Proposed Re-evaluation Decision PRVD2012-02, Linuron

Pest Management Regulatory Agency
27 July 2012
ISSN: 1925-0967 (PDF version)
Catalogue number: H113-27/2012-2E-PDF (PDF version)

Summary

Table of Contents

• Proposed Re-evaluation Decision for Linuron
• What Does Health Canada Consider When Making a Re-evaluation Decision?
• What is Linuron?
• Health Considerations
• Environmental Considerations
• Value Considerations
• Regulatory Status in Organisation for Economic Cooperation and Development Countries
• Proposed Measures to Minimize Risk
• Next Steps
• Other Information

Proposed Re-evaluation Decision for Linuron

After a re-evaluation of the herbicide linuron, Health Canada's Pest Management Regulatory Agency (PMRA), under the authority of the  Pest Control Products Act, is proposing to phase out the sale and use of all linuron products in Canada. This is because an evaluation of available scientific information found that, under the current conditions of use, the human health and environmental risks estimated for linuron do not meet current standards.

This Proposed Re-evaluation Decision is a consultation document^{Footnote 1} that summarizes the science evaluation for linuron and presents the reasons for the proposed re-evaluation decision. Before making a final re-evaluation decision on linuron, the PMRA will accept and consider written comments on this proposal received up to 60 days from the date of its publication. Please forward all comments to Publications. The PMRA will then publish a Re-evaluation Decision^{Footnote 2} on linuron, which will include the decision, the reasons for it, a summary of comments received on the proposed re-evaluation decision and the PMRA's response to these comments.

Once the final re-evaluation decision is made, registrants will be instructed on how to address any new requirements.

The information is presented in two parts. This summary describes the regulatory process and key points of the evaluation, while the Science Evaluation provides detailed technical information on the human health, environmental and value assessments of linuron.

What Does Health Canada Consider When Making a Re-evaluation Decision?

Regulatory Directive DIR2001-03, PMRA Re-evaluation Program, presents details of the re-evaluation activities and program structure. The key objective of the Pest Control Products Act, to prevent risks of concern to people and the environment from the use of pest control products. Health or environmental risk is considered acceptable if there is reasonable certainty that no harm to human health, future generations or the environment will result from use or exposure to the product under its conditions or proposed conditions of registration^{Footnote 3}. The Act also requires that products have value^{Footnote 4} when used according to the label directions. Conditions of registration may include special precautionary measures on the product label to further reduce risk.

To reach its decisions, the PMRA applies hazard and risk assessment methods as well as policies that are rigorous and modern. These methods consider the unique characteristics of sensitive subpopulations in both humans (for example, children) and organisms in the environment (for example, those most sensitive to environmental contaminants). These methods and policies also consider the nature of the effects observed and the uncertainties present when predicting the impact of pesticides. The re-evaluation draws on data from registrants, published scientific reports, information from other regulatory agencies and any other relevant information. 

For more information on how the PMRA regulates pesticides, as well as the assessment process and risk-reduction programs, please visit the Pesticides and Pest Management portion of Health Canada's website at healthcanada.gc.ca/pmra.

What is Linuron?

Linuron is a selective systemic herbicide. It is registered for pre- and post-emergence control of annual and perennial broadleaf and grassy weeds on both crop and non-crop sites. Linuron is often used in tank mixtures with other herbicides to broaden the weed control spectrum. For some uses, only tank mixtures are registered and no treatment with linuron alone is registered. Linuron may be applied as a broadcast spray or directed spray using ground or aerial equipment. The rate of application of linuron ranges from 0.20 to 4.50 kg a.i./ha depending upon the use site. The rate of application can also vary with soil texture and organic matter content: a lower rate is applied to light coloured and low organic matter soils and a higher rate is applied to dark coloured and high organic matter soils.

Based on the PMRA's records, a total of eight linuron products are registered under the authority of the Pest Control Products Act, including three technical grade active ingredients and five commercial class end-use products. Of the five commercial class end-use products, three are formulated as suspensions and two as wettable granules (one of which is in water soluble bags). No domestic class end-use products containing linuron are registered in Canada.

Uses of linuron belong to the following use site categories: terrestrial food crops, terrestrial feed crops, industrial oil seed crops and fibre crops, forests and woodlots, and industrial and domestic vegetation control for non-food sites.

Health Considerations

Can Approved Uses of Linuron Affect Human Health?

Risks of concern were identified for both dietary and occupational exposure to linuron.

Potential exposure to linuron may occur through diet (food and water), when handling and applying the product or by entering treated sites. When assessing health risks, two key factors are considered: the levels at which no health effects occur in animal testing and the levels to which people may be exposed. The dose levels used to assess risks are established to protect the most sensitive human population (for example, children and nursing mothers). Only those uses where exposure is well below levels that cause no effects in animal testing are considered acceptable for registration.

Linuron was of low to slight acute toxicity by the oral and inhalation routes of exposure in rats, and of low acute dermal toxicity in rats and rabbits. In rabbits, linuron was non-irritating to slightly irritating to the skin and mildly irritating to the eyes. Linuron was not considered to be a skin sensitizer in guinea pigs.

The most sensitive endpoint that could result from acute exposure to linuron is an effect on red blood cells (methemoglobinemia), as observed in the short-term dog studies. Linuron was carcinogenic in rats and, as a result, a cancer risk assessment was conducted. Exposure to linuron resulted in adverse effects in the developing fetus. When linuron was administered to pregnant animals, the developing fetus was more sensitive to linuron than the adult animal. In addition, generational sensitivity was observed with linuron targeting the male reproductive tissues at lower doses in offspring compared to parental males. Due to the nature of these endpoints and their potential implications on the health of the young, extra protective factors were applied during the risk assessment to further reduce the allowable level of human exposure to linuron.

Residues in Water and Food

Dietary risks from food and water are of concern.

Reference doses define levels to which an individual can be exposed over a single day (acute) or lifetime (chronic) and expect no adverse health effects. Generally, dietary exposure from food and water is acceptable if it is less than 100% of the acute reference dose or chronic reference dose (acceptable daily intake). An acceptable daily intake is an estimate of the level of daily exposure to a pesticide residue that, over a lifetime, is expected to have no significant harmful effects.

Human exposure to linuron was estimated from residues in treated crops and drinking water, including the most highly exposed subpopulation (for example, infants <1 year old). Risk concerns were identified from exposure to linuron from food commodities only, as well as exposure to drinking water only (refer to Appendix VI). Aggregate exposure from food and drinking water represents approximately 2800% of the acute reference dose and 2500% of the chronic reference dose for the most sensitive population infants <1 years old and is of concern. The cancer risk (food and drinking water) was 1 × 10^-4 for the general population and is also of concern. A lifetime cancer risk that is below 1 × 10^-6 (one in a million) is generally considered an acceptable risk for the general population when exposure occurs through pesticide residues in or on food, and to otherwise unintentionally exposed persons. Further information on how the potential cancer risks from pesticides are assessed can be found in Science Policy Notice SPN2000-01. A Decision Framework for Risk Assessment and Risk Management in the Pest Management Regulatory Agency. Due to the risk concerns identified, mitigation measures and refinements were considered in the aggregate assessment. However, even with the use of refined estimates based on the mitigation scenario where all crops except those with the lowest rates (wheat, barley, and oats) are removed, the aggregate exposure from food and drinking water remains of concern.

The drinking water concentrations used in the assessment are based on water modeling and are likely conservative due to modeling inputs and assumptions such as maximum half-life values for soil and aquatic biotransformation, maximum number and rates of application, yearly application in the same field for multiple years, etc. In addition, the modeled concentrations for surface water scenarios were used to estimate residue concentrations in all sources of water. This may be conservative for the chronic and cancer assessment as surface water modeled estimates are much higher than groundwater modeled estimates, and an individual is likely to drink from multiple sources of water over a lifetime. Concentrations from monitoring data are lower, but were not used to refine the assessment, as monitoring data on transformation products of concern were not available.

The  Food and Drugs Act prohibits the sale of adulterated food; that is, food containing a pesticide residue that exceeds the established maximum residue limit (MRL). Pesticide MRLs are established for Food and Drugs Act purposes through the evaluation of scientific data under the Pest Control Products Act. Each MRL value defines the maximum concentration in parts per million (ppm) of a pesticide allowed in/on certain foods. Food containing a pesticide residue that does not exceed the established MRL does not pose an unacceptable health risk.

MRLs in/on all commodities treated with linuron are currently regulated under subsection B.15.002(1) of the Food and Drugs Regulations, which requires that residues not exceed 0.1 ppm. Additional details regarding MRLs can be found in the Science Evaluation of this consultation document.

It should be noted that any on-going food use for linuron would require a modern residue chemistry data base in order to validate assumptions made in this risk assessment and to meet the basic regulatory requirements for a food use pesticide.

Risks in Residential and Other Non-Occupational Environments

Non-occupational risks are not of concern.

There are currently no registered residential uses of linuron; as such a risk assessment for this scenario was not required.

Potential exposure and risk from activities at pick-your-own facilities was considered negligible, as linuron is applied early in the season, long before harvest and it is not directly applied on the fruit or on the crop foliage.

Occupational Risks From Handling Linuron

Occupational non-cancer risks to handlers are of concern for most uses of linuron.

The majority of non-cancer risk estimates associated with applying, mixing and loading activities for linuron did not reach the target Margin of Exposure (MOE) even with maximum personal protective equipment (PPE) and engineering controls, and are therefore of concern. Non-cancer risk estimates associated with applying, mixing and loading activities for wheat, oats, barley, asparagus, parsnip, celery, carrots, coriander, caraway, sweet white lupins and dill reached the target MOEs provided mitigation measures such as additional engineering controls, PPE and restrictions on amount handled per day are considered.

Occupational cancer risk to handlers are not of concern.

Most occupational cancer risks are not of concern for agricultural scenarios, provided mitigation measures such as additional engineering controls, personal protective equipment and restrictions on amount handled per day are employed.

Occupational non-cancer and cancer risks for postapplication workers are of concern for most uses of linuron.

Occupational postapplication risk assessments consider exposures to workers entering treated agricultural sites. Based on the current use pattern for linuron, non-cancer risks to workers performing activities, such as thinning, pruning and harvesting of all crops except wheat, barley and oats, did not meet current standards and are of concern. Postapplication cancer risks for workers performing activities, such as irrigating or scouting, thinning, pruning or harvesting of most crops did not meet current standards. The restricted-entry intervals (REIs) required to mitigate postapplication risk range from 21 to 74 days and may not be agronomically feasible.

Environmental Considerations

What Happens When Linuron Is Introduced Into the Environment?

Linuron poses a potential risk to terrestrial and aquatic organisms.

Linuron enters the environment when it is used on various crops as an herbicide. In the terrestrial environment, linuron appears to be slightly to moderately persistent, whereas, in the aquatic environment, linuron appears to be non-persistent to slightly persistent. Linuron is soluble in water and has medium to slight mobility in soil. An increased mobility may occur in some specific environmental conditions, such as soil with low organic matter content and coarse texture. Based on its persistence and mobility linuron appears to have the potential to contaminate ground water and surface water through runoff, leaching and spray drift. Linuron was detected in Canadian ground water and surface water through water monitoring.

Linuron breaks down into several transformation products through biotransformation at rates that depend on environmental conditions. The major transformation products, identified in biotransformation studies, were desmethoxy linuron, norlinuron and desmethoxy monolinuron. These transformation products have a mobility being low to immobile in the environment.

Linuron poses risks to both terrestrial and aquatic organisms. Birds and small wild mammals are at risk in and around the site of application due to the consumption of contaminated food items and the risk cannot be fully mitigated through label statements.

Value Considerations

What is the Value of Linuron?

Linuron contributes to weed management in a variety of crop and non-crop sites.  

Linuron is a key herbicide for both carrot and potato production in Canada. Furthermore, 25 new uses of linuron are identified in the Canadian Grower Priority Database (a database of priority needs for registration of new uses identified by growers) and many of them are identified as "high priority and no alternatives".  

Active ingredients other than linuron are registered for use on all sites listed on the current linuron labels. The viability of these active ingredients as alternatives to linuron needs to be confirmed via feedback from stakeholders during the proposed re-evaluation decision consultation period. Stakeholder feedback to date suggests linuron is a key use in carrot and potato production and a future priority in herb and spice production. The number of alternatives for use on some of the minor crops is very limited.  

Linuron may play a role in delaying the development of herbicide resistance in weeds when used in rotation or combination with active ingredients from other herbicide site-of-action groups. 

In light of the proposed phase out of all uses listed on the linuron labels, the PMRA requests the following information:

• potential impact of the proposed phase out of each site listed on the linuron labels;
• identification of those critical uses of linuron with few alternatives that require transition strategies to replace active ingredients;
• extent of current use of linuron for each site listed on the linuron labels;
• availability, viability and extent of use of alternative active ingredients to linuron;
• availability, effectiveness and extent of use of non-chemical weed management practices for any of the registered uses of linuron; and
• typical crop area per farm and percent crop treated for the minor crops coriander, choke cherry, dill and parsnip.

Regulatory Status in Organisation for Economic Cooperation and Development Countries

The European Union's overall conclusion as of 2 December 2002 from the re-evaluation of linuron is that it may be expected that plant protection products containing linuron will fulfil the safety requirements laid down in Article 5(1)(a) and (b) of Directive 91/4147/EEC and therefore is authorized for use at a maximum application rate of 0.95 kg a.s./ha.

The  United States Environmental Protection Agency (USEPA) published a Reregistration Eligibility Decision in March 1995 and a Tolerance Reassessment Progress and Risk Management Decision in May 2002. The USEPA concluded that most registered uses were eligible for reregistration except for the following uses: cotton, potato, non-cropland (rights of ways) and sweet corn. The USEPA was concerned with the potential for linuron on postapplication/re-entry exposure to workers and handlers (mixers/loaders/applicators) exposure. Therefore, in order to reduce the postapplication/re-entry exposure risks, the USEPA established a 24-hour REI, required postapplication/re-entry exposure data, and minimum PPE for handlers of all end-use products containing linuron.

The following differences should be noted between the PMRA's risk assessment and those of the USEPA: toxicological end points, use pattern. The USEPA considered linuron a Group C carcinogen but did not conduct a quantitative cancer risk assessment.

Proposed Measures to Minimize Risk

The PMRA has assessed the available information and concluded that the use of linuron and associated end-use products in accordance with the label presents an unacceptable risk to health and the environment. Therefore, the PMRA is proposing to phase out the sale and use of all linuron products in Canada.

It should be noted that as risk concerns were identified from all areas of the health risk assessment, a large and refined data set that addresses all areas of the risk assessment (in other words, toxicology, exposure, and residue chemistry) would be required to potentially revise the assessment. While certain conservatisms are present in the assessment, the product database was sufficiently complete to conduct a quantitative evaluation of the health risks. Furthermore, the conservatisms present do not fully account for the magnitude of the risk concerns identified.

Next Steps

Before making a re-evaluation decision on linuron, the PMRA will consider comments received from the public in response to this consultation document. The PMRA will then publish a Re-evaluation Decision, which will include the decision, the reasons for it, a summary of comments received on the proposed decision and the PMRA's response to these comments.

Other Information

At the time that the re-evaluation decision is made, the PMRA will publish an Evaluation Report on linuron in the context of this re-evaluation decision (based on the Science Evaluation of this consultation document). In addition, the test data on which the decision is based will also be available for public inspection, upon application, in the PMRA's Reading Room (located in Ottawa).