Every registered chemical that is intended for use on food crops will have a Maximum Residue Level (MRL) and an associated withholding period set for it. This is the minimum permissible time between the last application of an agrichemical to a crop and harvesting for human consumption, or grazing by animals. The withholding period may also be known as the Pre-Harvest Interval (PHI).
A withholding period can also be defined by stages of crop growth rather than a specific length of time. For example, some products should not be sprayed after flowering.
The withholding period allows sufficient time for the residue level of the chemical on the crop to drop to a level below its MRL. The withholiding period is determined from chemical decay rates observed in actual, local field trials and compared with work overseas.
Withholding periods assume you applied the agrichemical in accordance with the instructions on the label. The critical factors in ensuring MRLs are not exceeded are:
- following withholding periods
- not applying agrichemicals at above label rates (eg poor calibration).
MRLs are different for different countries and different crops. You need to check the MRL for your market and determine the last date that a specified agrichemical can be used on product destined for that market. Your industry organisation will be able to provide details of PHIs and MRLs.
The re-entry interval (REI) (or re-entry period) is the minimum amount of time that must pass between the time the agrichemical was applied to an area or crop and the time that people can go into that area without protective clothing and equipment.
Re-entry times are set to protect people against poisoning by agrichemicals if they enter a treated area too soon after application without proper protective equipment. If there is no re-entry time specified on the label, recommended practice is to wait until spray has dried.
Unfortunately, it is relatively easy to be exposed to agrichemicals after they have been applied to their target. For example:
- Breathing in vapours, dusts or mists
- Touching residues on the sprayed plants
- Getting the agrichemical in your eye through vapours, dusts or mists, or by rubbing your eyes with your hand, a glove or clothing that is contaminated with residue
- Eating food that has been treated or eating without first washing hands
When treated plants are touched during work activities such as weeding, thinning, or brushing against plants, some agrichemical residue may be transferred to skin. Workers in a field can also cause residues on plants and on the soil surface to 'fly up' as a dust - the dust settles on the worker's skin and/or are inhaled. People in treated areas may also breathe fumes (vapours) from a recent agrichemical application.
Signage should be used to advise on re-entry restrictions.