A Case of Bad Gas?
Bottom Line: While it is not common, the risk of breathing contaminated air from a scuba tank is a real threat to divers, especially when air fills are done by a source whose reputation is unknown or questionable. Because some contaminants are impossible to detect on your own, technology is required to insure the air you breathe is as fresh as a morning breeze.
The stank seemed so obvious to one diver. This was clearly a case of some bad gas, and all of the warning signs were there: A rickety scuba shack belching a hazy plume of exhaust from a cranky, gas-powered air compressor in a country where standards, testing and inspections for compressed air quality were most likely inconvenient suggestions.
His buddy sampled the same suspect air from the equally suspicious aluminum 80 tank with a prolonged whiff and declared, “I don’t smell anything.” Great. Now what? Civilization as we knew it was 500 miles away and so were—as is described by PADI—any “reputable scuba air sources” for compressed gas.
Here was our problem: How do we know that the gas we are about to breath at depth is free of contaminants? The truth is we can’t know without some technological help. There are several possible impurities in compressed air. Passing the so-called “sniff test” (a nose check for any odor from the tank air) or a "taste test" (a sort of flavor check of your air—there should be none) has important but limited screening values as one of the most insidious contaminants in a scuba cylinder is invisible to all of our senses.
According to the Centers for Disease Control and Prevention (CDC):
Carbon monoxide (CO) [is] an odorless, colorless gas, which can cause sudden illness and death... [It] is produced any time a fossil fuel is burned.
What makes CO so toxic is that it essentially replaces oxygen in the bloodstream at a rate of 240-to-1, and “[t]he net effect is the reduction in the oxygen-carrying capacity of the blood.”
According to the Navy and Marine Corps Public Health Center:
CO is the deadliest of the toxic gases commonly found in compressed air.
An acceptable level of CO at sea level can prove deadly at recreational diving depths because of Dalton’s law of partial pressures. But that begs this question: What is an “acceptable level” of CO? The short answer is not much, simply because our CO tolerance is very low considering the very small amounts we are regularly exposed to.
How small, you ask? Well, measurements of CO are in parts per million (ppm), where one molecule of CO is detected among 999,999 molecules of other gases, or 0.0001%. The average global concentration of outdoor CO levels is somewhere between 0.04 ppm and 0.12 ppm (0.000004% to 0.000012%)., 
The Compressed Gas Association, Inc. (CGA) and the American National Standards Institute (ANSI) established a standard for—believe it or not—air. The standard known as ANSI/CGA G-7.1-2011 requires that the compressed air in scuba tanks have 10 ppm or less of CO, or .001%. Even at recreational diving’s maximum operating depth of 130 feet/40 meters (about 5 bar/ata), the partial pressure of CO (PCO) of 10 ppm would be at an acceptable level of .005%, or equivalent to 50 ppm of CO in surface air.
Table 1 gives examples of scientific research on the effects of CO and the related levels for symptoms that range from a headache and nausea to unconsciousness and eminent death. A contaminated tank containing 800 ppm of CO might not immediately present symptoms at the surface, but at a depth of 99 feet/30 meters (4 bar/ata), the same tank of air could prove deadly within 30 minutes (0.08% x 4 bar/ata = PCO 0.32 bar/ata, or equal to breathing 3,200 ppm [0.32%] of CO in surface air).
Meanwhile, back at that desolate dive destination with that funky tank of gas… What do we do? Luckily, there are solutions available to analyze tanks for CO on the go. Here are a few options that might be worthy of consideration depending on your future dive itinerary. Prices range from $7.95 to $420.00:
DE-OX SAFE Carbon Monoxide Analyzer
DE-OX CO is able to test a tank fill for carbon monoxide content in any breathable gas mix, including air, or it can be connected to a gas or diesel compressor for in-line continuous monitoring.
POCKET CO SCUBA
In addition to being a general purpose CO detector, Pocket CO SCUBA has been configured to allow easy detection of extremely low levels of CO in SCUBA tanks, down to 2 ppm.
The world’s first dual gas (oxygen and CO) analyzer and data logger. Programmable via smartphone. Compatible with existing My Nitroxbuddy smartphone app.
A one-day use device that detects the presence of CO in breathing air. Tear open the packet and fill included balloon with tank air. If the air is contaminated by CO, the sensor button in the balloon will change color.
SENSORCON CO DETECTOR FOR SCUBA TANKS
Kit includes the waterproof Carbon Monoxide Inspector, carrying case, yoke adapter and male and female connectors for sampling air or CO.
OxyCheq Expedition CO Analyzer w/ Alarm
Expedition Carbon Monoxide Analyzer with alarm and carrying case. The alarm will sound when the level of CO reached 10 ppm (+/- 1ppm).
NUVAIR PRO CO ANALYZER
Fast response compact water resistant container. Range of 0-50 ppm. Alarm goes off at 10 ppm CO (BC flow adapter sold separately.)
DIVENAV monOX DELUXE
Carbon monoxide detector for scuba divers. Smartphone controllable.
 PADI Open Water Diver Manual (2013, p. 191).
 Journal of the American Board of Family Medicine. http://www.jabfm.org/content/11/6/481
 Discussed in detail in PADI’s Encyclopedia of Recreational Diving (2008, pp. 4-32, 33).
 Agency for Toxic Substances and Disease Registry, Centers for Disease Control and Prevention. https://www.atsdr.cdc.gov/ToxProfiles/tp201-c2.pdf
 World Health Organization, Regional Office for Europe. http://www.euro.who.int/__data/assets/pdf_file/0020/123059/AQG2ndEd_5_5carbonmonoxide.PDF
This page is provided for informational purposes only. It is solely the opinion of ATA/BAR DIVERS and is provided without compensation, affiliation or consideration of any kind. No endorsement of, or professional experience with, any equipment profiled on this page should be inferred or implied. Amazon Associates links go to support this site.
Table 1. Effects of Carbon Monoxide.
Figure 1. De-Ox Safe CO Detector
Figure 2. Pocket CO SCUBA
Figure 3. DiveNav COOTWO
Figure 4. CO-PRO™
Figure 5. Sensorcon SCUBA CO Detector
Figure 6. OxyCheq Expedition
Figure 7. Nuvair Pro CO
Figure 8. DiveNav monOx DELUXE.