The Six Million Dollar Stalk

On August 19, 2009, the German news magazine Der Spiegel carried in its online version an article about someone who found a stalk of common ragwort in prepackaged rocket salad. According to the article, the facts of the matter are these: On 4 August, someone found one stalk of a plant with a yellow flower in a 125g pack of rocket salad. The plant was subsequently identified as liver-damaging ragwort, containing 2.5 mg of toxins.

Since then, the article goes on, many discounters like Plus have banned rocket completely, and sales have plummeted. There is some theater being played between the person who discovered the plant, the government agency in charge, and a NGO that fights against ragwort. At least four large providers of rocket are facing existential problems because no-one wants to buy their rocket any more.

What is going on here? There have been no reported human deaths from eating ragwort. The lethal doses for animals are: 2 kg of fresh plant matter per kg body weight for sheep, 0.14 kg/kg body weight for beef, 0.05 kg/kg for chickens, mice apparently need an astonishing 1.5 kg/kg, and so on. No numbers exist for humans. So the consequences of eating a single stalk of this plant are far from clear. So why is there so widespread panic?

I believe it has to do with the presentation of the threat, and the deeper one digs, the more unclear the real nature of the threat becomes. The Fedral Institute for Risk Assessment, BfR, is the agency charged with, well, assessing risk, and have published a report about a similar incident in 2007, where again one stalk of ragwort was found in salad. They write (translation mine):

From cases of poisoning we know that ragwort poisoning can lead to life-threatening liver damage in humans and animals. […] There is no scientific evidence to support the idea that there exists an amount of plant that can be consumed without any dangers to one’s health. […] It is the opinion of the BfR that acute to medium-term liver damage cannot be excluded as the result of consuming the contaminated salad. A grown-up weighing 60 kg, eating contaminated salad every day, would consume about 220 to 349 micrograms of [toxins] daily. This exceeds the maximally tolerated exposition of 0.1 micrograms of [toxins] for medicine without accepted applications by orders of magnitude.

There are several things wrong here, and they smell of ass-covering rather than of good solid science. First, if there is no harmless maximal dose, where do the 0.1 micrograms come from? Either there is a maximal harmless dose or there is none. Second, they are comparing apples and oranges. Since there haven’t been many reported cases of ragwort in salads—in fact there have been only two, one in 2007 and one in 2009—the risk assessment that is based on the assumption that a grown-up eats those toxins each day, every day, in perpetuity is not comparable to the threat by one-time exposure that the contaminated salad presumably posed.

So the official government agency uses double-talk to cover its ass, presumably because it doesn’t want any actual deaths blamed on it. And the people, believing in that assessment, only see the apparent 2500 times greater intake than the “daily allowance” and that there is no safe maximum that you can consume. So they stop eating rocket.

But let us for the moment assume that eating a single stalk of this plant was invariably lethal. Then there would have been two deaths in three years that would be directly attributable to ragwort. Let us make a back-of-the-envelope calculation of what it would cost to reduce this amount to one death every three years.

Some numbers: the Spiegel article says that 600 hectares are devoted to rocket in Germany. In another aricle we learn that rocket can be harvested between the end of April and the end of October. Rocket needs five weeks from seeding to harvesting, and the yield can be estimated as about one ton of rocket per hectare.

That means that in any particular week, 6005 = 120 hectares are ripe for harvesting, yielding 120 tonnes of rocket per week. From beginning of May to end of October are roughly 24 weeks (which is an understatement to account for the time to fill the production pipeline). So Germany produces an astonishing 120×24 = 2880 tons of rocket per year. Let’s be conservative and say that there are really only 1000 tons. Let’s further assume that one leaf rocket weighs 1 g. Then the objective is to find one stalk of ragwort among 1 billion rocket leaves with an error rate of one stalk every three years.

For this task we take 100 super-humans. These super-humans will need to sort through 10 million rocket leaves a year and will also need to have such a low error rate that the combined error of all 100 together is one ragwort every three years. What would someone like that cost? Let’s assume you’re lucky and you get them for 20,000 Euros each. In that case, saving a single human life would cost 6 million Euros in three years.

This is the kind of number you get every time you want to make an already-improbable event even more improbable: At some point the costs rise to such heights that exposure to such risks (two persons among 80 million dying every three years due to ragwort consumption) should simply be seen as a fact of life.

For comparison, the families of 911 victims got USD 2 million per victim, on average. But that’s the top of the line: victims of Hurricane Katrina received USD 0, on average.