Sargassum health effects: the rotten-egg smell, asthma and when to get care

Sargassum is a brown seaweed that floats at sea in large mats. Since 2011, massive strandings on Caribbean coastlines have increased, according to clinical studies from Martinique. The health problem starts when it piles up on shore and decomposes.

As it rots, sargassum releases hydrogen sulfide (H₂S) and ammonia. According to ATSDR, the CDC’s toxic substances agency, hydrogen sulfide is a gas that smells like rotten eggs and is produced when bacteria break down organic matter. A paper in PAHO’s public health journal notes that decomposing sargassum produces the gas after 48 hours.

According to the University of South Florida (USF) Sargassum Watch System, large amounts of sargassum have appeared in the Caribbean Sea every summer since 2011 except 2013, and Caribbean shores see strandings in nearly every spring and summer. In the Atlantic sargassum belt, the amount typically peaks in June or July and is lowest in winter. In its September 2026 bulletin, USF reported that sargassum amounts kept declining in almost every region but were still relatively high compared with the same month in previous years.

Amounts vary a lot from year to year and coast to coast, so we don’t publish conditions for specific beaches. Before you go, check the National Weather Service in San Juan (opens in a new tab) and the satellite imagery from the USF Sargassum Watch System (opens in a new tab), which gives satellite data (virtual buoys) for eastern and western Puerto Rico, Culebra, Vieques and Mona and publishes a monthly bulletin.

This is what ATSDR says about the gas rotting sargassum gives off:

In Martinique, a study of 154 patients seen in 2018 for complaints attributed to sargassum found mostly neurological, digestive and respiratory symptoms. A larger study of 850 patients seen at Martinique’s university hospital between 2018 and 2024 describes a picture similar to low-level hydrogen sulfide exposure.

Those studies come from areas with massive strandings and weeks or months of exposure, for example people living on the shore. A short visit to a beach with a little sargassum is not the same. People with asthma or other lung disease are the ones who most need to take care.

ATSDR advises reducing exposure by avoiding areas that are sources of the gas. In practice:

If you have irritated eyes, a scratchy throat, a headache, nausea or your asthma is worse than usual, but no emergency signs, Dr. Diana Nieves Castro, a Puerto Rico–licensed general practice physician with several years of emergency-department experience, can see you by video. We evaluate, diagnose and treat. If appropriate, the doctor prescribes and we send your prescription to your pharmacy.

If you only need more of the inhaler you already use, that is a prescription refill. On vacation here? We see visitors too: see care for visitors. For red eyes, read about pink eye.

A doctor visit has the same price for every age, with no membership; your card is charged when you request it, with a full refund if the clinic can’t see you. Doctor online 24/7: video doctor visits 24/7 — book online and choose your time right away. We send you a secure video link that works on your phone, tablet or computer. For patients under 21, a parent or legal guardian stays on camera with photo ID. You need to be physically in Puerto Rico during the visit. Book your visit.

For your safety, with any of these signs call 911 or go to an emergency room. See when to go to the ER.