Florida red tides and adverse birth outcomes: Investigating harmful algae as a novel environmental exposure.

Algal blooms have been linked to adverse human health, but little is known about early life exposures. This ecologic study leveraged the 2016-2018 'red tide' blooms to investigate effects on birth outcomes among Florida Gulf coast residents. We estimated K. brevis concentrations and prevailing wind directions from June through October annually. We spatiotemporally linked these data with monthly counts of preterm, term low birthweight, and total live births to residents of coastal census tracts. Tract residents were considered 'exposed' when K. brevis in local waters exceeded various thresholds and winds blew onshore. Using a staggered difference-in-differences design, we compared preterm birth and term low birthweight birth rates pre- and post-exposure under different latency scenarios. Among 200 coastal tracts, 85 % neighbored waters where K. brevis exceeded 10,000 cells/L. Elevated K. brevis with onshore winds was associated with a concurrent decrease of 9.52 (95 % CI: 14.70, -4.70) preterm births per 100 live births, though lagged associations were statistically insignificant. Models of term low birthweight, which generally showed null associations for K. brevis elevations, had parallel trends violations, such that the results should be interpreted with caution. Since red tides occur in the warm season and high temperatures can increase the risk of preterm birth, it is possible that bloom-related odors and respiratory irritation prompted pregnant residents to stay indoors, avoiding the heat and potentially explaining the observed protective effect. However, the lack of individual-level, spatiotemporally-refined exposure measurements likely introduced misclassification. Future work should focus on improving exposure assessment methods and controlling for confounding by weather to clarify health risks from red tide exposures during pregnancy.