Assessment of hydrogen sulfide exposure in shoreline cleanup workers during the 2025 Atlantic Sargassum bloom

• Hydrogen sulfide (H 2 S) exposure was evaluated among Sargassum cleanup workers. • Atmospheric H 2 S concentrations exceeded 1 ppm in 46.3% measurements, exceeded 10 ppm in 1.7%, and reached peaks of 50.8 ppm • Self-reported common symptoms include itching, burning sensations, headaches, dermatitis, and fatigue. • An 8-hour permissible exposure limit of no more than 1 ppm of H 2 S is recommended to protect workers. Since 2011, massive blooms of pelagic Sargassum spp. have emerged as a recurrent environmental crisis across the tropical Atlantic. Along affected coastlines, thousands of tons can accumulate annually, releasing toxic gases during anaerobic decomposition and posing potential health risks. Hydrogen sulfide (H₂S) is of concern due to its adverse effects on the respiratory, neurological, gastrointestinal, and ocular systems. This study quantified H₂S exposure among Sargassum cleanup workers in the Mexican Caribbean during the unprecedented 2025 bloom, the largest recorded to date. 35 workers were equipped with a portable personal sensor (1 ppm resolution; 1-100 ppm detection range) that recorded H 2 S concentration at 1-s intervals throughout Sargassum removal activities. H 2 S concentrations ranged from <1 to 50.8 ppm. Overall, 46.3% of readings exceeded the Mexican occupational standard of 1 ppm for an 8-h shift, 1.7% exceeded 10 ppm, and short-term spikes surpassed 50 ppm. Questionnaire data indicated that workers reported dermatological, respiratory, neurological, ocular, and gastrointestinal symptoms consistent with known H₂S toxicodynamics, suggesting acute health impacts even at concentrations below <5 ppm. These findings support adopting a permissible exposure limit (PEL) of ≤1 ppm (8-hour time-weighted average) and a short-term exposure limit (STEL) of 5 ppm (15 minutes), in line with Mexican and UK safety standards. This study provides the first direct evidence of acute H₂S exposure among Sargassum cleanup workers and highlights the urgent need for continuous monitoring, protective equipment, and strengthened occupational safety measures in regions affected by massive Sargassum strandings.