An analysis of data from the Kaiser Permanente Southern California hospitals collected between 2001 and 2014 found that children whose gestational weeks 1-10 and 30-37 coincided with periods of very high nighttime temperatures had a 13-15% higher risk of being diagnosed with autism by age 5. The paper was published in Science of the Total Environment.
Autism, or autism spectrum disorder (ASD), is a neurodevelopmental condition that affects how a person communicates, interacts socially, and experiences the world. It is called a spectrum because it can appear in many different forms and with very different levels of support needs.
Common features include differences in social communication, restricted or repetitive patterns of behavior, and strong or unusual sensory sensitivities. Some autistic people speak fluently, while others speak little or not at all. Many autistic individuals have intense interests, strong attention to detail, or preferred routines that help them feel safe and organized.
Causes of autism are not fully understood, but research shows that genetic influences play a major role. However, there is emerging evidence that certain prenatal environmental exposures can increase the risk of autism. Studies have so far linked increased risk of autism with maternal exposure to air pollution, per- and polyfluoroalkyl substances (PFAS), maternal health conditions such as fever, diabetes, and hypothyroidism, and other factors.
Study author David G. Luglio and his colleagues investigated the association between exposure to extreme temperatures and autism risk across 37 weeks of pregnancy in a large group of children from Southern California. They note that the increasing number and severity of heat waves is becoming a worldwide concern, already implicated in increased hospitalizations and higher mortality rates.
Previous studies have already reported links between prenatal heat exposure and conditions such as neural tube defects, neurodevelopmental delay, or reduced language acquisition. In their study, these authors examined the links between weekly averages of daily minimum (nighttime) and maximum (daytime) temperatures during gestation and autism in children.
They analyzed data from Kaiser Permanente Southern California hospitals collected between 2001 and 2014. Their data came from an expansive cohort of 294,937 mother-child pairs involving singleton pregnancies.
The children were followed from birth until age 5 to track autism diagnoses. Study authors derived temperature and relative humidity exposure estimates from the gridMET dataset. GridMET is a dataset containing daily highly spatially resolved surface meteorological data for the contiguous United States that is used widely across various studies. From these data, study authors derived daily maximum and minimum temperatures at the residential address of each study participant during the pregnancy.
Weekly mean maximum and minimum daily temperatures were calculated for each participant across all weeks of each individual mother’s pregnancy. They also adjusted the data to account for other influencing factors, such as fine-particle air pollution, vegetation, and neighborhood conditions.
Results showed that over 80% of the 4,076 children with an autism diagnosis were boys. Autistic children were more likely to be born to mothers who did not have children previously and who suffered from chronic conditions such as diabetes and obesity.
Study authors defined extreme heat as temperatures hitting the 90th percentile (3.6°F hotter than average) and the 99th percentile (5.6°F hotter than average) compared to the 50th percentile. Children whose gestational weeks 1-10 coincided with extreme nighttime heat periods were 15% more likely to be diagnosed with autism by age 5 compared to children who experienced average heat. Similarly, the risk was 13% higher in children whose 30-37 gestational weeks coincided with periods of intense nighttime heat.
Interestingly, researchers found no such association for daytime temperatures. They hypothesize this discrepancy is due to pregnant women spending more time away from home or in air-conditioned environments during the day, making it difficult to measure actual daytime heat exposure. Conversely, nighttime heat exposure is easier to track and is known to disrupt the sleep-wake cycle; previous research shows that too little sleep for pregnant women can be linked to neurocognitive delays in their children.
“Exposures to high nighttime temperature during early and late pregnancy were associated with autism risk in children, a result of concern in a warming world. Further research is needed to understand why daytime temperature was not associated with autism risk,” the study authors concluded. The findings are particularly relevant given that nighttime temperatures in places like California are rising three times faster than daytime temperatures.
The study contributes to the scientific understanding of environmental risk factors for autism. However, it should be noted that the design of the study does not allow any direct causal inferences to be derived from the results, and the study could not account for individual access to indoor air conditioning.
The paper, “Prenatal exposure to extreme heat and autism in children,” was authored by David G. Luglio, Xin Yu, Jane C. Lin, Ting Chow, Mayra P. Martinez, Zhanghua Chen, Sandrah P. Eckel, Joel Schwartz, Frederick W. Lurmann, Nathan Pavlovic, Rob McConnell, Anny H. Xiang, and Md Mostafijur Rahman.