A study by researchers at The University of Texas Health Science Center at San Antonio reveals a multiple-digits increase in the rate of autism for every 1,000 pounds of mercury released environmentally in Texas counties.
The study does not prove causation of autism by mercury, researchers are quick to point out.
The study compared mercury totals reported for 2001 in the 254 Texas counties to the rate of autism and special education services in nearly 1,200 Texas school districts. The districts, which range from urban to small metro to rural, enroll 4 million Texas children.
“The main finding is that for every 1,000 pounds of environmentally released mercury, we saw a 17 percent increase in autism rates,” said lead author Raymond F. Palmer, Ph.D., associate professor in the Health Science Center’s department of family and community medicine.
Large-scale mercury exposures such as accidental spills long have been implicated with developmental disabilities, but this study is among the first to examine the relationship between potentially chronic, low-dose mercury exposure and a developmental disorder such as autism, Dr. Palmer said.
Mercury is the third-most frequently found toxic substance nationwide, after arsenic and lead. Coal-burning power plants, which supply energy to cities and generally are in close proximity to population centers, release more mercury than any other source in the United States. Texas is fourth among the states in reported mercury releases, after California, Oregon and West Virginia.
Using statistical modeling, the researchers showed that increases in the rate of special education services were associated with higher mercury release levels. However, “it is the increase in autism that explains this relationship” in Texas, Dr. Palmer said.
The authors cautioned that the study is an ecological investigation based on county level and school district data. This type of study does not lend itself to interpretation at the level of the individual. This is only a first step in identifying areas for further investigation. The study also does not assess changes in mercury levels over time as a predictor of rates of change in developmental disorders. While the current study precludes causal interpretation, the team is working on a second report that will investigate the longitudinal association between mercury and autism rates.
Autism is a developmental disorder that varies in severity in individuals and is characterized by impaired ability to engage in normal social behavior and by behavior patterns such as repetitive motions and sounds. Autism is estimated to occur in as many as 1 in 200 children and is reported to be rising in prevalence, although statistics vary.
The study authors note that the new research “has implications for toxic substance regulation and prevention policies. The effects of differing state policies regarding toxic release of mercury on the incidence of developmental disorders should be investigated.”
The study is published in the peer-reviewed journal Health & Place, an Elsevier Ltd publication. Co-authors are Claudia S. Miller, M.D., from the department of family and community medicine at the Health Science Center; Zachary Stein from San Antonio; Stephen Blanchard, Ph.D., of the department of sociology at Our Lady of the Lake University; and David Mandell, Ph.D., of the University of Pennsylvania’s Center for Mental Health Policy and Services Research.