Scientists from Rutgers University (USA) analyzed data on nearly 900,000 births and concluded that exposure to wildfire smoke during pregnancy is associated with a higher likelihood of an autism diagnosis in the child. The work was published in the journal JAMA Pediatrics.
The study concerns microscopic solid particles formed during combustion. They are called PM2.5 - particles with a diameter of no more than 2.5 micrometers, that is, less than one thirtieth the thickness of a human hair. They are so small that they penetrate deep into the lungs and enter the bloodstream.
The researchers examined records from the Medicaid database, a government health care program for the poor that covers more than 40% of births in the country. The sample included 893,212 mother-child pairs with conception no earlier than January 1, 2007, and births before 2014. The average age of the mothers was about 25, and there were roughly equal numbers of boys and girls among the children.
The level of wildfire smoke at each woman's place of residence was estimated using a computer model that calculated particle concentrations for each square kilometer of US territory. These data were then matched with the children's medical diagnoses, which were tracked until the end of 2018.
During the observation period, autism was diagnosed in 8,357 children, and attention deficit hyperactivity disorder (ADHD) in 22,302. This is a condition in which a person has difficulty sustaining attention for long, restraining impulses and staying calm.
It turned out that an increase in the average concentration of fire particles over pregnancy of 0.75 micrograms per cubic meter of air (the difference between the lower and upper quartiles of the measured values) was associated with a 30% increase in the frequency of autism diagnoses. This is a statistical association that in itself does not prove that the smoke was the cause. The authors emphasize that the study is observational and that other factors could have influenced the result.
No such association was found with ADHD: the increase in risk was only 3% and was statistically insignificant. The association with autism was most pronounced in the second half of pregnancy, especially after about the 15th week.
"Even a moderate association can matter at the population level when many pregnancies are exposed, but additional information is needed to translate this result into an absolute increase in risk," explained lead author Stefania Papatheodorou.
The scientists note limitations: smoke levels were determined by postal code rather than by actual inhalation, and they did not account for how much time women spent outdoors or whether they used air purifiers. In addition, the data relate to 2007-2014 and to low-income families, so the results may not be fully transferable to other groups and to current conditions.
"Our results do not prove that wildfire smoke causes autism," Papatheodorou emphasized. "But protecting people from such smoke is important even far from an active fire."