A nationwide study of 893,212 mother-child pairs has associated exposure to wildfire smoke during pregnancy with a higher rate of autism diagnoses in children. The relationship was strongest when exposure occurred during the middle and later stages of pregnancy, particularly after approximately the 15th week. The researchers found no meaningful association between prenatal wildfire smoke exposure and ADHD diagnoses.
Published in JAMA Pediatrics, the study analyzed U.S. Medicaid health records for pregnancies conceived from 2007 through 2014. The children’s medical records were followed through the end of 2018. During that period, 8,357 children were diagnosed with autism and 22,302 with ADHD.
The researchers estimated wildfire-related PM2.5 exposure using a machine-learning system that mapped daily fine-particle concentrations across the contiguous United States. The analysis separated pollution produced by wildfires from background particle pollution and assigned weekly exposure estimates according to each mother’s residential ZIP code.
An increase of 0.75 micrograms per cubic meter in wildfire-specific fine particles across pregnancy was associated with a 30% relative increase in the rate of autism diagnosis. The study described this as a hazard ratio of 1.30, with the plausible range extending from a 7% to a 59% increase. The researchers noted that the relationship was not strictly linear, with estimates increasing at low and moderate exposure levels before leveling off at the highest concentrations.
The association varied by timing. Autism risk estimates rose from early to late pregnancy and were strongest during mid-to-late gestation. The authors said this period may be important because it includes rapid fetal brain growth, the formation of connections between neurons and the maturation of neural circuits.
By contrast, the same exposure increase was associated with only a 3% relative increase in ADHD diagnoses, a result that was not statistically significant. The study also found that non-wildfire background particle pollution was associated with a smaller, 10% relative increase in autism risk but was not associated with ADHD.
Wildfire smoke contains gases and fine particles measuring 2.5 micrometers or less in diameter. These particles can travel long distances, enter the lungs and move into the bloodstream. The study’s lead author, Stefania Papatheodorou of the Rutgers School of Public Health, said smoke may expose pregnant people hundreds or even thousands of miles from an active fire.
The researchers adjusted their models for factors including maternal age, race and ethnicity, previous births, conception season, folic acid prescriptions, smoking, alcohol or substance abuse, nutrition, temperature, background pollution and neighborhood socioeconomic characteristics. The findings remained similar in sensitivity analyses that extended the pregnancy window or limited diagnoses to those made after age 5.
The study found elevated autism estimates across racial and ethnic groups. Overall pregnancy-wide estimates were about 31% higher for both boys and girls, although the estimate for girls was less precise. The authors also reported regional differences, but the estimates were imprecise and their ranges overlapped.
The researchers emphasized that the study was observational and does not prove that wildfire smoke causes autism. Exposure was estimated from outdoor pollution at the mother’s residential ZIP code rather than from the amount of smoke each individual inhaled. The data also could not account fully for moves during pregnancy, time spent outdoors or the use of indoor air filters.
The study population consisted of low-income Medicaid beneficiaries, and diagnoses were identified through health care records. Differences in access to assessment and diagnostic services may therefore have influenced which children received a recorded autism diagnosis. The authors said autism has a complex, multifactorial basis and that the findings should not be interpreted as identifying a single cause.
Papatheodorou said the results suggest wildfire smoke during pregnancy may have implications for children’s longer-term neurodevelopment in addition to its established respiratory and cardiovascular effects. She also said protecting people from wildfire smoke requires cleaner indoor air, accessible information and support for families rather than placing the responsibility solely on pregnant individuals.
Sources: PsyPost.