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Food Focus November 2007

Making MRLs work for you

The idea of NZFSA considering a change to a maximum residue limit (MRL) may cause some people to get a little nervous about the imagined implications. But MRLs are constantly under review and new ones set. Though the reasons may not always be immediately obvious, much research and investigative work is done behind the scenes by NZFSA’s risk managers before relevant stakeholders are consulted on the idea. Food Focus looks at the work involved

People have used pesticides to prevent damage to their crops for thousands of years. In most countries, as in New Zealand, to sell or use a pesticide, it must be approved by a government agency.

We all know now that using pesticides carries some risk. Proper pesticide use minimises these risks to an acceptable level. In New Zealand we have various monitoring programmes to check that a pesticide is being used correctly and only approved pesticides and veterinary medicines can be used.

Such monitoring programmes are run by NZFSA and measure ‘maximum residue limits’ (MRLs), which are the upper levels allowed of any chemical that has been used on crops or livestock, when a farmer has followed Good Agricultural Practice (GAP). With normal and responsible application of chemicals, having performed their role, the traces of chemicals that remain at harvest slaughter should be no higher than the MRL that has been set for that chemical.

Making a change to the levels of residues allowed on food may make some people nervous if they don’t fully appreciate the safety calculations around the use of agricultural and veterinary compounds in food.

The MRL standard

Under the Food Act 1981, all imported and domestically produced food sold in New Zealand (except for food imported from Australia as there is mutual recognition of MRLs) must comply with the New Zealand (Maximum Residue Limits of Agricultural Compounds) Food Standards 2007. In the case of imported food, the standard allows for it to comply either with the international MRL standard (Codex) or domestic MRLs.

The MRL standard is amended regularly by NZFSA’s Agricultural Compounds and Veterinary Medicines Group to take account of new compounds coming on to the market and/or changes in use for those compounds already available.

In proposing an MRL, data are considered from residue studies that represent the range of crop or animal growing conditions. In each residue decay trial, samples are taken and analysed at regular intervals from the time of application until harvest, slaughter or milking.

For example, if an insecticide is effective for 14 days, then to keep protecting a ripening crop the insecticide would be applied at 14-day periods. As the last application has to protect the crop right up until harvest, it should be applied 14 days prior to harvest. This will let the insecticide run its course and achieve the function it was designed to do.

If the residues at harvest were approximately 0.2mg per kilogram of the crop, then an MRL of 0.2mg/kg is likely to be proposed. Any breaches of this MRL would indicate the crop was harvested too soon (eg, before the 14 days were up) or the insecticide was applied at a dose above that given on the label.

A breach in an MRL (or changing the level of an MRL), does not indicate the food is (or will become) unsafe to eat. Rather it is an alert system that a farmer or grower may not have followed GAP – normal and accepted use of a chemical. It is also illegal to sell food with a residue above the MRL.

What would trigger an MRL change?

There are many reasons why NZFSA would consider setting or changing an MRL. It could be because the compound is:

• new to New Zealand

• deemed to be exempt from an MRL (for instance, the active ingredient may not be chemical in nature)

• to be used in a different way, ie: under different growing conditions (field versus glasshouse), on a new crop or animal, or at different times of the growing year, or is no longer effective for the crop or animal at the usual dose.

As well, NZFSA risk managers are constantly scanning data which may necessitate a change to an MRL. This could be because:

• there is new information on the compound and it needs to be re-assessed

• agricultural practices have changed

• the compound is no longer used and therefore its MRL can be removed.

Changes to the MRL must take into account the total amount of that chemical you eat over a normal daily diet – to make sure that allowing a higher dose of that chemical doesn’t push you over the Acceptable Daily Intake (ADI). For example, farmers might want a shorter withholding period for a veterinary chemical so they can move their stock to slaughter sooner, or they may want to register a chemical for use on cows that had previously only been used on sheep.

Before NZFSA changes an MRL, we look at our dietary surveys and determine how much of the chemical someone would eat, and whether a change in MRL might push someone near the ADI for that chemical. If we were concerned, we would not allow this use, or would approach other industries to see if they can lower their use of that chemical to allow a new use or a higher dose in other foods.

Safe daily intake

How much of a chemical residue we can safely consume is given by the ADI. This is extrapolated from animal studies with a very conservative estimate of how much would be safe for people. If you wanted to calculate how much of a particular chemical you were consuming in your normal daily diet, you need to know what the MRL is for that food, and then calculate your intake using the ADI, which gives you the amount of chemical you can safely consume relative to your body weight, per day, over your lifetime.

The ADI is determined from animal studies with safety margins added in. Varying doses of a compound are given to laboratory animals. Researchers can observe the first, or slightest, signs of a toxic effect. They then use the dose just below this level – the ‘no observed adverse effect level’ (NOAEL) to help estimate the ADI.

To compensate for the uncertainty of how animals versus humans respond to chemicals, and to account for more sensitive people in the population, the NOAEL is lowered by at least a factor of 100. The result is the estimated ADI – how much of this chemical we can safely consume per day, per kg of our body weight, for our entire life without showing any toxic effect. The ADI is expressed in milligrams, per kilogramme of body weight, per day (mg/kg bw/day).

To calculate our actual intake of a residue, you need both the ADI and the MRL.

For example: The ADI for the fungicide iprodione (used on kiwifruit) is 0.06mg/kg bw/day. If you are a 70kg adult, you could safely eat up to 4.2mgs of iprodione every day, for ever, without showing any toxic effect. You also need to know how much residue is in kiwifruit. For iprodione the MRL is 5 mg/kg of fruit. Therefore to come close to reaching the ADI for iprodione from kiwifruit, you would have to eat nearly a kilogram of kiwifruit (each with the maximum residue level of iprodione – itself very unlikely), every day, for the rest of your life, and there would still be no signs of a toxic effect from this chemical!

More information on MRLs and chemicals in food, can be found on NZFSA’s website www.nzfsa.govt.nz/publications/hot-topics/chemical-residues.htm.

What is GAP

The term ‘good agricultural practice’ (GAP) describes the safe uses of a chemical under actual conditions necessary for effective use. The term is used internationally and regulators around the world use this as part of their management of pesticides and veterinary medicines.

GAP may encompass a range of use patterns necessary to achieve the desired effect without excessive use, with the chemical being applied in a manner that leaves a residue, which is the smallest amount practicable.