Investigating factors affecting unit-to-unit variability in non-systemic pesticide residues by stochastic simulation modelling

The estimation of acute dietary intake plays a key role in the safety assessment of pesticide residues and needs knowledge of the unit-to-unit variability in residues. There is limited knowledge of contributions of factors to often observed large unit-to-unit variability in residues. A stochastic simulation study was conducted to investigate the effects of sample size and heterogeneity in factors driving residue dissipation of non-systemic pesticides on the unit-to-unit variability among individual apples. The heterogeneity in driving factors was expressed in terms of variability in three dissipation parameters. The variability factor (VF), calculated as the ratio of residues in individual fruit at the 97.5th percentile and the mean residue, was used to represent the unit-to-unit variability. As the rate of dissipation increased, the variability in the corresponding parameter led to larger increases in the variability VF of residues over time. Thus, under field conditions, the relative in pesticide residues is expected to increase with time although the absolute level of residues decreases. The coefficient of variation (CV) was used to describe residue variability in samples of small sizes (5, 10, 20, 40, and 80). When sample size was fewer than 40 fruit, the sample CV increased steeply with decreasing sample size. Measuring residues as concentrations, instead of per fruit, also led to an average 4-8% increase in the CV of residues because of variability in fruit expansion.