Ocean water is a complex solution containing a high concentration of dissolved salts and an array of microscopic organic and inorganic material. While a small, accidental mouthful during a swim rarely causes harm, ingesting larger quantities introduces significant risks to the body’s internal balance. The human body is not equipped to process seawater, which can rapidly lead to distress.
The Primary Danger: Salt Content and Dehydration
The most immediate danger from swallowing a substantial amount of ocean water is the effect of its high salt concentration on the body’s fluid balance. Seawater is a hypertonic fluid, meaning it contains roughly three to four times more salt (sodium chloride) than human blood, with a salinity around 35 parts per thousand. This extreme concentration creates a severe osmotic imbalance when ingested.
To manage the sudden influx of excess sodium, the kidneys must work overtime to excrete the salt. Because the kidneys can only produce urine with a salt concentration lower than that of seawater, they must pull water from the body’s existing fluid reserves to dilute the salt for excretion. This process leads to hypernatremia.
The result is a net loss of water from the body, causing greater dehydration than before drinking the seawater. This counterproductive attempt to hydrate draws water out of cells, including brain cells, to dilute the bloodstream. This leads to symptoms like weakness, muscle cramps, and profound thirst, as the body uses stored fresh water to maintain electrolyte equilibrium.
The Threat of Infectious Agents
Beyond the salt content, ocean water harbors a diverse population of microorganisms, some of which are pathogenic to humans. Ingesting this water can introduce bacteria, viruses, and parasites that cause recreational water illnesses. Common culprits include bacteria from the Vibrio genus, which naturally occur in marine environments and can cause gastrointestinal distress.
Other significant risks come from fecal contamination, often introduced through sewage runoff, storm drains, or agricultural discharge into coastal areas. These contaminants may include norovirus, which causes acute gastroenteritis, and parasites like Cryptosporidium and Giardia. Swallowing water contaminated with these agents typically results in diarrhea, vomiting, and abdominal pain.
The risk of infection is higher in coastal areas near dense populations, especially after heavy rainfall, which increases runoff. While the vast volume of the ocean dilutes microbes, high concentrations of infectious agents in near-shore waters can easily cause illness upon ingestion.
Risks from Chemical Pollution and Physical Matter
Ocean water can also contain non-biological contaminants, including industrial chemicals and physical debris. Coastal waters are the repository for runoff containing heavy metals like mercury and lead, pesticides, and various industrial chemicals. While often present in small amounts, high-concentration exposure in polluted areas can pose chronic health risks.
Physical matter such as microplastics is ubiquitous in marine environments and can be ingested with seawater. These tiny plastic particles can absorb persistent organic pollutants (POPs) and other toxic compounds, potentially carrying them into the digestive system. Ingesting large amounts of sand, sediment, or other debris can also cause physical irritation or blockages in the gastrointestinal tract.
Identifying Symptoms and When to Seek Medical Attention
Illness following the ingestion of ocean water typically manifests as gastrointestinal distress, caused by either pathogen infection or the immediate physiological shock of the high salt load. Common symptoms include nausea, vomiting, stomach cramps, and diarrhea, which may be severe or persistent. Signs of dehydration, such as dizziness, dry mouth, or dark urine, can also appear rapidly due to the hypernatremia.
If symptoms are mild and resolve quickly, rest and rehydration with fresh water or an electrolyte solution are usually sufficient. Medical attention should be sought immediately if an individual, particularly a child or elderly person, experiences persistent vomiting or diarrhea that prevents them from keeping fluids down. Other criteria for seeking professional help include high fever, bloody stool, or neurological symptoms like confusion or delirium, as these may signal a severe infection or advanced dehydration.