The common experience of opening one’s eyes underwater in the ocean often results in an immediate, sharp sting. This uncomfortable sensation signals that the external environment is disrupting the delicate balance of the eye’s surface. While human tears are naturally saline, the high concentration of salt in saltwater creates a significant physiological imbalance. Understanding this reaction requires a look into how water moves across cell membranes.
The Science Behind the Sting (Osmotic Pressure)
The eye’s surface is protected by the tear film, which includes an aqueous component. This layer is isotonic, maintaining balance with the corneal cells at an average osmolarity of approximately 300 mOsm/kg. This concentration is equivalent to roughly 0.9% sodium chloride, which is why commercial saline solutions are used for eye care.
Natural seawater is highly concentrated, or hypertonic, with a salinity of about 35,000 ppm, making it nearly four times saltier than human tears. When the eye is submerged, osmosis dictates that water moves from a lower salt concentration to a higher salt concentration. Consequently, the salty seawater draws water out of the corneal epithelial cells.
This rapid dehydration causes the cells on the eye’s surface to shrink. The cellular shrinkage irritates the fine nerve endings embedded in the cornea, which the brain interprets as stinging and burning. This hypertonic effect is the primary mechanism responsible for the pain experienced in ocean water.
Immediate Results of Exposure
The cellular irritation caused by osmotic pressure leads to several predictable, short-term symptoms. The most obvious result is temporary redness, or hyperemia, as blood vessels on the eye’s surface dilate in response to the irritant. The initial stinging is quickly followed by dryness and grittiness, related to the surface cells losing moisture.
The body attempts to counteract the imbalance by initiating excessive reflex tearing. This helps flush the hypertonic solution, diluting the salt and restoring the tear film’s balance. Temporary blurry vision also occurs, resulting from slight irregularity on the corneal surface due to osmotic stress. These symptoms are fleeting and resolve quickly once the eyes are removed from the saltwater environment.
Distinguishing Saltwater from Other Water Sources
Irritation from natural saltwater differs significantly from that caused by other common swimming environments, such as pools or freshwater bodies. Seawater causes pain through osmotic dehydration due to its high salinity. Conversely, chlorinated pool water and natural freshwater are hypotonic, meaning they have a lower salt concentration than the eye’s tear film.
Exposure to hypotonic water reverses the osmotic process, causing water to rush into the corneal cells. This leads to temporary swelling and puffiness that impairs vision. Pool water also contains chemical irritants like chlorine and chloramines, which cause a separate chemical irritation known as chemical conjunctivitis. Saltwater pools are less irritating than the ocean, as their salinity is much lower (around 3,500 ppm), but they still contain chlorine.
Freshwater bodies, like lakes and rivers, lack the high salinity of the ocean and the chemical disinfectants of pools. While they cause temporary blurring from cell swelling, they present a higher risk of infection from waterborne pathogens, such as bacteria and Acanthamoeba. The mechanism of irritation is distinct for each water type, ranging from osmotic dehydration to chemical exposure or osmotic swelling.
Minimizing Irritation and Post-Exposure Care
The most effective way to prevent irritation from saltwater is to use protective eyewear, such as swim goggles or a diving mask. Keeping the eyes closed during submersion is the next best preventative measure. If the eyes have been exposed, immediate post-exposure care can minimize discomfort and recovery time.
The first step is to rinse the eyes thoroughly with clean, cool tap water or, ideally, a sterile, preservative-free saline solution. This helps wash away the concentrated salt and restore the eye’s osmotic balance. After rinsing, applying lubricating eye drops, often called artificial tears, can help replenish the tear film and soothe the surface. Avoid rubbing the eyes, as this can worsen irritation and potentially cause micro-abrasions on the corneal surface. If redness, pain, or blurred vision persists for more than 48 hours, seek professional medical attention.