§ The Countess of Marasked Her Majesty's Government:
Whether they still consider measurements of cholinesterase activity to be a reliable method for assessing levels of exposure to organophosphates, particularly those used in sheep dips, and if not whether they will commission research to find an appropriate indicator.
Lord LucasMeasurement of blood cholinesterase activity is considered to be a useful method of monitoring acute exposure to organophosphates under certain circumstances. Guidance in the government bookletSheep dipping AS29 (Rev), recommends monitoring cholinesterase levels in contract sheep dippers who may have regular and frequent exposure to sheep dip chemicals. Measurement of cholinesterase 79WA may also be a useful diagnostic tool in cases of severe acute poisoning, where levels are expected to be clearly below the normal range. As the activity may recover rapidly, it is necessary to test the blood immediately after exposure. It would not however be the most suitable method for assessing levels of exposure among those, such as sheep farmers, who use organophosphates sporadically.
An alternative method is to monitor urinary excretion of the breakdown products of organophosphates. This has been used by the Health and Safety Executive to estimate exposure of sheep dippers to organophosphates and compare it with that of other workers. The levels found were similar to those found in other agricultural workers exposed to organophosphates and lower than those found in workers manufacturing pesticides. Although very sensitive for measuring exposure to diazinon, it cannot be used to determine exposure to propetamphos, because the urinary marker metabolite measured is not formed by this organophosphate. Diazinon and propetamphos are the two main active ingredients in organophosphorus sheep dips.
These two methods (the measurement of cholinesterase activity and of urinary metabolites) provide workable tools for monitoring acute exposure to organophosphates.
There is no method for measuring chronic exposure. Development of such methods would depend on prior knowledge of a biochemical defect or of any biomarker associated with chronic exposure. There is currently no evidence of such a defect or of any biomarker suitable for assessing such exposure. (The enzyme neuropathy target esterase (NTE), which is associated with delayed neuropathy, is not affected by diazinon, so this would not be a useful measure).
We take the concerns over possible long-term health effects of OP exposure very seriously and are commissioning new research to investigate whether chronic exposure poses a health risk to sheep dippers. It is expected that an announcement about the award of a contract will be made shortly.
A number of scientific papers have been published on methods for the assessment of human exposure to organophosphates, and copies have been placed in the Library of the House.