Seawater is a complex, naturally occurring solution with an average salinity of about 3.5%, meaning every kilogram contains approximately 35 grams of dissolved salts and minerals. The primary component is sodium chloride, which accounts for about 85% of the dissolved solids. Seawater also contains various other ions, including magnesium, sulfate, calcium, and potassium, as well as trace elements and dissolved gases. The human body interacts with this unique environment in diverse ways, from topical effects to profound internal reactions.
Topical Effects on Skin and Hair
The interaction between seawater and the body’s exterior layers is primarily driven by its mineral content. Seawater’s dissolved elements, such as magnesium, potassium, and calcium, can be absorbed topically by the skin. This absorption can strengthen the skin barrier and help with hydration by promoting the production of certain proteins in the epidermis.
The combination of salt and minerals can provide a mild exfoliation for the skin, helping to remove dead cells. For individuals with inflammatory skin conditions like eczema or psoriasis, the high magnesium content has been shown to reduce inflammation and roughness. However, the drying effect of salt crystals left on the skin and hair after evaporation can counteract these benefits. Salt is hygroscopic, meaning it draws water out of whatever it touches, leading to dehydration if not rinsed off.
This dehydrating effect is particularly noticeable on hair, where the salt can roughen the outer cuticle layer. The result is hair that feels brittle, is prone to tangling, and is difficult to manage. Furthermore, the salt can irritate the scalp in sensitive individuals or those with pre-existing conditions.
Respiratory System Interaction
Inhaling aerosolized seawater, often referred to as sea mist or sea spray, exposes the respiratory tract to tiny droplets containing salt ions and marine biogenic molecules. This natural saline effect can be beneficial, acting similarly to a commercial saline rinse by helping to clear the nasal passages and sinuses. The salt particles may help to thin mucus, improving mucociliary clearance in the upper airways.
The sea spray aerosols also carry microscopic organisms and bioactive compounds from the ocean, which may influence the body’s immune system. Exposure to low levels of airborne marine bacteria and their byproducts can mildly activate immune receptors in the lungs. Research suggests that this low-level exposure may help regulate immune responses.
However, direct inhalation of larger amounts of seawater, such as during a submersion event, is dangerous because of the water’s hypertonic nature. When hypertonic saltwater enters the lower respiratory tract, it creates an osmotic gradient that draws water out of the surrounding lung tissues and into the air sacs, or alveoli. This influx of fluid can lead to irritation, inflammation, and the dilution of pulmonary surfactant, which is necessary for maintaining lung function.
Osmotic Stress and Internal Fluid Balance
The most severe internal consequence of seawater exposure comes from ingestion, which causes profound osmotic stress on the body. Seawater, with an average salinity of about 35 parts per thousand, is a hypertonic solution, meaning it has a much higher concentration of dissolved salts than human blood plasma.
When a person drinks seawater, the high concentration of sodium chloride enters the digestive system and is quickly absorbed into the bloodstream. The body attempts to restore balance by diluting this excess salt, a process that requires water. Because the seawater is hypertonic, water is drawn out of the body’s cells and tissues through osmosis into the bloodstream and intestines to neutralize the high salt content.
This mechanism leads to cellular dehydration, ironically making the person thirstier despite having consumed liquid. The kidneys must work overtime to filter waste and excrete the massive sodium load. Since kidneys can only produce urine less concentrated than seawater, excreting the salt from one glass requires sacrificing more than one glass of the body’s fresh water stores. This leads to a net loss of fluid, rapidly accelerating systemic dehydration, increasing blood salinity, and placing severe strain on the kidneys, potentially leading to organ failure.
Pathogen Exposure and Contamination
Seawater, particularly in coastal areas, harbors various biological and chemical threats that can cause illness. Pathogenic bacteria, such as those from the Vibrio species, are naturally occurring in marine and estuarine environments and can infect humans through the ingestion of contaminated water or seafood, or through exposure to open wounds. Vibrio parahaemolyticus and Vibrio vulnificus are among the most common causes of seawater-related bacterial infections.
Viruses are also a concern, with enteric viruses like norovirus frequently detected in coastal waters, often due to sewage discharge or runoff. These viruses can cause acute diarrheal gastroenteritis and are a major public health risk, especially when concentrated by filter-feeding shellfish. Viruses and bacteria can enter the body through accidental ingestion, contact with mucous membranes in the eyes and nose, or by entering the bloodstream through cuts and scrapes.
Coastal waters may also contain chemical pollutants from agricultural runoff, industrial discharge, and microplastic fragments. These contaminants introduce substances like pesticides, heavy metals, and plasticizers into the marine environment, which can interact with the human body upon exposure. The presence of these agents complicates the overall health risk profile of recreational seawater exposure.