They are not explicitly labelled as containing organophosphates; instead the names of the specific chemicals used as active ingredients indicate the presence of organophosphates. The commonest of these chemicals is malathion; others include dichlorvos, paraphion and diazinon. The manufacturers insist their products have been approved by the proper authorities and deny that they pose a danger.
"Nuvan Top is fully licensed and has been on the market for around 30 years," said Philip Dobson, veterinary surgeon and technical manager for Ciba Animal Health, makers of Nuvan Top. "No significant side-effects have been noted. There has been the odd complaint but nothing to warrant a contra-indication on the label."
He said that whenever there have been complaints there have always been full investigations but "no direct link" has been found between the product and the symptoms some people experience.
Liquid Malathion has been on the market since 1956. Bob Daniels, Research and Development Manager for Fisons plc horticulture division, makers of Murphy liquid Malathion believes it is a "good old-fashioned killer".
"When used according to the label, there is no cause for concern," he said. "It has been approved by the authorities. The risks involved are extremely small."
A spokesman for the British Agrochemical Association agrees that there are no problems if the products are used correctly. "If used as directed, they are not dangerous. If you do something silly with them like have a bath in them or swallow them, then they are dangerous. If you swallowed a bottle of Domestos, you wouldn't be particularly happy either."
Although he could not comment on organophosphates in general, Graham Collyer, technical director for the Seton Healthcare Group, the makers of Derbac M Liquid and Prioderm, said: "Head lice preparations are classified as medicines. All safety data have to be reviewed by the Committee on Safety of Medicines. They are controlled by the medicines control agency."