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The Basics of Food Tolerances and Maximum Residue Limits

Written by Wiley A. Hall, 4th, Ph.D.
Originally Published in Safe Food Alliance, Testing & Analysis

An MRL (Maximum Residue Limit) or a food tolerance, as it is known in the USA, is the maximum amount of a pesticide residue that can be found in or on food. While any food with a pesticide residue under an MRL should be safe to eat, it is important to remember that an MRL is not a safety limit. That being said, a residue over an MRL is not necessarily a safety hazard, but rather a violation of good agricultural practices (GAP).

If you are growing, packing, importing/exporting, or otherwise involved in the domestic or global trade in agricultural commodities, it is critical that you be aware of any MRLs that may affect your commodity and the regulations around them. This article is meant to give you a basic introduction of pesticides and their limits, but no single article can cover all the complexities and local variations in MRLS. If you have questions about your specific situation, reach out to your PCA, local farm advisor, grower group, or a Safe Food Alliance representative.

Let’s start with some definitions.

Pesticide

A pesticide is defined by the EPA as: “Any substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest.”, “Any substance or mixture of substances intended for use as a plant regulator, defoliant, or desiccant.” or “Any nitrogen stabilizer”.

Active Ingredient

The active ingredient (AI) in a pesticide is the chemical or chemicals that actually have the desired effect and do the heavy lifting (the AI in Round Up is glyphosate). The rest of a pesticide product is the inert ingredients such as solvents (e.g. water) and adjuvants which are additives that improve pesticide performance (spreaders, wetting agents).

Pesticide Residue

A pesticide residue is the trace amount of any pesticide remaining on the crop after treatment
(and any post treatment steps required by the label, such as a reentry period). It is important to note that an MRL may refer to an AI, adjuvant, or any other residue that results from a pesticide application, like metabolites. Remember, all MRLs are specific to a pesticide and crop combination. For example, if you find the fungicide Imazalil on oranges, and the market does not have an MRL for it, then any Imazalil residue found on an orange is illegal (a positive list system or PLS).

Default MRLs

Some markets, like Europe or Japan, have default MRLs where if there’s no specific MRL for a given crop/ pesticide, but the pesticide isn’t specially banned, the MRL defaults to a given value (usually 0.01ppm). These default MRLs are generally set to around the current limit of detection for most pesticides, though, so there’s often little difference between a default MRL and no MRL.

Residue Definition

The last important definition we’ll discuss is the residue definition. The residue definition tells you what chemicals need to be measured to determine if your commodity conforms with the MRL. For example, the residue definition for the herbicide oxyfluorfen is just the chemical oxyfluorfen, but for fosetyl-aluminum both the parent compound, fosetyl-aluminum and the chemical it degrades into in the environment, phosphonic acid, needs to be measured. This can also change based on the commodity and market: the residue definition for glyphosate for wheat in the United States is different than the definition for almonds in Australia.

How are MRLs made?

While an MRL is not a safety limit, the setting of an MRL begins with safety testing.

How It’s Done

  1. The registrant of a pesticide performs animal studies to determine the smallest amount of the active ingredient that the animal can be exposed to before having an adverse reaction. This amount is the no observable adverse effect level or NOAEL.
  2. From there, they take the NOAEL and multiply it 100 times to give a range (Safety Margin) to include as much of the population as possible.
  3. The safety margins and two toxicological values are then applied: the smallest amount that it is acceptable for humans to consume at one sitting (acute reference dose or ARfD) and the smallest amount that it is acceptable for humans to consume on a long-term basis (acceptable daily intake or ADI) are calculated.
  4. The registrant carries out field trials, where the pesticide is applied to the commodity at the highest proposed rate and the crop is then handled according to GAP.
  5. Finally, the MRL is set based on statistical analysis on the range of pesticide residues found on the crop from the field trials, after all applicable harvest activities. What that means is they analyze how much of the crop is left after harvest and measure if it might get people sick.

Following The Directions

Again, while the MRL needs to be at a safe level (all sources of consumption are summed and the MRL is only approved if the public’s estimated consumption is under the ADI and ARfD), it is really a way of ensuring that GAP was followed in the application of the pesticide. If the instructions in the pesticide label are followed, the pesticide residue, by the time the commodity reaches the consumer, should be under the MRL. It is important to follow the label, as an MRL violation is a sign that GAP was not adhered to, possibly risking harm to the environment, workers and those who live nearby the spray site.

How to Stay Informed on MRLs

The world of pesticide regulations is large and constantly changing. Even the basic information
covered in this article may seem like a lot to have to keep track of, without getting into the specifics of how MRLs work in each individual market. Europe alone is worth its own (much longer) article, one that might go out of date in the time between writing and reading. The good news is that all the information you need to stay informed on the state of MRLs is publicly available if you know where to look.

For the most up to date information, make sure to stay in communication with any PCAs, farm advisors, or grower groups who may have firsthand knowledge of your crop, growing methods, and target market.

Second, visit an online database where you can find information on what MRLs there are for your crop and market. Online databases are available for the United States, Europe, Japan, Korea, Canada, for starters, but that can still be a lot to have to keep straight. Bryant Christie Inc. keeps an excellent MRL database that covers over “…1000 pesticides, 875 commodities, and 125 markets”; due to a grant from the federal government. Access is free to users within the United States.

Remember, the key to avoiding pesticide residue issues is to stay informed. By knowing where there may be potential issues ahead of time and testing for residues before you ship your commodity, you can manage your pesticide risk.

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