Exposure to mercury in the womb and lead after birth can lead to Attention Deficit Hyperactivity Disorder or ADHD in children, says a new study from Canada.

The study was based on data available from 208 children who were part of the Cord Blood Monitoring Program (1993-1998) which recorded the levels of various compounds in children's blood at the time of their birth. These children, along with their mothers, participated in the present study that assessed their neurological development. Additional data were collected through teachers' assessment of the kids.

Researcher Gina Muckle, from Laval University, Canada, and colleagues from Canada and the U.S. found that higher levels of mercury at birth resulted in higher symptoms of attention deficient hyperactivity disorder (ADHD). Similarly, children with higher levels of lead in current blood samples were more likely to have behavioral problems.

ADHD is a common childhood disorder and children with ADHD have difficulty controlling their actions and paying attention. They are easily distracted and cannot focus on a single task, which can affect school performance and children with ADHD also have difficulty handling social situations.

Methyl mercury can be found in contaminated fish and cooking does not cut down the mercury exposure risk. Recently, a health group recommended that tuna fish should be removed from school lunchrooms after high levels of mercury was detected in these fish.

"It's quite an interesting sort of finding, really. In the Seychelles they found that the more mercury exposure before birth, the higher the child's IQ. In the Faroe Islands, the more mercury, the worse off the kids were," Justin Williams, MD, psychiatrist, University of Aberdeen in Scotland told WebMD. Williams was not involved in the study.

Williams explained that the difference in health benefits could be attributed to the presence of fatty acids in the fish in Seychelles that protected children against the effects of mercury, WebMD reports. "As always, you have to look at these studies and be very cautious, really. What we may find is that mercury is a marker for someone else. It's not necessarily a causal relationship," Williams told WebMD.

According to the Centers for Disease Control and Prevention, exposure to lead is common and preventable. According to the CDC, lead based paint and dust are the common source of lead exposure. Kids under 6 years of age must not be allowed near peeling walls or chewable surfaces that may have lead based paints. In May this year, CDC lowered the acceptable lead levels from 10 micrograms per deciliter to 5 micrograms.

The study was published in Environmental Health Perspectives.