Carbon Monoxide

[overandover]

Is there an actual permissible level to be used for tank testing in SCUBA diving?

I am setting up a portable Carbon Monoxide detector. It has low and high level alarms, as well as Time Weighted Average and Short Term Exposure Limit alarms.

"The U.S. National Ambient Air Quality Standards for (Carbon Monoxide in) outdoor air are 9 ppm for 8 hours, and 35 ppm for 1 hour."

My depth limit is 130 fsw since I am only a recreational diver.

Figuring on using four atmospheres for my partial pressures calculations I end up with 2.25ppm for my lower alarm limit and 8.75 for my upper alarm limit. Thus I could dive at 130fsw for 8 hours with a surface reading of 2.25, or I could dive at 130fsw for 1 hour with a surface reading of 8.75, and still be within the U.S. National Ambient Air Quality Standards limits.

Any thoughts?

[Fotoz4FX]

On April 30, 1971 the EPA promulgated identical primary and secondary NAAQS (National Ambient Air Quality Standards) for CO at 9ppm for 8 hours and 35ppm for a one hour average NOT TO BE EXCEEDED more than once a year. The scientific basis for the standard was a study suggesting that low levels of CO exposure resulting in carboxyhemoglobin (COHb) concentrations of 2-3% were responsible for neurobehavioral effects in exposed subjects (Beard & Wertheim 1967).

The standards by which you are calculating your ambient ppm (under 4atm) are based on a 1991 document published by the US EPA which has since been modified as a result of information obtained up until 1999. The study has indicated that the information made available by the Beard & Wertheim study should no longer be considered a sound and scientific basis for the standard. Additional medical evidence acquired since 1970 has indicated that aggravation of angina pectoris and other cardiovascular diseases would occur at COHb levels as low as 2.7 to 2.9%. On August 18, 1980 the EPA proposed changes to the standard based on the findings of the revised criteria.

The proposed changes include: (1) retaining the 8 hour primary standard of 9ppm, (2) revising the 1 hour standard from 35ppm to 25ppm, (3) revoking the secondary standards because no evidence has been presented or reported at or near ambient pressures, (4) changing the form of the primary standards from deterministic to statistical and (5) adopting a daily interpretation for exceedances of the primary standards so that exceedances would be determined on the basis of the number of days on which the 8 or 1 hour average concentrations are above the standard levels.

See 1.2 Regulatory Background

You will also want to see 1.4.9 of the above for Health effects at altitude (conversely and unfortunately, there is no sufficient data to supplement your CO monitoring short of assumptive safety levels at ambient levels that multiply in their complexity at depth). As most AOW divers know, even O2 becomes toxic below 218ft/66m therefore the ppm of CO needs to be considered for it's effect at depth with volume/inspiration x 4 and I don't presume to have any data to support the standard effects on COHg under pressures.