Even just a few mercury-contaminated fish dinners during pregnancy could impact your child's IQ forever.

Premature births and ADHD have long been known causes of low IQ in children, but a new study recently found that exposure to common environmental contaminants could be even bigger causes behind a societal brain drain.

Previous studies have shown that mercury, lead, and organophosphate pesticides cause developmental problems in children. This latest study, published in the journal Environmental Health Perspectives, shows the collective magnitude of the effects these environmental contaminants have on a population's IQ.

Mercury pollution comes largely from coal-burning electricity plants and builds up in many of the fish pregnant woman and children eat. Old paint dust is a common household source of lead exposure. Organophosphate pesticides are used to kill bugs by damaging their neurological processes, and are still commonly sprayed on non-organic fruits and vegetables. "Each one of these reduce IQ, even at the very lowest levels of exposures," says pediatrician Phil Landrigan, MD, professor and chair of preventive medicine at Mount Sinai Medical School in New York. "They silently erase IQ."

Looking at the data of more than 25 million children under the age of 5 and using exposure levels common in everyday living, the analysis found that lead exposure was responsible for 23 million lost IQ points among the group of 25 million children. Organophosphate pesticide exposure was linked to 17 million lost IQ points, while mercury caused a 300,000-point reduction. Pre-term birth was previously believed to be the biggest factor leading to IQ reduction, but this study finds premature birth accounts for about 35 million lost IQ points, less than lead exposure.

While about a 1½-point loss per child may not seem like much, it collectively causes a major societal problem. IQ points are worth money: Economists note that an IQ point is worth $10,000 over the course of a lifetime. "If each of the 4 million babies born each year loses a few IQ points due to lead, mercury, or certain pesticides, very quickly your economic losses go into the billions," says Dr. Landrigan.

Lower IQ scores in children and adults means lower productivity at school and work, and some researchers believe lower IQs could stress the criminal justice system because people with lower IQs are more likely to be in trouble with the law because they have poor decision-making skills and an inability to focus.

Since exposure to these chemicals can cause lifelong damage starting even at the beginning of a pregnancy, Dr. Landrigan notes it's important not just for children, but also women who are pregnant or intend to become pregnant within a few months, be greatly protected from exposure to pesticides, mercury, and lead. Steps for doing that include:

Avoid the "bad" fish. Certain fish are contaminated with mercury, but the good news is there are plenty of healthy options loaded with beneficial omega-3s—without the toxic additives. But take your seafood choices seriously: Dr. Landrigan says even a small number of bad fish meals could make a difference in your child's lifelong health.

Remodel smartly. Dr. Landrigan says pregnant women and children should never be in areas where people are sanding old paint.

Eat organic. Scientists have clearly shown that eating organic can reduce pesticide presence in the diet by about 90 percent.