While health problems from childhood exposure to lead and mercury are on the decline, these and other toxic chemicals continue to take a toll, a new study reports.

The progress likely owes to decades of restrictions on use of heavy metals. But researchers from NYU Grossman School of Medicine in New York City said that exposure to other toxic chemicals — especially flame retardants and pesticides — led to more than 1 million cases of intellectual disability in the United States between 2001 and 2016.

Unlike the heavily restricted metals, toxic chemicals are subject to fewer limits, the researchers noted.

"Our findings suggest that our efforts to reduce exposure to heavy metals are paying off, but that toxic exposures in general continue to represent a formidable risk to Americans' physical, mental, and economic health," said lead investigator Abigail Gaylord, a doctoral student at NYU Langone Medical Center.

"Unfortunately, the minimal policies in place to eliminate pesticides and flame retardants are clearly not enough," she added.

The study team reported that IQ loss from toxic chemicals dropped from 27 million IQ points in 2001-2002 to 9 million IQ points in 2015-2016. Though researchers called this sharp decline good news, they remain concerned about childhood toxin exposure.

Among children, the percentage of cognitive (mental skills) loss attributed to chemicals in flame retardants rose to 81% in 2015-2016, up from 67% in 2001-2002, the study found.

The substances analyzed are commonly found in household items and can build up in the body, damaging organs, the study authors explained in an NYU news release. For example, lead, mercury, flame retardants and pesticides can all interfere with the thyroid. Exposing a young child to any of these substances can lead to learning, developmental and behavioral impairments, they said.

The researchers noted that childhood exposure to toxins has resulted in $7.5 trillion in lost productivity and other costs.

According to senior study author Dr. Leonardo Trasande, a professor of pediatrics, environmental medicine and population health at NYU Langone, "Although people argue against costly regulations, unrestricted use of these chemicals is far more expensive in the long run, with American children bearing the largest burden."

The findings were published in the Feb. 15 issue of the journal Molecular and Cellular Endocrinology.