Scuba Q and A:
How Deep Can You Dive Using Air?
Q. How deep can someone scuba dive using normal air?
There are a number of factors that determine the Maximum Operating Depth (MOD) of any gas mixture. These include:
Inert gas narcosis using air begins to become debilitating through anaesthetic effect below 30m/100ft. This is due to the high lipid solubility of the nitrogen component (79%) in air. The correlation between the lipid solubility of a gas and its anaesthetic property is defined in the Meyer-Overton Law of Lipid Solubility.
Read more about Narcosis in this article: Nitrogen Narcosis – Perceptions of Susceptibility
CNS Oxygen Toxicity
Divers have to limit their oxygen exposure to a maximum partial pressure of 1.4 ppO2 in order to avoid Central Nervous System (CNS) toxicity. At high pressure, oxygen interferes with neural function and will cause convulsions. This 1.4 ppO2 limit equates to a 56.6m/185ft maximum depth when breathing air (21% O2).
Read more about Oxygen toxicity in this article: Oxygen Exposure Mangement For Divers
The issue of Gas Density and its role in CO2 retention (hypercapnia) is woefully under-educated by many scuba training agencies.
Gas density has been well studied and it is proven that a greater density of breathing gas reduces respiratory efficiency that leads to CO2 retention.
Studies by the US Navy Experimental Dive Unit (NEDU) have illustrated a huge uptake in CO2 retention when breathing gas density exceeds 6g per litre.
When breathing air, that 6g/L limit is reached at just 36m/120ft.
To date, most of the major recreational dive agencies have simply ignored those scientific findings. They don’t teach gas density at all, to either instructors or technical divers.
If big agencies DID acknowledge that issue and applied appropriate safety measures, then it’d severely curtail recreational deep diving – and that’d impact industry profits.
There are some (more ethical) agencies that actively teach gas density management – and they apply a 30m/100ft limit to air/nitrox diving. Beyond which they use helium (trimix) mixtures to mitigate both gas density and inert-gas narcosis risks.
Simon Mitchell, a leading decompression and hyperbaric physics researcher, gives an excellent lecture on gas density on this YouTube video. Every dive instructor and deep diver should watch it…
About the Author
Andy Davis is a RAID, PADI TecRec, ANDI, BSAC and SSI qualified independent technical diving instructor who specializes in teaching advanced sidemount, trimix and wreck exploration diving courses across South East Asia. Currently residing in ‘wreck diving heaven’ at Subic Bay, Philippines, he has amassed more than 9000 open circuit and CCR dives over 27 years of diving across the globe.
Andy has published many magazine articles on technical diving, has written course materials for dive training agency syllabus, tests and reviews diving gear for major manufacturers and consults with the Philippines Underwater Archaeology Society.
He is currently writing a series of books to be published on advanced diving topics. Prior to becoming a professional technical diving educator in 2006, Andy was a commissioned officer in the Royal Air Force and has served in Iraq, Afghanistan, Belize and Cyprus.