5 Things More Dangerous Than Sharks When You’re 47 Meters Down: A Diver’s Perspective
We recently saw 47 Meters Down. The new shark thriller starring Mandy Moore is designed to get your heart pumping, for sure. The fact that the silver screen has a habit of making us unnecessarily scared of sharks aside, the things that made us fear for the survival of the novice diving duo at the center of this horror flick had nothing to do with the animals that surrounded their dive cage.
1. Inexperienced divers going to significantly deeper depths than those for which they are trained can be a recipe for disaster even without a shark in the mix. Due to the effects of pressure, the volume of air a diver consumes in a given time typically increases with their depth—go deeper breath more. Other factors can also cause a diver’s air consumption rate to increase including exertion while swimming, speaking and cold water, just to name a few. Stress is also a major contributor to air consumption so two inexperienced divers being harassed by sharks like those portrayed in the film would result in a drastic increase in air consumption. At 47 meters even an experienced diver with a fantastic air consumption rate would quickly drain their scuba cylinder, probably in less than 20 minutes. An inexperienced diver swimming, talking and being harassed by sharks MIGHT make it five minutes at that depth.
2. The full face mask system used in the movie would be extremely dangerous for these inexperienced divers. Humans convert 5% of the oxygen we breathe into carbon dioxide which is then exhaled. The masks in the film do not have separate chambers for new and exhaled gas which would result in the diver breathing in carbon dioxide. The effects of breathing carbon dioxide on the surface are rapid breathing, rapid heart rate, fatigue, nausea and headache. Other, more severe effects can develop if the carbon dioxide exposure is not reversed. Underwater all of these effects would be more even sudden and life threatening.
3. According to the US Navy dive decompression tables a diver may spend up to five minutes at 160’ (47 meters) without needing to decompress during their ascent. The longer a diver stays underwater the greater their exposure to “the bends” becomes. The bends is a layman’s term for decompression sickness, which is caused by the accumulation of nitrogen breathed under pressure. This does not pose a problem until a diver begins to surface and nitrogen bubbles form in the bloodstream. The bubbles can cause the bends. The amount of time it would take a diver to safely surface increases as the amount of time s/he spends at that depth increases. It would actually take more than four hours to safely surface from a 60-minute dive at a depth of 160 feet.
4. Another risk often faced by divers at depths exceeding 80 feet is nitrogen narcosis. Commonly referred to as “rapture of the deep,” nitrogen narcosis makes a diver feel and act like they are inebriated, lacking inhibitions and generally numb to any concerns. Luckily nitrogen narcosis has not been known to leave any lasting effect so long as divers recognize it and ascend enough to limit its effects. It is easy to imagine the havoc nitrogen narcosis could play on a diver who needs to be very mindful of their surroundings and sharks!
5. Even in warm water hypothermia is a risk. Not wearing a proper fitting full coverage wetsuit can set up a diver for problems. Water can pull heat away from a diver’s body 25 times faster than air. Hypothermia’s effects on the body start with shivering which can become violent and lead to loss of coordination and function. In bikini-style suits the likelihood divers at 47 meters down would experience more than just a mild hypothermia is high.
Given the depth, equipment, clothing and stress factors, it would be a challenge for the most experienced divers to survive under these circumstances.
— Halle Minshall, Dive Safety Coordinator, & Erik Helgesen, Exhibit Diver