A study tracking the development of more than 300 Mexican American children living in the farming region of Salinas Valley has provided another disturbing proof linking exposure to a pesticide - widely used in Australian agriculture - to rising cases of child Attention Deficit Hyperactivity Disorder (ADHD).

The US-based study shows a mother's exposure to organophosphate (OP) pesticides during pregnancy is a strong predictor of attention-related conditions in her child, especially boys at age five.

The new findings followed the release of another US-based study that discovered children with higher OP residue levels were about twice as likely to have ADHA, based on the study involving 1,100 ages eight to 15.

OP pesticides disrupt neurotransmitters, specifically acetylcholine which is responsible for maintaining attention and short-term memory.

These studies provide a growing body of evidence that OP pesticide exposure can impact human neurodevelopment, particularly among children, said Professor Brenda Eskenazi, from the University of California, Berkeley, and lead researcher on the later study.

The latest findings did not come as a surprise to Australian toxicology experts as they backed up the emerging OP-ADHD association.

Chris Winder, Professor of Toxicology at the University of NSW said OP pesticides are widely used in Australian agriculture and all are known to be neurologically active.

He said, It has been known for many years that hyperactivity conditions can arise in children from chemical exposures such as lead, tobacco smoke, phthalates, sodium benzoate, food dyes and some pesticides.

The development of a child's brain starts early in pregnancy and resumes even after birth, said Prof Winder. It is not surprising to see symptoms of ADHD in pre-school children found to be connected with exposure to OP pesticides during the prenatal period.

The findings also highlighted the evidence of harmful health effects influenced by human-made synthetic hormone disrupters called xenohormones, said Dr Irina Pollard, Associate Professor of Biological Sciences at Macquarie University in Sydney.

She calls for precautionary steps to be taken to reduce children's exposure to the man-made synthetic hormones and other pollutants that pose long-term adverse health implications.

California Medicine