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 ATSDR, breathing low levels of hydrogen sulfide for a long time can irritate the eyes, nose and throat, and cause headaches and breathing difficulty in people with asthma. Very high levels affect the lungs, heart and nervous system. One important detail: your nose stops noticing the smell after a few minutes at high levels, so not smelling it any more doesn’t mean the air is clean.

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.

We don’t publish conditions for specific beaches; use the live official sources:

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. We do not prescribe controlled substances.

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 costs US$79, the same price for every age, with no membership; your card is charged when you request it, with a full refund if the visit can’t take place. Request a visit online 24/7 — we confirm your time by email or text. 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.