As sargassum mats continue to wash ashore on Miami Beach, environmental engineering professor Helena Solo-Gabriele and her team at the University of Miami are answering the question that worries every parent building castles in the sand: Is it safe?
The answer, according to new research published in Exposure and Health, is reassuringly nuanced. Overall health risks from sargassum are low—but the seaweed poses a measurable threat that families shouldn't ignore, particularly for children who spend extended time on the beach or have pica, a condition that involves consuming nonfood items.
The culprit is arsenic, a naturally occurring contaminant that sargassum accumulates from ocean water. When massive amounts of seaweed wash ashore and begin to decompose, this arsenic doesn't stay locked in the rotting vegetation. Instead, it migrates into the surrounding environment—seeping into water, transferring into sand, and even entering the air. Solo-Gabriele's lab studies found that noncancer risks from arsenic exposure during casual beach play are minimal. However, the research identified small but measurable increased cancer risks in certain scenarios, particularly from skin contact and accidental ingestion. The risks compound with long-term, repeated, and prolonged exposures, making it crucial for beachgoers to understand how their behavior affects their exposure.
How sargassum is managed on shore matters enormously. When beach maintenance crews remove the seaweed entirely, arsenic exposure drops. But when sargassum is mixed into sand rather than taken away, arsenic levels in that sand climb significantly—a finding that underscores how mitigation strategies directly shape public health outcomes.
The decomposition itself is a hazard. As seaweed rots, it releases sulfur-containing gases, particularly hydrogen sulfide—that unmistakable rotten-egg smell. Lab studies show these emissions spike within the first couple of days of breakdown, and large, soaked accumulations can quickly become an air quality concern. Mechanical engineering assistant professor Jiayu Li points out that the odor is merely a warning sign. "The smell is only part of the story," Li explains. "If beachgoers notice a strong rotten-egg odor or feel eye, throat, or breathing irritation, that is a good sign to move away from the area." For sensitive individuals or those with respiratory conditions, this matters enormously.
The good news is that simple precautions work. Washing hands before eating at the beach and taking a shower after returning home significantly reduce exposure. Children should be discouraged from consuming sargassum, and families can avoid accumulations that emit the telltale hydrogen sulfide smell. Solo-Gabriele emphasizes that sargassum remains "an important part of ocean and coastal ecosystems"—the challenge is learning to coexist safely as seasonal blooms intensify.
The research paints sargassum not as a simple nuisance but as a dynamic environmental system with cascading effects on air, water, and sand. As another heavy sargassum season unfolds on Miami's shores, this evidence-based guidance offers families the clarity they need to enjoy the beach responsibly while respecting the limits of what their bodies can safely absorb.