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Several studies have tied chemicals to negatively affecting brain development in children. Over the past few years, due to many factors, such as better screening methods, the incidence rates for autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD) and dyslexia have risen. According to a new study, researchers believe that the chemical environment could have played a huge role in increasing these rates as well.

For this study, a team of researchers from the Harvard School of Public Health and the Icahn School of Medicine at Mount Sinai reported identifying six more industrial chemicals that could detrimentally harm cognitive development. These chemicals are manganese, fluoride, chlorpyrifos, dichlorodiphenyltrichloroethane, tetrachloroethylene and polybrominated diphenyl ethers. The team, made up of Dr. Philip Landrigan at Mount Sinai School of Medicine in New York and Dr. Philippe Grandjean from Harvard School of Public Health in Boston stated that the number of toxic chemicals has doubled within the past seven years and could continue to increase if nothing is done.

"We know from clinical information on poisoned adult patients that these chemicals can enter the brain through the blood brain barrier and cause neurological symptoms," said Grandjean according to CNN. "When this happens in children or during pregnancy, those chemicals are extremely toxic, because we now know that the developing brain is a uniquely vulnerable organ. Also, the effects are permanent."

The team stated that manganese, which is found in drinking water, is tied to reducing math test scores and increasing children's hyperactivity levels. Increased exposure to fluoride from drinking water as well is tied to an average of a seven-point reduction in children's IQ (intelligence quotient) scores. The other four chemicals can be found in solvents and pesticides and have been tied to aggressive behaviors and social development issues.

"The consequence of such brain damage is impaired [central nervous system] function that lasts a lifetime and might result in reduced intelligence, as expressed in terms of lost IQ points, or disruption in behavior," the authors wrote reported by TIME.

The team of researchers had previously identified five harmful chemicals in a 2006 study. Those chemicals were lead, methylmercury, polychlorinated biphenyls, arsenic and toluene. The researchers had tied exposure to these chemicals to lower IQ scores, disruptive school behaviors, poor school performance and neurological abnormalities.

The researchers reported that there are two huge barriers that must be stripped down in order to protect children from increased exposure to these chemicals. First, there is not enough research and testing on industrial chemicals and their potential effects on children's brain development. If these tests were conducted before the chemicals were used, it could reduce exposure levels significantly. Second, researchers currently have to provide a huge amount of evidence before regulatory agencies would agree to step in and limit or restrict the use of certain chemicals.

"Our very great concern is that children worldwide are being exposed to unrecognized toxic chemicals that are silently eroding intelligence, disrupting behaviors, truncating future achievements and damaging societies, perhaps most seriously in developing countries," the authors wrote. "A new framework of action is needed."

The study was published in the journal, Lancet Neurology.