus tubaeformis, as well as mushroom species such as Suillus variegatus, Cantharellus cibarius, Boletus edulis, and Cortinarius caperatus.”

“But we have not yet analysed the relationship between the activity in soil samples and mushrooms. That may reveal interesting research results, such as how caesium travels between earth and organisms.”

What are results of the mushroom samples that you’ve received from the students?

"What we see in the mushroom samples is that the signal from Chernobyl is clear. But the activity is quite low. It’s what to be expected, since there has been no fallout of radioactive caesium in Sweden since 1986. Caesium-137 has a half-life of 30 years, and now, with just over a half-life passed, we can see that there are very few mushroom samples that currently have an activity above the threshold value, which is 1500 becquerels per kilo for selling mushrooms.”

“However, the Swedish Food Agency has set a limit of 10 000 becquerels per kilo for how much you should consume. And no samples show such high levels. But there are a couple of samples just above 1500 becquerels per kilo.”

What do your results imply when it comes to eating mushrooms? 

"We have created a so-called mushroom sandwich index. In this we make comparisons between how much Swedish mushrooms a person must eat in a year to reach the same levels one is exposed to from natural background radiation in Sweden. It turns out that you have to eat more than one hundred kilos of mushrooms from the most active mushroom sample in a year, in order to get the same amount of exposure that you naturally get from radioactive substances in soil and rock during the same time. This can also be compared with the dose every Swede on average gets from medical examinations every year. That equals a daily consumption of 100 grams of our most active mushroom. Most of the samples we’ve received have much lower activity and those you can eat a lot more of.”

“The limit values are set at low levels for there to be little risk. Even if you include the mushrooms whose activities exceed the limit, a high intake of those mushrooms would still be a very small contribution to the total radiation dose.”

“The greatest danger with mushrooms is that there are many poisonous mushrooms. The risk of picking the wrong species of mushroom poses a significantly greater risk than radiotoxic risks.”

Is there a difference between the mushrooms in terms of their amount of becquerel?

"We believe there are differences, but the mushroom samples we’ve received are dried and typed by school classes. To verify the typing requires DNA sequencing, which we plan to do. There may be genetic differences between different mushroom species. This we plan to follow up in further studies.”

How does the concentration of caesium differ in various parts of Sweden?

 “In short, it’s as you would expect. The coast of Norrland received the biggest fallout from Chernobyl due to, among other things, wind and rain. Areas in the counties of Västmanland, Uppsala, Gävleborg, and Södermanland still have higher levels than, for example, Skåne and the West coast where there were no big fallouts. That is represented quite well in our materials, there are no big surprises.”

What do you hope that the upper secondary school students that have participated take away from the mass experiment?

 "I hope they feel that they’ve contributed to research. With so many getting together, each and every one contributing in a small way, it leads to an enormous amount of material to use for research. The students' efforts have meant a lot to our work.”

Read more:

The mass experiment Radiant Earth

Mushroom sandwich index Radiant Earth