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Lead exposure from vehicle exhaust during childhood may have dramatically increased the number of psychiatric disorders in the U.S. and altered the state of mental health across the population, according to an observational study.

Over 75 years, researchers found that exposure to leaded gasoline -- which peaked in use during the 1960s, 70s, and 80s -- may have contributed to an estimated 151 million excess psychiatric disorders, according to Michael McFarland, PhD, of Florida State University in Tallahassee, and co-authors.

This corresponded to a 0.13-standard deviation increase in the total liability to mental health across the U.S. population during that time, they reported in the .

"America's history of lead use and American children's history of lead exposure has likely changed the mental health of our country for the worse over the last 75 years," co-author Aaron Reuben, PhD, of Duke University in Durham, North Carolina, told MedPage Today.

Following on childhood lead exposure, Reuben said this study was designed to collect a full picture of the effect of lead exposure in the U.S., especially during the period when lead exposure was highest in regards to vehicle exhaust, which wasn't banned until 1996. He added that numerous studies have linked lead exposure to worse mental health problems, including increases in attention deficit-hyperactivity disorder (ADHD) and depression.

Using available data on childhood blood lead levels, leaded gas use, and population statistics, the authors determined the estimated effect of lead exposure on population-level mental health. They calculated the effect using general psychopathology factor points, or mental illness points, which work similar to IQ points, Reuben said.

According to this analysis, the total U.S. population had gained 602 million factor points by 2015.

"We all have a sense that every IQ point you have matters, and across the population losing millions of IQ points is a bad thing," Reuben said. "Here, we're doing the reverse. We're saying how many mental health symptom points did the population gain?"

"You want as few of those as possible," he added.

Even with numerous studies showing the effects of lead exposure on overall health, Reuben noted that it is still important to quantify and understand the magnitude of risk for increased mental health issues caused by lead exposure.

"What our study says is, for the U.S. population, lead has been a significant contributor to that risk, and it's been an invisible one that almost no one has thought about in this way," Reuben said. "If you're trying to understand changes in the population mental health, you're trying to improve, you're trying to prevent disorder, you're trying to treat disorder, we would say lead needs to be considered in the pantheon of problems."

While this analysis focused on the full U.S. population, Reuben added that considering lead exposure for individual patients can offer some benefits as well, since clinicians can order testing and offer some .

To conduct the analysis, the authors used serial, cross-sectional blood lead level data from the National Health and Nutrition Examination Surveys and historic leaded gasoline data to estimate U.S. childhood blood lead levels from 1940 to 2015. They then calculated the population mental health symptom elevations from known lead-psychopathology associations.

To quantify that elevation the authors used the general psychopathology points scaled to match IQ scores; symptoms of anxiety, depression, and ADHD using z-scores; and differences in the personality traits of neuroticism and conscientiousness.

The analysis also revealed population-level changes across several specific disorders over the 75-year study period:

• 0.64-standard deviation increase in internalizing symptoms
• 0.42-standard deviation increase in ADHD symptoms
• 0.14-standard deviation increase in neuroticism
• 0.20-standard deviation decrease in conscientiousness

Lead exposure-associated mental health and personality trait differences were most pronounced for individuals born from 1966-1986, and peaked in 1966-1970.

The study had several limitations. First, the authors did not establish a causal link between leaded gasoline-based exposure and mental health issues. Instead, to prove the validity of their total population estimates in the absence of a proven causal association. Second, the analysis relied on consumption levels of leaded gasoline from 1940-1975 to estimate blood lead levels and did not capture lead exposure from other sources, so the authors noted that the result of this analysis likely identified the floor of estimated mental health issues related to lead exposure, rather than the ceiling.

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