An analysis of data collected from Kaiser Permanente Hospital in Southern California between 2001 and 2014 found that children whose gestational weeks 1 to 10 and 30 to 37 coincided with periods of very high nighttime temperatures had a 13 to 15 percent higher risk of being diagnosed with autism by age 5. This paper is Total environmental science.
Autism, or autism spectrum disorder (ASD), is a neurodevelopmental condition that affects how a person communicates, interacts socially, and experiences the world. This is called a “spectrum” because it can manifest in different ways and with different levels of support needs.
Common features include differences in social communication, restricted or repetitive behavioral patterns, and strong or unusual sensory sensitivities. Some people with autism speak fluently, while others speak very little or not at all. Many people with autism have strong interests, a strong attention to detail, or preferred routines that help them feel safe and ordered.
The causes of autism are not completely understood, but research shows that genetic influences play a large role. However, new evidence is emerging that prenatal exposure to certain environments may increase the risk of autism. Previous studies have linked increased risk of autism to maternal exposure to air pollution, per- and polyfluoroalkyl substances (PFAS), maternal health conditions such as fever, diabetes, hypothyroidism, and other factors.
Study author David G. Ruglio and colleagues investigated the link between exposure to extreme temperatures over 37 weeks of pregnancy and autism risk in a large group of children in Southern California. They point out that an increase in the number and intensity of heatwaves is a global concern and is already associated with increased hospitalizations and higher mortality rates.
Previous studies have already linked prenatal heat exposure to conditions such as neural tube defects, delayed neurodevelopment, and reduced language acquisition. In their study, these authors examined the association between weekly average daily minimum (nighttime) and maximum (daytime) temperatures during pregnancy and autism in children.
They analyzed data from Kaiser Permanente Hospital in Southern California collected between 2001 and 2014. Their data were derived from a broad cohort of 294,937 mother-infant pairs, including singleton pregnancies.
Children were followed from birth until age 5 to track autism diagnosis. Study authors derived temperature and relative humidity exposure estimates from the gridMET dataset. GridMET is a dataset containing daily highly spatially resolved surface weather data for the continental United States that is widely used in a variety of studies. From these data, the study authors derived the daily maximum and minimum temperatures in each study participant’s place of residence during pregnancy.
For each participant, we calculated weekly average maximum and minimum temperatures across all weeks of each mother’s gestation period. We also adjusted the data to account for other influencing factors, such as particulate air pollution, vegetation, and neighborhood conditions.
They found that of the 4,076 children diagnosed with autism, more than 80% were boys. Children with autism were more likely to be born to mothers who had never had children and had chronic illnesses such as diabetes or obesity.
The study authors defined extreme heat as temperatures reaching the 90th percentile (3.6°F above average) and 99th percentile (5.6°F above average) compared to the 50th percentile. Children whose gestational weeks 1 to 10 coincided with an extreme nocturnal heat cycle were 15% more likely to be diagnosed with autism by age 5 compared to children who experienced an average heat cycle. Similarly, children whose gestational age between 30 and 37 weeks coincided with a period of intense nocturnal heat had a 13% increased risk.
Interestingly, the researchers found no such association with daytime temperature. Researchers hypothesize that this discrepancy is due to pregnant women spending more time away from home and in air-conditioned environments during the day, making it difficult to measure their actual daytime heat exposure. Conversely, nocturnal heat exposure is easier to track and is known to disrupt sleep-wake cycles. Previous studies have shown that too little sleep in pregnant women can lead to neurocognitive delays in children.
“Exposure to high nighttime temperatures during early and late pregnancy was associated with autism risk in children, a worrying outcome in a warming world. Further research is needed to understand why daytime temperatures are not associated with autism risk,” the study authors concluded. The findings are particularly significant given that in places like California, nighttime temperatures are rising three times faster than daytime temperatures.
This study contributes to the scientific understanding of environmental risk factors for autism. However, it should be noted that the study design does not allow direct causal inferences to be made from the results, and the study could not account for individual access to indoor air conditioning.
The paper, “Prenatal extreme heat exposure and childhood autism,” 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.
