core temperature, can lead to unconsciousness or heart failure. Dehydration from prolonged exposure to hypertonic salt water—or, less frequently, salt water aspiration syndrome where inhaled salt water creates foam in the lungs that restricts breathing—can cause loss of physical control or kill directly without actual drowning. Hypothermia and dehydration also kill directly, without causing drowning, even when the person wears a life vest. Blunt trauma in a fast moving flood or river water can kill a swimmer outright, or lead to their drowning. Adverse effects of swimming can include: Exostosis, an abnormal bony overgrowth narrowing the ear canal due to frequent, long-term splashing or filling of cold water into the ear canal, also known as surfer's ear Infection from water-borne bacteria, viruses, or parasites Chlorine inhalation (in swimming pools) Heart attacks while swimming (the primary cause of sudden death among triathlon participants, occurring at the rate of 1 to 2 per 100,000 participations.) Adverse encounters with aquatic life: Stings from sea lice, jellyfish, fish, seashells, and some species of coral Puncture wounds caused by crabs, lobsters, sea urchins, zebra mussels, stingrays, flying fish, sea birds, and debris Hemorrhaging bites from fish, marine mammals, and marine reptiles, occasionally resulting from predation Venomous bites from sea snakes and certain species of octopus Electrocution or mild shock from electric eels and electric rays Around any pool area, safety equipment is often important, and is a zoning requirement for most residential pools in the United States. Supervision by personnel trained in rescue techniques is required at most competitive swimming meets and public pools. Lessons Traditionally, children were considered not able to swim independently until 4 years of age, although now infant swimming lessons are recommended to prevent drowning. In Sweden, Denmark, Norway, Estonia and Finland, the curriculum for the fifth grade (fourth grade in Estonia) states that all children should learn how to swim as well as how to handle emergencies near water. Most commonly, children are expected to be able to swim —of which at least on their back – after first falling into deep water and getting their head under water. Even though about 95 percent of Swedish school children know how to swim, drowning remains the third most common cause of death among children. In both the Netherlands and Belgium swimming lessons under school time (schoolzwemmen, school swimming) are supported by the government. Most schools provide swimming lessons. There is a