Continued Carbon Monoxide - Cozumel

[cruiser]

Had 2 tanks filled from my LDS - 0 ppm CO

 

[John C. Ratliff]

I personally carry a combination O2 and CO monitor I designed myself around a lab regulator that gives 2lpm output. There's a cleanable stainless 10 micron filter inline. The regulator lets me determine how full the tank is. The O2 is a typical nitrox analyzer. For the CO I use a cheap Korean CO detector which I calibrate against a lab CO monitor. The CO detector sets off a buzzer at 2ppm CO and does not read out the actual CO level.

Why the 2ppm? Early in life I put myself through school in part by working as a fireman. I understand they don't do it anymore, be we were subjected to CO at several levels, not so we would understand or be able to detect CO, but rather to give us real world experience in the effects. I never want to experience those higher levels again and you just can't detect lower levels at all until it's too late. So I'm not going to accept anything that is less than trace above ambient.

One point, that I haven't heard yet and was the reason I started carrying a CO monitor, is that a friend had his own compressor and never had a problem until one day. One time we hooked up tanks and detected exhaust fumes. After further checking he discovered that the telephone company was right outside his shop with a truck that had an exhaust stack that directed it's output right at his roof mounted intake. That was years ago and he bought a lab CO monitor. The point is that even a good shop can have problems.

Why the 2 ppm recommendation? I'm pretty sure you know, but in case (and for the others that may not), I'll explain it a bit better. It goes back to some basic physics, and Dalton's Law of Partial Pressures.

Dalton's law (also called Dalton's law of partial pressures) states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture. This empirical law was observed by John Dalton in 1801 and is related to the ideal gas laws.

This has special significance to us as divers. The "Partial pressure in diving breathing gases" is also explained in a different page in Wikipedia.

In recreational diving and professional diving the richness of individual component gases of breathing gases is expressed by partial pressure.

I'll explain more on this after supper...stay tuned...

...It is later, and I've got my chores done for the day. I've played around with my old Healthwasy SCUBA regulators too. So it's been an interesting evening for me. Now, where were we? Oh yes, discussing CO poisoning and why a 2 ppm limit for CO in scuba air for diving. You have the links above for Dalton's law. So each component of air exerts its own partial pressure according to the sum of all the components in the air. As depth changes, the absolute pressure changes too. So a partial pressure of 10 ppm at the surface (1 atmosphere absolute, or 1 atm) is 10 ppm x 1 atm = 10 ppm. But as we go deeper, the partial pressure increases according to Dalton's law too, so that at 33 feet of sea water (34 feet of fresh water) we have half the volume but twice the concentration in that volume, which is 2 atm absolute. At 66 feet it is 3 atm absolute, which means 1/3 the volume but 3 times the concentration (or density) of the gases. 99 feet is 4 atm absolute, or 1/4 the volume but 4 times the density of the gas (4 times the number of molecules per unit volume, such as the lungs).

We can apply this to CO as a contaminant in the air too. So if we dive to the sport diving limit, which many used to feel was 132 feet, or 5 atmospheres absolute pressure, and our scuba tank has 10 ppm as measured at the surface in ambient air (1 atm absolute), then at 132 feet that is the equivalent of 50 ppm at the surface. This is a significant load of CO. If we start out with a surface contamination level of 25 ppm (which could easily be attained), at 132 feet it is 25ppm x 5 atm absolute = 125 ppm.

I am an industrial hygienist, and so study the effects of chemicals on the human body. I have a book titled Casarett & Doull's Essentials of Toxicology. On page 417 of that book, they talk about carbon monoxide.

Carbon Monoxide Carbon monoxide is classed toxicologically as a chemical asphyxiant because its toxic action stems from its formation of carboxyhemoglobin, preventing oxygenation of the blood for systemic transport (see Chap. 11).

Analysis of data from air-monitoring programs in California indicates that 8-h average values can range from 10 to 40 ppm of CO. Depending on the location in a community, CO concentrations can vary widely. Concentrations predicted inside the passenger compartments of motor vehicles in downtown traffic were almost three times those for central urban areas and five times those expected in residential areas. Occupants of vehicles traveling on expressways had CO exposures somewhere between those in central urban areas and those in downtown traffic. Concentrations above 87 ppm have been measured in underground garages, tunnels, and buildings over highway.

No overt human health effects have been demonstrated for COHb levels below 2 percent, but levels above 40 percent can be fatal as a result of asphyxia. At COHb levels of 2.5 percent resulting from about 90-min exposure to 50 ppm CO, there is an impairment of time-interval discrimination; at approximately 5 percent COHb, there is an impairment of other psychomotor faculties. Cardiovascular changes also may be produced by exposures sufficient to yield COHb in excess of 5 percent.

"COHb" means "carboxyhemoglobin," and a 5% COHb level means that about 5% of the red blood cell receptor cites carry CO rather than oxygen. If you are a smoker, you already carry a sgnificant load of COHb in your blood. Similarly, if you drive through tunnels (Seattle area, for instance) you will gain a significant load of COHb. One fact not mentioned in the book is that CO has a much greater affinity for the red blood cells than does oxygen. The CO will preferentially go into the cites on the red blood cell over oxygen, so that if there is one molecule of CO and one of O2, it is the CO which gets the ride. The preference level is something like 8:1 (8 CO molecules will get the ride to 1 O2 molecule). Diving is strenuous, and so we need all the oxygen available to us (although it too can become toxic if it has a partial pressure of 1 atm or more, which is at about 198 feet if I remember correctly; this is why the O2 level is decreased for deep dives).

I hope this gives you a better feel for CO, and why it is important to limit the amount in air to below 2 ppm. Remember that at 132 feet of sea water, or 5 atm absolute pressure, the 2 ppm is as if you were breathing 10 ppm on the surface. This is tolerable. The American Conference of Governmental Industrial Hygienists (ACGIH) has threshold limit values (TLVs) for many chemical which they update yearly. The TLV for CO is 25 ppm. The TLV-TWA (threshold limit value--time-weighted average) is the "concentration for a conventional 8-hour workday and a 40-hour workweek, to which it is believed that nearly all workers may be repeatedly exposed, day after day, for a working lifetime without adverse effect..."

In the early 1950s, Jacques Cousteau, Frederick Dumas and Captain Tailliez almost lost their lives cave diving in the Fountain of Vaucluse, France. They were diving inside the cave to explore it and went about 400 feet inside,t a depth of 165 feet. Cousteau pulled Damas out by hanging onto a rope that Fargus tended, and finally decided to pull them up when he could not understand the signals. "Tailliez broke the surface, found a footing and walked out of thewater, erect and wild-eyed. In his right hand he held his dagger, upside down. His fingers were bitten to the bone by the blade and blood flowed down his sodden woolens. He did not feel it..." Cousteau, Jacques, The Silent World, page 82. Cousteau had to be helped out, and Dumas was unconscious but still breathing. After they had recovered, they had their air analyzed. According to Cousteau, "The next morning we sampled the cyclinders. THe analysis showed 1/2000 of carbon monoxide. At a depth of one hundred and sexty feet the effect of carbon monoxide is sixfold. The amount we were breathing may kill a man in twenty minutes. We started our new Diesel-powered free-piston air compressor. We saw the compressor sucking in its own exhaust umes. We had all been breathing lethal doses of carbon monoxide." (Cousteau, IBID). I did a calculation, and I believe that 1/2000 x 1,000,000 = 500 ppm that they were breathing. This was the equivalent of them breathing CO at 500 x 6 = 3000 ppm at 165 feet depth!

The tragedy is that this is still happening. In the 1980s I learned of a CO diving death from a heart attack; CO poisoning can initiate a heart attack in people with coronary artery disease. And we all have some load of CO from our daily activities.

Well, one more bit of information before bed. There are other means of analyzing for CO than buying a CO analyzer. They are called "detector tubes" and are used with a hand pump. The pump is calibrated to pull in through the tube a known amount of air, so the change in color of the chemical indicator in the tube will read out in ppm. These are available from several suppliers, and may be a more reliable means of checking for CO when in far field places (the South Seas, for instance). Here are two links to well-known brands:

http://www.sensidyne.com/colorimetric-gas-detector-tubes.php

http://www.afcintl.com/entity/tabid...egoryid/59/sename/detector-tubes/default.aspx

If nothing else, they are good for backup. We use them at work for our Emergency Response Team.

SeaRat