Drowning - Cause

Cause

Approximately 90% of drownings take place in freshwater (rivers, lakes and swimming pools) and 10% in seawater. Drownings in other fluids is rare, and often relates to industrial accidents.

People have drowned in as little as 30 mm of water lying face down, in one case in a wheel rut. Children have drowned in baths, buckets and toilets; inebriates or those under the influence of drugs have died in puddles.

Drowning can also happen in ways that are less well known:

• Deep water blackout - caused by latent hypoxia upon ascent from depth, where the partial pressure of oxygen in the lungs under pressure at the bottom of a deep free-dive is adequate to support consciousness but drops below the blackout threshold as the water pressure decreases on the ascent. It usually strikes upon arriving near the surface as the pressure approaches normal atmospheric pressure.
• Shallow water blackout - caused by hyperventilation prior to swimming or diving. The primary urge to breathe (more precisely: to exhale) is triggered by rising carbon dioxide (CO₂) levels in the bloodstream. The body detects CO₂ levels very accurately and relies on this to control breathing. Hyperventilation artificially depletes this, but leaves the diver susceptible to sudden loss of consciousness without warning from hypoxia. There is no bodily sensation that warns a diver of an impending blackout, and victims (often capable swimmers swimming under the surface in shallow water) become unconscious and drown quietly without alerting anyone to the fact that there is a problem; they are typically found on the bottom.

• Secondary drowning - Inhaled fluid can act as an irritant inside the lungs. Physiological responses to even small quantities include the extrusion of liquid into the lungs (pulmonary edema) over the following hours, but this reduces the ability to exchange air and can lead to a person "drowning in their own body fluid". Certain poisonous vapors or gases (as for example in chemical warfare), or vomit can have a similar effect. The reaction can take place up to 72 hours after a near drowning incident, and may lead to a serious condition or death.