While the author's acknowledged that such an approach is not a direct risk assessment, per se, the did urge consumers to consider the relative rankings when making their purchasing decisions. They also concluded that consumers should continue to eat ample quantities of fruits and vegetables, that consumers wash or peel fresh fruits and vegetables, and that consumers should try to purchase organically grown peaches, apples, grapes, pears, green beans, winter squash and spinach if such products are available. Further into the report, however, the authors did engage in some elementary risk assessment practices and the results of such risk assessments served to drive many of the headlines generated in tonight's news. Not surprisingly, peaches had, by far, the highest TI calculated in the report, and 90 percent of this TI was represented by methyl parathion residues. Let's break this issue down a bit more.
Methyl parathion is one member of a large class of insecticides known as organophosphates, or OPs.
Members of this class of chemicals exert their toxicity in mammals by interfering with an enzyme in the nervous system known as cholinesterase. Significant inhibition of this enzyme can lead to toxic effects such as increased sweating, vomiting, abdominal discomfort, and pinpointing of the pupils. Among the OPs, methyl parathion is considered to be a fairly toxic member but not as potent as a cholinesterase inhibitor as several other members of the family such as its ethyl parathion relative. Ethyl parathion is considered to be more toxic because its two ethyl ester groups provide a greater steric hindrance to a normal metabolic detoxification reaction, hydrolysis, than is provided by the two smaller methyl ester groups of methyl parathion. Such an additional uncertainty factor may seem prudent for estimating acute exposures, but I disagree with EPA's philosophy that the additional 10-fold uncertainty factor should apply for the chronic reference dose since chronic exposures concern an entire lifetime and not just the brief periods of infancy and childhood.