Inveresk Clinical Research laboratories in Edinburgh have reportedly been asked by German and Danish firms to test the pesticides Methamidophos and Malathion on human volunteers.

Environmental groups say the chemicals should not be tested on humans.

Malathion is a widely used insecticide, introduced in 1950. It attacks the nervous systems of insects and is used to fight head and body lice in humans.

In low levels, it is not thought to be toxic, but some scientists believe it could cause birth and genetic defects.

There have been some cases in the USA of children having respiratory problems after breathing in high levels of the insecticide.

Methamidophos is highly toxic if swallowed, touched or inhaled. It can cause headaches, nausea and vomiting.

Studies in China have found that it reduces men's sperm viability and sperm count.

Safety

Inveresk Clinical Research, which has been involved in previous controversy over testing of organophosphates, says any tests will be well monitored with the volunteers' safety a main concern.

It says: "Volunteers are screened to ensure that they are healthy and free from other medications.

"Their family doctors are informed before their participation. An independent committee advises on the ethical aspects of the trial."

It adds that volunteers are given "all relevant information" on the tests which it describes as "very open".

It says it cannot discuss individual clients or trials.

The issue of humans being used as guinea pigs in pesticide trials hit the headlines in July when US firm Amvac Chemical Corporation asked a Manchester University company, Medval, to conduct tests of a toxic insecticide.

The trials were revealed by the Environmental Working Group in Washington which said they were unnecessary, although they are not illegal in the UK.

Brain damage

Normally pesticides and insecticides are tested on rats and mice.

EWG believes that, by using humans, the companies can increase the amounts of pesticides on crops and in food, water and air.