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current methods of assessing short-term intake of pesticide residues by humans need to be modified. So concludes a study conducted by an international team of researchers. Their assertion is significant, as single exposure to some pesticide residues can be unsafe. A single high dose of organophosphates, for instance, can lead to chronic headaches. Ironically, to date attention has been mostly focussed on the long-term intake of pesticide residues.
The researchers, led by Denis Hamilton of Australia's Queensland department of primary industries, analysed worldwide data on pesticide residues in fruits and vegetables. The data had been collected via field studies and market-place surveys. Based on the information, the researchers assessed both short-term and long-term risk from pesticide residue exposure. They found that short-term exposure had been underestimated in many cases. Reason -- the pesticide residue level in an individual unit of fruit or vegetable had not been given due consideration. Conventionally, the concentration of pesticide residues in a consignment or batch of foodstuff is assessed on the basis of a small sample size. Such methodology may not be appropriate -- the residue level in a single fruit or vegetable was found to be many times higher than the average residue of all units combined. A comparison of individual units of medium-sized crops showed a residue variation of about 30 per cent.
Typical of the difference were tomatoes from Hungary. They had been treated with mancozeb and zineb pesticides. As per the researchers, the highest residue levels in a single tomato were 2.4 and 3.6 times more than the mean residue of all the tomatoes. In another assessment, the researchers found that single apples collected from Switzerland contained residues 2.2 to 2.8 times more than the mean residue concentration of all the fruits. The position of the fruits on the trees seems to have influenced their residue levels -- the average residues in apples from the bottom and middle foliage were three times more than the levels of fruits from the top.
The researchers opine that such factors should be included in the assessments of residue exposure. This is not always the case due to the use of either the probabilistic method of residue assessment, or the deterministic approach. In the probabilistic method, the consumption patterns and the distribution of possible residues in foodstuff are combined in 'repeated probabilistic calculations' to derive the risk exposure level. In the deterministic method, the intake is estimated on the basis of 'high residue' factor -- high residue means that the pesticide was used at the highest recommended rate, the crop was harvested at the smallest interval after treatment and the residue in the edible portion was the highest found in any of the supervised trials. The deterministic calculation also includes a variability factor for foods consumed as units.
The researchers' analysis showed that the deterministic method calculates higher exposure risk than the probabilistic system. For instance, when exposure data pertaining to the commonly used insecticide chlorpyrifos was assessed for the entire us population, it was found that deterministic estimates showed exposure level 'two orders of magnitude' greater than those resulting from probabilistic estimates.
The findings, published in the journal Pest Management Science (Vol 60, No 4, April 2004), may help risk assessors worldwide to take better decisions. In their concluding remarks, the researchers state that the probabilistic and deterministic methods should be considered complementary to each other in order to accurately derive the exposure limits.