( Health.com ) -- Attention deficit hyperactivity disorder ( ADHD ) appears to be on the rise in the United States , and in the search for explanations researchers have begun to scrutinize fetal exposure to a wide range of toxins , including lead , tobacco , pesticides , and chemicals such as PCBs . Mercury , a metal that affects the nervous system , is among the latest suspects to be investigated . And in a new study , researchers report that children who are exposed to higher levels of mercury in the womb are more likely to exhibit attention problems , hyperactivity , and other ADHD symptoms when they 're 8 years old . The study included roughly 600 mothers and children from New Bedford , Massachusetts . The researchers measured prenatal mercury exposure by analyzing samples of the mothers ' hair shortly after birth , and found that a child 's risk of ADHD symptoms increased by 40 % to 70 % past a certain exposure threshold ( 1 microgram per gram ) . The association was seen primarily in boys , which was n't unexpected , since previous research has shown that boys seem to respond differently than girls to chemicals that disrupt the endocrine system . Health.com : What causes ADHD ? 12 myths and facts . `` This study and one other recent study -- which both implicate prenatal mercury exposure [ in ] the development of ADHD -- suggest that the impact of mercury is much greater than previously recognized , '' says Dr. Bruce P. Lanphear , M.D. , a professor of children 's environmental health at Simon Fraser University , in Vancouver , British Columbia , and the author of an editorial accompanying the new study . The other recent study , which was conducted in Inuit children in QuÃ©bec and published earlier this fall , produced similar results : Children who were exposed to higher prenatal levels of mercury ( as measured by samples of umbilical-cord blood ) were more likely to exhibit ADHD symptoms between the ages of 8 and 14 . The new study , which appears in the Archives of Pediatrics & Adolescent Medicine , looks only at ADHD symptoms , rather than official diagnoses . And it shows only an association , not a cause-and-effect relationship . That said , it would be difficult to design a mercury-exposure study more rigorous than this one , since for ethical reasons pregnant womencould not be selectively exposed to high levels of mercury , which is known to be toxic to the developing fetus , says Dr. Elza Vasconcellos , M.D. , a pediatric neurologist at Miami Children 's Hospital . Health.com : Food additives that may affect ADHD . Aside from on-the-job exposure ( which is common in mining and certain types of manufacturing ) , people are most likely to be exposed to mercury by eating fish that have ingested the metal in contaminated waters . This is why the U.S. Food and Drug Administration recommends that pregnant women eat no more than two six-ounce servings of low-mercury fish per week . In the study , however , fish consumption was not independently related to ADHD symptoms . In fact , when the authors conducted a second analysis among the same group of mothers and children , they found that the offspring of mothers who reported eating more than two servings of fish per week while pregnant actually had a 60 % lower risk of ADHD symptoms . How to reconcile these seemingly contradictory findings ? One possibility is that the nutritional benefits of fish may offset the harmful effects of mercury , the authors suggest . Fish is a good source of omega-3 fatty acids , healthy fats that are essential to brain development , Vasconcellos says . Health.com : Quiz : Do you have adult ADHD ? `` It 's possible to eat fish low in mercury and high in nutritional value , and it 's possible to eat fish high in mercury and low in nutritional value , '' says Dr. Susan Korrick , M.D. , the senior author of the study and an assistant professor of medicine at Brigham & Women 's Hospital , in Boston . `` What really matters is the kind of fish you 're eating . '' But Korrick and her coauthors had no information on what type of fish these women ate . And even if they did , there 's no reliable way to estimate which fish are high in mercury and which are n't . As a general rule , a fish 's mercury content depends on the size of the fish ( larger fish tend to contain more mercury ) , where it was caught , and how far along it is in its lifespan . Smaller , oily fishes such as salmon , herring , mackerel , and sardines tend to be low in mercury and yet high in omega-3s . Shark , swordfish , and fresh tuna , on the other hand , are more likely to contain high levels of mercury and a relatively modest amount of omega-3s , though there are exceptions to all of these rules . The potential link between mercury and ADHD will need to be explored further in future studies . In the meantime , Lanphear says , avoiding mercury-laden fish is only a short-term precaution . A more permanent solution to the contamination problem , he says , will require societies to reduce mercury emissions across the board . More ADHD news from CNN Health .
ADHD symptoms were noted when the children were 8 years old . This research shows associations , not cause-and-effect . Fish consumption is not independently related to ADHD symptoms . Shark , swordfish and tuna are more likely to contain high levels of mercury .