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Natalie Gibb

Bad Air

By March 9, 2011

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Yesterday, I made my triumphant return to diving after a serious ankle injury - almost triumphant. Since nearly two months had passed since I last used my scuba gear, I set up my equipment carefully, double checking every piece for damage and ensuring that it functioned properly. I breathed from the regulators and tasted the air, and everything appeared normal. Thrilled to return to the caves, I jumped in the cenote and began my dive, giggling into my regulator with delight.

The cave dive I chose for my first dive back is an easy, shallow cave with beautiful formations. My plan was to make a relatively short, hour-long dive so as not to stress my newly healed ankle, but my dive did not last even that long. As I followed the contour of the cave downward from fifteen to about twenty-five feet, I began to feel a bit odd. While I wasn't frightened, my heart rate and breathing rate quickened, and I began to taste a strange, metallic flavor in my mouth. My head hurt and I felt nauseated. Recognizing that these signs could possibly point to contaminated air, I turned around and headed out of the cave.

Contaminated air is extremely rare in recreational diving. Most divers will never receive a contaminated scuba tank because tank fill stations follow very strict regulations to ensure that the compressed air a diver receives is clean and dry. However, freak accidents do occur. Filters may break without notice, trace amounts of lubricant can be pumped into tanks, or contaminants such as car exhaust can enter the compressor through the uptake vent. Again, this is extremely, extremely rare, but it does happen and divers should know how to recognize that a tank is filled with contaminated air because diving with contaminated air may lead to unconsciousness and death.

Before diving:

Smell and taste the air in the scuba tank before ever breathing from it underwater. All divers are trained to test their tank in this manner during their open water course, but rarely perform the check after certification. Smelling the air is particularly effective because a diver's sense of smell is more sensitive to contaminants than his sense of taste. Report any unusual odors or flavors to your dive guide, especially odors reminiscent of smoke, burning plastic, and exhaust. When in doubt, do not dive with the tank.

If you pick up your tanks from air fill station, check out the compressor. Make sure that the compressor's air intake is located where it can take in fresh air uncontaminated by car exhaust and other pollutants. One trick I have used is to play stupid and ask the technician filling that tanks to explain the compressor to me. This helps to learn how the compressor works and allows a diver to confirm that maintenance is up to date.

During a dive:

Be warned that the most common contaminants found in compressed air are carbon monoxide and carbon dioxide. These are both odorless, tasteless gases. Normally, these gases are present in a tank because exhaust or another pollutant has been compressed along with the air, and a diver can recognize that the air is polluted by its unusual smell. However, it is conceivable that that these gases could be present without exhaust and other contaminants. In this highly unlikely scenario, a diver must understand how to recognize the signs and symptoms of contaminated air since he won't be able to recognize the contamination by the smell. Some signs and symptoms are:

cherry red lips and fingernails (remember underwater they might look dark green)
paranoia and confusion
accelerated heart rate
accelerated breathing rate
nausea and vomiting
visual disturbances such as tunnel vision

Many of these signs and symptoms are similar to those of oxygen toxicity and narcosis. Thankfully, the solution is the same - end the dive as quickly as is safe. If you suspect contaminated air, try breathing from a buddy's tank or a different tank (although if your tank is contaminated, chances are that your buddy's tank is as well).

I recognized several of these symptoms in myself and ended the dive before I had a problem. I am still not sure why I didn't smell or taste something in the air before the dive, but I can only assume that it was such a small amount of contamination that it wasn't noticeable until I breathed from the tank under pressure. My ability to recognize my symptoms kept me safe.

While the chance that the average recreational diver will ever receive tainted air is almost zero, it does happen. Always checking tank air before a dive and learning to recognize the signs and symptoms of breathing contaminated air is a logical precaution that will help to keep a diver safe in the highly unlikely even that he does receive bad air.

More Diving Safety Articles

What is a PFO and why should divers know about it?
How can you avoid problems with carbon dioxide?
Are you (still) medically fit to scuba dive?

Speak up! Have you ever received contaminated air? How did you know?

Image copyright istockphoto.com, cbpix


March 10, 2011 at 8:01 am
(1) One of the Mike's says:

Becareful girl.

I have been lucky that never had any problem with contaminated air. And I have been a little lax and don’t check air near as much as I should.

March 10, 2011 at 1:48 pm
(2) Richard says:

Assume you followed up on this, and talked to the fill station folks. If one tank was bad, there could have been dozens of others. Did you track down the cause?

March 10, 2011 at 2:03 pm
(3) Natalie L Gibb (Editor) says:

@ Richard. Yup. Apparently the carbon filter (?) broke somehow. They caught the problem and emptied all the tanks and refilled them with clean air, but mine must not have been 100% emptied. Perhaps someone opened them and simply didn’t drain them completely.

March 10, 2011 at 5:33 pm
(4) Denise says:

It’s great that you’re back diving Natalie. What a comeback entrance. You have these experiences to train us and I appreciate the reminders.

March 11, 2011 at 1:20 am
(5) Sharon & Jordan says:

Glad to know you’re finally getting back underwater after your ankle injury. Hope one of these days we’ll have the opportunity to dive with you on the dive we were going to do together that day!

Thanks for sharing this scary story and helpful info. Stay safe!

March 13, 2011 at 12:32 am
(6) Phing says:

Great article. Very good tips. I teach scuba diving in Thailand and have been lucky not to ever have contaminated air. You seem to have handled it very well without panicking. It made my heart race just reading it.

March 13, 2011 at 2:34 pm
(7) Jean says:

Scary stuff. Did you fill up at that station in the middle of the jungle near Playa?

November 24, 2012 at 6:18 am
(8) Butch Winkler says:

I made dive this weekend that did not go well. Half way through the dive my breathing became labored and fast. I was getting a headache and constantly coughing with an agitated throat. I did smell or taste the air until in the water and terminated after 40 minutes. After surfacing I did not feel well and that lasted mildly through the night. I have been diving since I was 19 and am now 67. I run 6km in 33 min 4x a week, lift 3 x and have not experienced any health problems. The dive was only 25 ft. I will of course see a doctor next week but wondered if you though this might be the cause if nothing medical shows up. I terminated my weekend diving plans and will notify dive shop. I am also certified divCon with ssi and padi dive master and make50 to 100 dives per year. BUT…. With that said I DID NOT smell the air.
Embarrassed but still growing after all these years


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