Quantifying the public health impacts of <i>Karenia brevis</i> (Florida red tide) algae bloom exposure along Florida’s Gulf Coast

Florida red tide (Karenia brevis) blooms release brevetoxins, potentially affecting respiratory, gastrointestinal, and neurological health. Multiple studies have identified associations between red tide exposure and various health conditions, particularly respiratory. However, there remains a need to elucidate the causal mechanisms underlying these associations and to more precisely estimate the magnitude of their effects. This study aimed to estimate the causal effect of K. brevis exposure on acute respiratory, gastrointestinal, and neurological conditions using a large cohort, clinically validated diagnoses, and a comprehensive set of potential individual- and community-level confounders. An observational cohort was derived by using electronic health record data from a large clinical research data network covering >60% of health care providers in Florida. West coast residents within 5 km of the shoreline were included. Monthly K. brevis concentrations were categorized and linked to acute diagnoses from 2012 to 2019. Generalized estimating equations, incorporating an adjustment set calculated from an expert-reviewed directed acyclic graph, were used to estimate the causal effects of red tide on the diagnoses. Based on the assumption of correct model specification and the absence of unmeasured confounding or selection bias, red tide exposure showed a weak effect on increasing risk of acute respiratory conditions (adjusted odds ratio = 1.001 per log cells/L higher, p = .012) and gastrointestinal conditions (adjusted odds ratio = 1.002 per log cells/L higher, p = .010). No appreciable effect was found for neurological conditions. The interaction between wind and red tide suggested that onshore winds combined with higher red tide levels may exacerbate the risk of respiratory and gastrointestinal illness. This causal analysis brings further evidence that red tide exposure may increase the risk of acute respiratory and gastrointestinal illness, albeit with small effect sizes, contingent on the validity of our causal model assumptions. Thus, environmental policies apt to reduce K. brevis growth in the Florida west coast might also produce positive public health effects.