What's up with SDHIs?

In 2018, some scientists sounded the alarm about so-called SDHIs that would be used on a massive scale for the protection of agricultural crops, eventually leading to the entry of these substances into the human and animal food chain. On 7 November 2019, an additional study was published. Below, we will address a number of issues in this regard.

  1. What does ‘SDHI’ stand for?
  2. How do crop protection products containing SDHI work?
  3. Can SDHIs have an effect on people?
  4. So what's the problem then with those SDHIs?
  5. Should nothing then be done about SDHIs?
  6. Which active substances belong to SDHIs?
  7. Which crop protection products based on SDHIs are authorised in Belgium? In which crops are they most frequently used?
  8. Are SDHIs used on a large scale?

1.What does ‘SDHI’ stand for?

SDHI is short for succinate dehydrogenase inhibitor.

Succinate dehydrogenase (SDH) is a specific enzyme, and an enzyme is a protein that is produced by living beings to enable vital processes in the cells. Enzymes ensure that the necessary chemical reactions are continued. There are many different enzymes, each with a specific effect. The enzyme succinate dehydrogenase plays a role in the respiration in the cells of living beings.

An inhibitor, on the other hand, is a substance that prevents enzymes to function. So an SDHI is a substance that prevents the enzyme succinate dehydrogenase from working.

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2.How do crop protection products containing SDHI work?

A crop protection product contains several substances, but only the so-called active substances affect the targeted crop diseases or pests.

Active substances must be approved at European level, and can then be used for the production of plant protection products, which must be approved at national level.

An active substance can have an effect in many ways. So an SDHI acts on a specific enzyme; other active substances will act on other enzymes or life processes of the harmful organisms, e.g. the transmission of the nerve impulses.

On 14/04/2020, there were 12 approved active substances that are SDHIs. These are all active substances that are used to combat fungal diseases. The SDHI appears to be effective only against fungi and not against other crop pests, or even against other organisms in general. If the crop is sprayed with an SDHI, the life processes of the fungi are stopped, upon which they die. The crop is thus protected against these fungi.

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3.Can SDHIs have an effect on people?

Any substance will have an effect on any living creature at a certain dose. Even substances that we use daily, such as sugar or salt, can cause adverse effects if we overdose on them. Doses that do not have an immediate effect can still cause problems in the long run.

Unlike sugar and salt, synthetic active substances of plant protection products are specially designed to cause effects on living organisms. That is why they are thoroughly examined for possible effects on humans and the environment before approval is granted for sale or use. In order to meet all standards, synthetic active substances are therefore designed to have a very specific effect on harmful organisms and much less on beneficial ones.

The hundreds of health effect studies assessed at European level showed that no adverse effects of SDHIs are to be expected in humans at the normal dose of use. The studies submitted involving experimental animals allow this to be studied.

In addition, the overall dietary exposure level is low and safe, as for most of the active substances examined by the EFSA (2019).

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4.So what's the problem then with those SDHIs?

Nothing really. These active substances were examined like all other approved active substances and meet all the legal criteria to be approved for use. The effects on human health have been examined, as have those on the environment and on beneficial organisms such as bees, other insects, earthworms, birds, mammals,... and there are no indications to suggest that SDHIs would have any problematic effects.

The opinion published on 16 April 2018 states that the use of SDHIs would pose unacceptable risks. The report was drawn up by a scientific team of the French national research institute CNRS (Centre National de la Recherche Scientifique), among others. It is, however, a very concise communication that does not contain any significant new data, and certainly not a study in which new and extensive data are presented. Moreover, the publication was not checked by other experts in the same field, contrary to what is customary in scientific journals.

In short, this announcement indicates that SDHIs would be able to modify the activity of human SDH enzyme in tissues grown in the laboratory and directly exposed to the test substance. There is no new evidence whatsoever to support the claim that it makes the population ill.

There are very specific, rare hereditary conditions that cause people to exhibit a diminished effect of the SDH enzyme naturally. These conditions can, in turn, lead to the formation of specific tumours. On this basis, the authors of the opinion believe that the presence of SDHIs in our diet can lead to the development of such tumours. However, none of the available scientific studies on the prevention and spread of these specific tumours confirm this. Since some active substances have been used for a long time, it is unlikely that the increased occurrence of these tumours would not have been noticed, for example in farmers who commonly use these substances. This warning, therefore, seems premature. If SDHIs had indeed had the suggested effects, this would have been noted in the available studies and would have been taken into account by banning the active substances or restricting their use.

It should be noted that experts from the French authority (ANSES, 2019) have also analysed the problem and draw the same conclusions, namely that there is no reason to withdraw or restrict the approval of these active substances.

The study of 7 November 2019 suggests new evaluation methods that would be able to identify the effects of SDHIs in a better way, but furthermore only confirms that a malfunction of the SDH-enzyme can lead to adverse effects in laboratory trials. However, the data do not allow to contradict the other available studies that demonstrate that the effects of SDHIs under realistic conditions meet all legal safety standards. The publication has also been commented by ANSES, which still does not see any reason for concern.

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5.Should nothing then be done about SDHIs?

The studies submitted in support of the approval of an active substance must be carried out according to established European methodologies and the latest scientific knowledge. The approval of active substances must be renewed every 7 to 15 years to take account of scientific developments. The dossier for the renewal of the approval of an active substance is as extensive as the one for the original approval, precisely because so many new scientific findings are added. The above-mentioned opinion will also be taken into account. Moreover, the approval of an active substance can be reviewed at any time if it appears that potentially harmful effects are reported in the scientific literature. For this reason, the authorities are already following up on each report, and the relevant SDHIs are no exception to this rule. For the time being, there are no studies that support this theory, but the authorities are monitoring this closely so that immediate action can be taken where necessary.

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6.Which active substances belong to SDHIs?

The following active substances belonging to SDHIs were approved on 14/04/2020:

  • benzovindiflupyr
  • bixafen
  • boscalid
  • carboxin
  • fluopyram
  • flutolanil
  • fluxapyroxad
  • isofetamide
  • isopyrazam
  • penflufen
  • penthiopyrad
  • sedaxane

Their status can be consulted in the European database. This database also contains the start and end dates of the approval under the Plant Protection Regulation, as well as the main toxicological reference values (ADI: Acceptable Daily Intake, ARfD: Acute Reference Dose and AOEL: Acceptable Operator Exposure Level).

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7.Which crop protection products based on SDHIs are authorised in Belgium? In which crops are they most frequently used?

The authorised crop protection products as well as the crops in which they may be used can be found here, on the basis of the active ingredients mentioned under question 6.

Based on the sales figures for 2016 of the various authorised crop protection products and the permitted applications for each of these products, we can estimate that approximately 48% of the SDHIs are used in fruit and vegetable crops, 37% on cereals and 15% on potatoes. The actual sales figures per active ingredient are included in this general overview.

More detailed information on the use of these products may be available on request from the regional authorities.

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8.Are SDHIs used on a large scale?

The sales figures suggest that in the period from 2015 to 2017, an average of 65 tonnes of active substance of SDHI were sold per year. This represents less than one percent of all crop protection products combined, or about 2.5 percent of all fungicides. Therefore, it cannot be claimed that these products are used on a massive scale, certainly not compared to the other crop protection products.

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