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Maximum Residue Limits (MRLs) for Agricultural Compounds in Food: The Purpose and Procedure for Determining MRLs

4 Assessing MRL Proposals

When proposing an amendment of the MRL Standards, or indeed any standard under the Food Act, the following criteria are considered:

• the need to protect public health

• avoiding unnecessary restrictions to trade

• maintaining consistency between New Zealand food standards and those applying internationally

• the need to meet New Zealand's international obligations.

4.1 Protection of Public Health

Protection of public health is achieved under the risk assessment/risk management framework (i.e. determining the level of risk posed by identified hazards and then managing that risk). However, as a general rule, the conditions placed on the use of agricultural compounds are designed to ensure that residues are at the lowest level that can be achieved without compromising the product's intended purpose. Exposure to those compounds through consumption of food should not exceed the PDE_(food), if one has been established by the Environmental Risk Management Authority (ERMA), or international standards for Average Daily Intakes (ADIs).

Where a risk to public health from exposure to an agricultural compound residue is identified, the mechanism for managing that risk is not usually through the MRL itself, but rather through the conditions placed on registration of products containing those chemicals. These conditions control the use of the agricultural compounds. The MRL is used to monitor compliance with those conditions (although selling food that breaches an MRL is also an offence).

4.1.1 Risk Assessment

Risk assessment for agricultural compound residues consists of assessing the toxicological risk of exposure to those residues and identifying the consumption levels that the compounds should not exceed. Usually this assessment is undertaken by ERMA under the Hazardous Substances and New Organisms Act 1996 (the HSNO Act).

NZFSA’s Agricultural Compound and Veterinary Medicines (ACVM) Group reviews the residue data against the limits set by ERMA and combines this with known or expected consumption patterns to estimate potential daily intakes of the agricultural compounds. From this comparison, the ACVM Group can determine the acceptability of any proposed product use on food crops.

All companies wishing to market agricultural compounds for use in New Zealand must have their products approved under the ACVM Act and ensure that they are also covered by an approval under the HSNO Act. This process ensures that before a compound is imported, manufactured or used in New Zealand, it has been through a complete review and evaluation supported by data presented to international standards.

Significant scientific data are required to show that the compound can be used in a manner that does not have an adverse effect on public health. Where appropriate, the data should be generated in accordance with international guidelines, such as the OECD's Guidelines for the Testing of Chemicals and Good Laboratory Practice (GLP). Among other things, the studies must include data on acute toxicity, reproductive toxicity, (effect of the compound on reproduction), carcinogenicity, mutagenesis (ability of the compound to cause cancer or mutations), metabolism in plants and animals and environmental fate.

Risk assessment in the development of MRLs is based on:

• hazard identification - identification of known or potential adverse health effects in humans produced by agricultural compounds which may be present in a particular food

• hazard characterisation - evaluation of the nature of the adverse effects associated with agricultural compounds present in food, the level of the residue that is required to cause that effect, and also the level that clearly does not cause the effect. This is also referred to as the 'dose-response assessment' for chemical hazards

• exposure assessment - evaluation of the likely intake of agricultural compounds via food, as well as exposures from other sources where relevant

• risk characterisation - estimation of the severity and occurrence of known potential adverse health effects in a given population, based on hazard identification, hazard characterisation, and exposure assessment.

4.1.2 Hazard Identification and Characterisation

The metabolism and toxicological data supplied by applicants is evaluated to identify any adverse health effect that could occur if consumers were exposed to excessive levels of the agricultural compound. Hazard characterisation is carried out to estimate the level of exposure to a compound below which no adverse effects are expected.

In risk assessment, potential health hazards need to be identified first. In the hazard identification phase of the risk assessment, the results from each toxicity study are examined, with a focus on the relationship between chemical dose and toxic effect. Each study uses a range of chemical doses and measures the number and types of different toxicological responses in laboratory animals. From this, the dose producing no toxic effect in the study demonstrating the highest sensitivity is identified. This is the No Observed Adverse Effect Level (NOAEL). The NOAEL is divided by uncertainty factors to give a margin of safety, which usually includes a factor of 10 to account for potentially greater sensitivity of humans as compared with laboratory animals, and an additional factor of 10 to protect sensitive individuals. In this way the ADI is determined from the NOAEL. More information on the ADI is provided in Appendix 2.

4.1.3 Exposure Assessment

The PDE_(food) is used for dietary intake calculations where a value has been set. The PDE_(food) is a value set by ERMA and represents the proportion of the acceptable daily exposure to a substance via the food route as relevant to the New Zealand population. The methodology for calculation of these values is set out in the Hazardous Substances (Classes 6, 8, and 9 Controls) Regulations 2001 and can be found at: www.legislation.govt.nz.

NZFSA estimates exposure to agricultural compound residues using the internationally accepted approach to calculate the National Estimated Daily Intake (NEDI). The NEDI is calculated by multiplying the Supervised Trial Median Residue (STMR) by the estimated New Zealand daily per capita consumption of the crop. Estimates of dietary intake are based on the 1997 New Zealand National Nutrition Survey (or other data as appropriate). This assessment takes account of high risk consumers and sensitive population groups (e.g. children).

• NEDI = S (Fi x STMRi)

where

• S = the sum of (for all foods);

• Fi = New Zealand per capita food consumption for each given food commodity with that residue in it; and

• STMRi = the median residue observed in supervised trials for the food that F_i applies to.

The NEDI also takes other factors into account, such as levels of residues in edible portions and effects of processing, when this gives a more accurate estimate of exposure. In the past, exposure was estimated using the Theoretical Maximum Daily Intake (TMDI) model, which is the sum of the established or proposed MRL (or permissible proportions) multiplied by the estimated average New Zealand daily per capita consumption for each food commodity.

• TMDI = S (Fi x MRLi)

where

• S = the sum of (for all foods)

• Fi = New Zealand per capita food consumption for each given food commodity with that residue in it.

This was a very simple and straightforward approach but it greatly overestimates the true level of exposure to the population. It is very rare that residues will ever approach the MRL and often no residues whatsoever are detected. The NEDI is a more accurate estimate of actual exposure and therefore a more useful tool in determining whether residues pose any risks to public health. When assessing public health risks, NZFSA’s ACVM Group calculates the NEDI and compares this with the PDE_(food).

An agricultural compound’s use pattern is deemed acceptable provided that the NEDI, based on typical residues remaining in food produced in accordance with GAP, is not more than the PDE_(food). This approach also factors in dietary considerations for different consumption patterns of children and other population sub-groups to ensure that sensitive populations are protected.

4.1.4 Risk Characterisation

A risk characterisation is the expression of health risk in quantitative or qualitative terms. In this case, the expression is the proportion of the PDE_(food) represented by the NEDI. The health risks from agricultural compounds, and most other chemicals that are not regarded as primary carcinogens, have been shown in extensive toxicity testing to have a threshold dose below which no adverse effects are expected. The risk characterisation therefore relies on the PDE_(food) as the benchmark for protection of public health. As already discussed under the hazard identification step, the PDE_(food) adds a large margin of safety, resulting in a level well below that observed to cause any toxic effects in animal studies. Additionally, the actual intake of agricultural compound residues is overestimated because it is extremely unlikely that each and every item of food consumed over an individual’s lifetime will have been treated with a particular compound.

4.1.5 Risk Management

Until ERMA has set an appropriate PDE_(food) for any given agricultural chemical, an appropriate ADI is used in the absence of a PDE_(food). The ADI value is used in exactly the same way as the PDE_(food), however as a default position, NZFSA bases assessments on 50% of the ADI value.

Management of Toxicological Risks

MRLs for particular plant compounds are not based on the PDE_(food). However it is true that no MRL would be acceptable unless its potential daily intake was no more than the PDE_(food). The toxicology and residue data for those compounds have been assessed and conditions are placed on their registration to ensure that the MRLs are not exceeded. Such conditions may include limiting the permissible uses of the compound or extending the WHP.

Other means of Protecting Human Health

Other human health-related risks arising from the use of agricultural compounds are managed through separate processes. For example, risks of promoting the development of antibiotic resistant human pathogens through inappropriate use of antibiotics are managed through the conditions placed on the registration of veterinary antibiotics.

If a product is also a prescription medicine under the Medicines Act, it may not be registered as an agricultural compound until the Director-General of Health has granted consent. This gives the Ministry of Health the opportunity to identify public health hazards other than potential residues and indicate the kinds of controls that would be needed to manage the risks. The Ministry of Health has the function of improving, promoting, and protecting public health.

If a product is also a hazardous substance, it cannot be registered as an agricultural compound until approval has been granted under the HSNO Act by ERMA. The purpose of the HSNO Act is to protect the environment, and the health and safety of people and communities, by preventing or managing the adverse effects of hazardous substances and new organisms. ERMA considers the potential exposure to a hazardous substance through all exposure routes, including ingestion of food, and may impose, limits in air, water, soil and on surfaces that must not be exceeded when the substance is used.

NZFSA imposes conditions on the use of agricultural compounds (which could include limiting who can use a product and under what circumstances) to ensure that any identified risks are adequately managed. Where a risk identified by NZFSA, the Ministry of Health or ERMA cannot be managed adequately via a wide range of possible conditions then the product would not be registered, even though a MRL may have been set for its active ingredient.

4.2 Implications for Trade

Domestic Food Production

Domestic food production and trade is usually facilitated by new MRLs. The proposed MRLs permit residues up to the specified level, which in turn reflects GAP or appropriate use in food-producing plants and animals.

Imported Foods

Under the MRL Standards, imported food commodities may comply with either the appropriate Codex MRL, the limits specified in the MRL Standards, or the 0.1 mg/kg default level. Although additional MRLs do not necessarily affect imports, they could ease the importation of some foodstuffs because the new MRLs complement the Codex permission.

Exported Foods

New Zealand MRLs do not apply to exports. However, food produced with agricultural compounds used in accordance with GAP is unlikely to breach New Zealand MRLs, irrespective of whether the food is sold domestically or exported. Export food commodities must also comply with requirements of the country importing the produce. Occasionally, MRLs are set lower than GAP requires. This can ensure international market access for the food commodities listed.

4.3 Consistency with International Food Standards

There is no requirement that the MRLs for New Zealand domestic production are the same as those set by other countries or international bodies (e.g. Codex). However, maintaining consistency with food standards applying internationally is an important aspect of New Zealand food standards development. New Zealand's approach to MRL setting is aligned with international practice, and recognises Codex MRLs for imported food.

4.4 Implications for International Obligations

MRLs for agricultural compounds fall outside the scope of the joint Australia New Zealand food standard-setting system. This is because the MRLs reflect agricultural practices specific to the New Zealand climate, production systems and local pest and disease pressures.

Although the proposed MRLs are for foods sold domestically, under the Trans Tasman Mutual Recognition Arrangement (TTMRA) between Australia and New Zealand, New Zealand manufacturers can export foods to Australia if they meet New Zealand domestic requirements. Equally, foods meeting Australian domestic food requirements may be imported into New Zealand from Australia.

New MRLs are consistent with New Zealand's obligations under the World Trade Organization (WTO) Agreement on the Application of Sanitary and Phytosanitary Measures (the SPS Agreement). The MRLs are recommended on the basis of internationally approved protocols and criteria that are accepted by the WTO. In addition, under the MRL Standards, imported food commodities may comply with either the appropriate Codex MRL or the New Zealand standard. As a matter of course, New Zealand MRLs are notified internationally under the SPS Agreement.