The Joint Effects of Extreme Heat and Wildfire Smoke on Paediatric Acute Care Utilisation

As climate change worsens, instances of combined extreme heat and wildfire smoke events are becoming more frequent. Despite their increased vulnerability, investigations on the joint effect of wildfire smoke and extreme heat on children's health are limited.

To investigate the joint effects of extreme heat and wildfire smoke on paediatric acute care utilisation (ACU) in California from 2006 to 2019.

In this case-crossover analysis, we assembled a time-series dataset of warm seasons, (May-September) for 1772 ZIP Code Tabulation Areas (ZCTA) in California from 2006 to 2019 to evaluate daily ACUs in the paediatric population (0-19 years). For wildfire smoke, we identified ZCTA-days exposed using a previously developed time-series dataset. For extreme heat, we calculated the daily ZCTA-specific maximum heat index. There were four exposure types: exposed to extreme heat alone, exposed to wildfire smoke alone, exposed to both events simultaneously (compound event) and not exposed to either event. We quantified the synergistic effects by comparing a child's exposures on the day when an ACU occurred to the child's exposure during control days.

We found 1100-105,788 ZCTA-days where exposure to both extreme heat and wildfire smoke events occurred across eight combinations of event definitions. The relative excess risk due to interaction (RERI) ranged up to 0.11 (95% confidence interval [CI]: 0.03, 0.19) with thresholds of the 95th percentile for extreme heat and 35 μg/m³ for wildfire PM_2.5, indicating a synergistic effect of extreme heat and wildfire smoke on paediatric ACUs. Positive RERIs were consistently observed for infectious enteritis, heat-related illness, asthma, endocrine nutritional and metabolic disease, and respiratory disease.

Investigating the synergistic effects of extreme heat and wildfire smoke events in paediatric populations is necessary to develop effective health protection strategies.