WebMD) Exposure to industrial chemicals may be responsible for a "silent pandemic" of brain development disorders affecting millions of children worldwide, and not enough is being done to identify the risks. That is the contention of two researchers who have studied the effects of chemical exposures on brain development for many decades. In an essay published online in the journal The Lancet, the researchers identified 202 potentially harmful industrial chemicals that may be contributing to dramatic increases in autism, attention deficit hyperactivity disorder (ADHD), and other brain disorders among children. Roughly half of the chemicals are in common use, but very few have been tested to determine their impact on brain development. "The bottom line is you only get one chance to develop a brain," Philippe Grandjean, M.D., of the Harvard School of Public Health, tells WebMD. "We have to protect children against chemical pollution because damage to a developing brain is irreversible." Tip of the Iceberg Grandjean and co-author Philip Landrigan, M.D., of New York's Mount Sinai School of Medicine, noted that of the industrial chemicals known to be toxic to the human brain, only five — lead, mercury, arsenic, PCBs, and toluene — have been proven to cause damage to the developing brain. These chemicals have been identified not because they are necessarily more dangerous than the others, but because they have been studied the most, Grandjean and Landrigan contend. "The few substances proven to be toxic to human neurodevelopment should be viewed as the tip of a very large iceberg," they wrote. Grandjean spent decades documenting the toxic effects of mercury exposure on the developing brain, and Landrigan spent decades studying the effects of lead exposure in children. Lead and mercury are among the few chemicals that are now strictly regulated to protect children. But regulation came long after the dangers were first recognized. Lead-based paint was first linked to sickness in children more than a century ago, but lead was not removed from paint and gasoline in the U.S. until the late 1970s and early 1980s. "Despite those early pediatric warnings, the largely unchecked use of lead in petrol, paints, ceramic glazes, and many other products through much of the twentieth century caused continued risk of lead poisoning," the researchers write. A Generation Exposed Almost all children born in industrialized countries between 1960 and 1980 were exposed to substantial amounts of lead from gasoline. The researchers write that lead exposure in this population could be responsible for a substantial reduction in average IQ scores. "A generation of American children was exposed to this very dangerous neurotoxin while we were doing traditional risk assessment," Grandjean tells WebMD. "We can't afford to make the same mistake again." Annette Kirshner, Ph.D., of the National Institute of Environmental Health Sciences (NIEHS) agrees that more expeditious ways of identifying chemical exposures that put children at risk are needed. The prevailing thinking among researchers studying autism and ADHD is that both genetic and environmental factors play a role in the childhood brain disorders. "There is still no good evidence linking any single environmental exposure to autism and ADHD," Kirshner tells WebMD. "It will probably require a global effort to understand the combination of factors that lead to these disorders." But Grandjean and Landrigan argue that exposure to industrial chemicals appear to have created a "silent pandemic in modern society." "Although these chemicals might have caused impaired brain development in millions of children worldwide, the profound effects of such a pandemic are not apparent from available health statistics," they wrote.