by Madi Swayne
Originally published at ScientificAmerican.com
Every year, millions of people, including myself, make many millions of recreational scuba dives, very rarely with any ill effects. What goes unappreciated by many divers is the scientific basis for their safe exploration of the underwater world. The history of the science of safe diving begins with a research paper published in 1908 by a British physiologist John Scott Haldane.
The self-contained breathing apparatus (SCUBA) was still forty or so years in the future with the work of Jacques Cousteau and others. Haldane motivated his work by stating, “Men who have been working in compressed air, and in diving, preparing foundations of bridges, etc. under water, or making tunnels or shafts through water-bearing strata, are liable on their return to atmospheric pressure to a variety of symptoms generally known as ‘diver’s palsy’ or ‘caisson disease,’ but which may more conveniently be designated ‘compressed-air illness.’” This statement and others in that paper created the very vocabulary of diving accidents.
The origin of “the bends” as a euphemism for decompression sickness may have come from (or at least be reinforced by) Haldane’s observation that the goats he used to test decompression strategies would sometimes emerge with bent forelegs, either as a result of guarding against pain from bubbles in joints or possibly central nervous system injuries. Goats were readily available as experimental subjects and were considered usefully close to humans as models of the physicological responses to too-rapid deconcompression.
Decompression Sickness is caused by nitrogen absorbed in the body being released too quickly as the pressure surrounding the diver decreases, usually due to an unsafe and rapid ascent to the surface. The pressure at depth naturally causes the tissues within the body to become progressively saturated by nitrogen, the inert gas that makes up 78% of air.
As the diver goes deeper, the pressure increases. Greater time at depth loads more nitrogen into tissues, ultimately saturating them. If the diver ascends too rapidly, super-saturation of tissues ultimately leads to bubble formation in these tissues, leading to a myriad of symptoms ranging from pain or loss of sensation to, in extreme cases, death. This was reflected in Haldane’s work in a terrible toll of injured and deceased goats.
Because different systems within the body have varying levels of perfusion and hence rates of gas exchange, Haldane modeled the bodies of goats, and by extension humans, as consisting of various theoretical compartments with different halftimes for gas equilibration.
Each compartment with a halftime T on-gassed or off-gassed at a rate given by ln 2/ T. Haldane was able to fit his “goat data” using five theoretical compartments with halftimes of 5, 10, 20, 40 and 75 minutes. Although these compartments were purely mathematical constructs, one could associate efficiently perfused tissues like blood and brain with short halftime compartments, and slowly perfused tissues like bone and ligament with slower compartments. Haldane assumed, with empirical rather than theoretical justification, that tissues could withstand up to 2:1 over-pressurization during ascent without harm, and this provided a basis for cumbersome but straightforward calculations of decompression strategies based on keeping all five compartments within the 2:1 rule.
Later, it was realized the Haldane 2:1 rule was too conservative for fast compartments, and the U.S. Navy created their first dive tables in 1956 based on variations of Haldane’s scheme. The Navy Tables are based on six, rather than five compartments with a maximum halftime of 120 minutes. Instead of a uniform 2:1 ratio, each compartment had its own “M-value”, and these were as large as 4:1 for the fastest compartments. Some of the modern recreational dive tables are closely related to these Navy Tables, and thus their connection to Haldane’s 1908 work is apparent.
As I am training to be a science diver, I owe a debt to Haldane and his goats. Dive physiology and dive physics has been much of my classroom work for this program. And this has shaped my actual diving. Before making a dive my buddy and I plan our dive. The planning process includes what is known as a SEABAG pre-assessment. Each letter in the acronym refers to a different aspect of the dive plan: S-ite, E-mergency, A-ctivities, B-uoyancy, A-ir, G-ear and go. We then also look at a dive table that corresponds to the gas we are breathing (air or nitrox) to determine the maximum time we can remain at the planned depth without incurring a mandatory decompression stop. While this 5” x 7” plastic card is covered in numerical charts and may initially be very overwhelming, with practice it has become easy to use. Haldane bent his goats so that I can dive with reduced risk of the bends.
About the Author: Madi Swayne is a freshman working toward dual degrees, a BA in Environmental Studies, and a BS in Policy, Planning and Development. She has a strong interest in environmental policy and marine pollution. Her love for the ocean and her concern for its protection come from a lifelong passion for surfing. Madi is actively involved as a guide for SCOutfitters, a student-run outdoor adventure group on campus.
Editor’s note: Scientific Research Diving at USC Dornsife is offered as part of an experiential summer program offered to undergraduate students of the USC Dana and David Dornsife College of Letters, Arts and Sciences. This course takes place on location at the USC Wrigley Marine Science Center on Catalina Island and throughout Micronesia. Students investigate important environmental issues such as ecologically sustainable development, fisheries management, protected-area planning and assessment, and human health issues. During the course of the program, the student team will dive and collect data to support conservation and management strategies to protect the fragile coral reefs of Guam and Palau in Micronesia.
Instructors for the course include Jim Haw, Director of the Environmental Studies Program in USC Dornsife, Environmental Studies Lecturer Dave Ginsburg, SCUBA instructor and volunteer in the USC Scientific Diving Program Tom Carr and USC Dive Safety Officer Gerry Smith of the USC Wrigley Institute for Environmental Studies.