By: Clark Brenzil PAg., Provincial Specialist Weed Control
A Maximum Residue Limit (MRL) is the upper limit of residue of a particular pesticide that is “tolerated” in a particular food commodity. It is often used as a measure of appropriate use of a pesticide by regulatory bodies. The MRL is established during the registration of a pesticide by thorough combination of the Acceptable Daily Intake on all commodities and the expected exposure through the consumption of those crop products that have been treated with the pesticide.
The “Acceptable Daily Intake” is calculated by a process that follows this general method:
1) The toxicity profile of a pesticide is determined by finding the exposure level, measured in amount per unit of body mass in test animals, at which the pesticide has no effect (called NOEL – No Observable Effect Level). The measure of toxicity includes things like acute (one exposure) or chronic (extended exposure) poisoning, risk of cancer, birth defects and any other unusual change in body functioning compared to individuals not exposed to the pesticide, such as changes in organ weight or general animal growth rates. It’s measured over both short (90-day) and long-term (two-year) feeding periods. Since it would be unethical to do research directly on people, animal models are used. Rats are used in general toxicity testing, as they don’t take up much space and their metabolic functions are very similar to humans. Rabbits are typically used for skin and eye irritation studies.
2) A 10-fold uncertainty (safety) factor is applied in Canada, the United States and many other countries to arrive at an allowable level of exposure. This factor:
- Recognizes the differences that may occur when adapting animal models to human exposure (inter-species uncertainty); and
- Ensures that people who may be more vulnerable to exposure (children or the infirmed) are protected (intra-species uncertainty).
Some countries such as Japan multiply the intra- and inter-species uncertainties for an overall uncertainty of 100 fold.
The NOEL is divided by the overall uncertainty factor to get the Acceptable Daily Intake.
The other component of establishing MRLs is conducting a Dietary Risk Assessment based on the average consumption pattern of all of the commodities that might be treated with the pesticide. Studies are conducted to assess how much of a particular pesticide remains in the harvested material of a crop or forage, or how much is transferred to milk or meat of livestock when fed treated forage. All of the potential exposure to the test pesticide is calculated and a “risk basket,” or exposure risk profile, is created.
If a lot of a commodity is consumed, the residues present in that commodity need to be lower than in one where little is consumed in order to fall below the acceptable daily intake. For example; large amounts of lentils are consumed in countries such as India and Turkey, whereas much less are consumed in Canada. Therefore the MRL for a pesticide in India could be expected to be much lower than in Canada to fall under the same Acceptable Daily Intake established in the toxicology investigation phase.
If, when used as intended, the dietary risk of a pesticide in commodities proposed for registration cannot fall below the Acceptable Daily Intake value, the pesticide will not be registered for use. Usually the manufacturer will determine this early in the development process and not even apply for registration if the candidate pesticide will not be able to achieve its MRL.
If, however, testing indicates that Dietary Exposure Risk of registered crops falls below the established MRL value, then the pesticide will meet the food safety requirements for registration in Canada and, pending other regulatory requirements, become registered in Canada. Registration in Canada means that sprayed commodities may be consumed in Canada; however, for a commodity to be exported into other countries, a similar process needs to take place with the national pesticide regulator in those countries, and an MRL needs to be established for that commodity in that country. Even using the same science, different countries may establish different MRLs as a result of different national consumption patterns for a particular commodity.
Check the Guide to Crop Protection for individual product restrictions available at the time of publishing.
Also check out our article MRL Concerns for Oilseed Crops for specific oilseed issues.