Silent killer

Carbon monoxide poisoning is all too common. It is the leading cause of death by poisoning in the U.S. Boatowners are particularly at risk due to the small volumes of air in a boat, the fact that boats are necessarily tightly sealed (to keep the water out), and the fact that within these small, sealed spaces there are often several appliances capable of producing carbon monoxidein addition to the galley stove, there is likely to be an engine, some form of a non-electric cabin heater, and maybe a gas-powered water heater.

What makes carbon monoxide such an insidious killer is the fact that it is odorless, colorless, tasteless, and more-or-less the same weight as air (just a little lighter) so that it tends to hang around. It is absorbed by the lungs and enters the bloodstream, reacting with blood hemoglobin to replace critical oxygen molecules, forming something known as carboxyhemoglobin (COHb). The result is that the body is deprived of oxygen.

Carbon monoxide binds to blood hemoglobin far more readily than does oxygen in the air, even if oxygen is available. So the mere presence of carbon monoxide is dangerous, with or without plenty of fresh air. Once attached to the hemoglobin, carbon monoxide is relatively stable. As a result, very low levels can progressively poison people. For example, as little as 0.2% carbon monoxide in the air binds up red blood cells (i.e., forms carboxyhemoglobin) at the rate of 1% (of the body’s red blood cells) per minute. If someone is doing work, the rate can double to more than 2% per minute. Within 45 minutes the red blood cells are 75% taken up with carbon monoxide, resulting in a lethal concentration. (Dr. Ethan Welch, writing in Mid-Gulf Sailing, January 1995.)

At the other end of the scale, high doses of carbon monoxide can be lethal in a matter of minutes (see table on page 90). If a victim escapes death, he or she may still suffer permanent brain damage.

Typically, carbon monoxide poisoning produces a range of symptoms, beginning with watery and itchy eyes and a flushed appearance, and progressing through the following symptoms: an inability to think coherently, headaches, drowsiness, nausea, dizziness, fatigue, vomiting, collapse, coma, and death. Note how similar many of the early symptoms are to those of seasickness, flu, or food poisoningall too often people suffering from carbon monoxide poisoning fail to recognize the problem. Note also the manner in which the poisoning creeps up on people. It dulls the senses, causing a failure to recognize the problem, which enables the fatal punch to be delivered.

Boat/U.S. quotes an example of a family of three on a large sailboat with a washing machine and propane-fueled water heater. The washing machine malfunctioned, causing the water heater to stay lit. The heater consumed the air in the cabin and then produced carbon monoxide. The son fell, tried to get up, and fell again. Hearing the loud thump, the wife got up, was overcome, and collapsed.

“My husband saw me go down and thought I had fainted because of seeing our son (whose lip was bleeding). He stepped over me to assist our son and looking back he noticed the cat lying beside me. It clicked I might faint, but not the cat. He picked up the phone and pushed a pre-programmed button to call our neighbors and let them know we were in trouble. He tried to pull us out, but he, too, was going down.” All three were rescued by the local fire department and regained consciousness. What is particularly interesting about this case is that the boat’s hatches were wide open, and a 10-knot wind was blowing outside. The question is: what can be done to prevent such incidents?

Engine and appliance installations

All engine exhausts contain carbon monoxide, with gasoline engines producing far higher levels than diesels (for this reason, Onan, a major manufacturer of generators, is no longer selling gasoline-powered gensets in the marine market and has recently conducted an extensive advertising campaign warning of the dangers of carbon monoxide poisoning). Cold, poorly tuned, and overloaded engines produce more carbon monoxide than warm, properly tuned, and load-matched engines. So the first task in minimizing the potential for carbon monoxide formation is to ensure that the engine is properly matched to its task and, as far as possible, will be operated as designed.

There is always, however, the voyaging sailboat owner who uses the engine more for battery-charging at anchor than for propulsion, which results in the engine running long, underloaded hours below its designed temperature; there is not much that can be done about this, other than to make sure that the carbon monoxide gets out of the boat.

Standards such as the American Boat and Yacht Council’s (ABYC) guidelines on the installation of exhaust systems for propulsion and auxiliary engines cover safe engine exhaust installations in some detail. Of particular importance is an exhaust system that is gas-tight inside the hull. This, in turn, requires adequate support and strain relief built into the exhaust system (to absorb engine vibration without failure); the use of galvanically compatible materials (to lessen corrosion); proper marine exhaust hose in wet exhaust systems; and double-clamping of all hose connections with all stainless steel hose clamps. Each engine on a boat must have its own dedicated exhaust system, with nothing connected into this exhaust (with the sole exception of a cooling water injection line on a water-cooled exhaust).

Regardless of the quality of the initial exhaust installation, an exhaust system is a regular maintenance item. Leaking gasoline exhausts are way and above the leading cause of death from carbon monoxide poisoning. At least once a year the entire system should be inspected for any signs of corrosion or leaks. Warning signs include discoloration or stains around joints, water leaks, rusting around the screws on hose clamps, corrosion of the manifold discharge elbow on water-cooled engines, and carbon build-up within the exhaust (which will increase the back pressure and the probability of leaks). To check a discharge elbow, the exhaust hose must be removed, which will also enable a check to be made for carbon build-up.

Keeping the exhaust gases out of the engine compartment is only one side of the equation. The other is preventing any leaks that do occur from entering accommodation spaces and minimizing the extent to which an engine draws its combustion air from the accommodation spaces (this will minimize the likelihood of oxygen depletion down below). Engines should be given an independent source of combustion air, with all gaps around engine room plumbing, cableways, hatches, and access panels minimized. Checking engine ventilation systems, particularly if they depend on hoses or ducting, is another regular maintenance item.

When it comes to fuel-burning appliances, which are almost always in the accommodation spaces themselves, certain other protective measures need to be taken. The optimum situation is one in which the appliance has its combustion air ducted in from outside the accommodation spaces, with combustion occurring inside a sealed chamber that then exhausts through an external flue. Such an appliance cannot cause oxygen depletion, nor can it emit carbon monoxide directly into accommodation spaces. There are cabin heaters built in this fashion (notably those from Wallas Marin and Ardic, distributed by Scan Marine, Seattle), although most are not. The ABYC standard covering boat heating systems (A7) requires this kind of heater in engine or bilge spaces. The combustion chambers need to be checked as a part of a regular maintenance schedule to enure that there is no damage or corrosion, while both inlet and exhaust ducting should be inspected to make sure that it is gas-tight.

The next best option in terms of safety is represented by those appliances that, although they use cabin air for combustion, confine the combustion process to a chamber that is then vented via an external flue. As long as there is no backdraft down the flue (generally a matter of proper design, although there may be situations in which the airflow off sails on a sailboat will cause backdrafting with just about any flue), and as long as the flue is not obstructed (primarily a matter of design once again, in particular ensuring that the flue cannot trap water), even in a situation of oxygen depletion and carbon monoxide formation, the carbon monoxide will be vented outside accommodation spaces.

Safety can be enhanced by the addition of an oxygen-depletion sensor wired in such a way that it automatically cuts off the fuel supply to the heater in the event of oxygen depletion. Oxygen depletion sensors have been used for years by Force 10, a Canadian stove manufacturer; their sensors shut down the fuel supply if the oxygen in the combustion air falls below 95% of normal. However, it should be noted that, for a situation in which carbon monoxide is produced but there is still a good airflow through the cabin, the oxygen depletion sensor will do nothing to protect the occupants. Periodic maintenance once again includes inspection of the combustion chamber and flue (it was an improperly vented flue that killed tennis star Vitas Gerulaitas).

Then there are all those appliances that not only draw their air from accommodation spaces but also exhaust the combustion gases into the same atmosphere. These include all non-electric galley stoves, and also some cabin heaters and water heaters (it should be noted that cabin and water heaters of this type do not comply with ABYC standards). These appliances are potentially the most lethal of all; the boat-building industry needs to look at them and think again about using them. In the meantime, it is essential that the public understands that these appliances should never be used when unattended or when people are sleeping (carbon monoxide is especially dangerous when people are asleep since victims don’t feel any side-effects and may not wake up); that they should only be used in conjunction with adequate ventilation; and, in particular, that galley stoves are not to be used for cabin heat.

Finally, it should be noted that the ABYC’s guidelines on marine inboard engines and transmissions disallow the use of an engine-cooling air discharge duct(s) or openings from air-cooled engines as a direct source of cabin heat. In other words, it is not acceptable to pipe hot air from the engine compartment into accommodation spaces. If engine room air is to be used as a source of heat, it must be passed through some form of a heat exchanger, so that the hot air that is circulated through the accommodation spaces is kept isolated from the engine compartment air.

Hull design

Measures such as these will get exhaust gases and the combustion gases of most properly maintained appliances out of the boat, at which point certain design features, or external factors, may suck them right back in. Notable here is backdrafting (the station-wagon effect), created when underway by many square-transom power boats, particularly if the aft deck is enclosed with canvas.

Other design issues include the desirability of locating engine exhaust outlets close to the waterline, well outboard in the transom or in the side of the boat close to the transom, well away from any engine air intakes and well away from any other hull openings. Any drains that terminate overboard, such as those from sinks, sumps, showers, and air-conditioners, should be kept well away from exhaust outlets and should be constructed in such a way as to prevent the ingress of carbon monoxide (generally a P-trap is fitted). Ventilation ducting for accommodation spaces and air-conditioning ducting should not pass through engine spaces. If it must, it should be either rigidly constructed or else formed from multi-layered, reinforced flexible material, and it needs to be airtight. Even with these kinds of measures in place, people should be discouraged from sleeping when a power boat is underway, particularly in aft cabins.

This leaves certain potentially lethal situations that cannot be eliminated in the design and construction phase. Examples include running an engine with the boat up against a dockside so that the exhaust is deflected back into accommodation spaces, or running the engine when rafted to another boat, with similar results. The operation of auxiliary generator sets in such situations is of particular concern, particularly if run at night when people are sleeping.

Carboxyhemoglobin curves

The design of an effective carbon monoxide alarm is a complicated business. There are two substantial hurdles to be overcome. The first is the time-weighted effect of exposure to carbon monoxide; the second is the susceptibility of existing sensors to nuisance alarms (see graph on page 94).

Looking first at the time factor. Relatively low levels of carbon monoxide over an extended period of time can be just as lethal as high doses over a short period of time. Conversely, relatively high levels over a short period of time are not necessarily harmful. It is not unusual for there to be such relatively high levels from time to time, but for these to rapidly disperse (for example, when an engine is first cranked at dockside or when a boat in close proximity to other boats fires up its engine or generator, with the exhaust drifting across the other boats).

If an alarm is designed simply to respond to a given threshold level of carbon monoxide, in order to protect against long-term low-level contamination the threshold must be set at a very low level. This will cause the alarm to be triggered by any short-term rise in carbon monoxide levels, resulting in many nuisance alarms (these almost invariably result in the alarm’s being disconnected or by-passed by the boat operator, at which point the alarm is effectively useless). If, on the other hand, an alarm is set to respond to a higher threshold, it will provide no protection against low levels of carbon monoxide contamination sustained over long periods of time.

It is clear from these curves that an effective carbon monoxide alarm must have the ability to track carbon monoxide levels over time and to monitor in some fashion the likely impact on carboxyhemoglobin levels. It is equally clear that this is a complicated process, particularly since carbon monoxide concentrations, when present, will almost certainly be constantly changing. Newer devices incorporate microprocessors that enable them to keep track of time-weighted carbon monoxide concentrations (known as time-weighted averaging) and to calculate the corresponding carboxyhemoglobin levels in the blood. The question then is: at what level of carboxyhemoglobin should the alarm be sounded?

Underwriters Laboratories (UL) has developed a standard for carbon monoxide gas detectors for marine use. This was published in 1989, and it largely underlies a 1991 standard from the ABYC. In both standards, the requirement is for an alarm to be sounded at or below the 20% carboxyhemoglobin level. Thereafter, there are some minor differences between the standards, the most significant of which is a requirement in the ABYC standard that carbon monoxide detectors must not alarm until carbon monoxide concentrations reach at least 40 PPM, whereas UL1524 has no lower limit. The ABYCs purpose here is to lessen the number of nuisance alarms.

Both of these standards have been superseded to some extent by another UL standard that is the basis for carbon monoxide alarms sold for household use and for use in recreational vehicles. This standard, first published in 1992 and then revised in 1995, requires an alarm to be sounded at or below the 10% carboxyhemoglobin level, while also stating that the alarm must not sound until carboxyhemoglobin levels have reached the 2.5% level or higher. In other words, it considerably narrows the band within which an alarm must sound, increasing the technological challenge to manufacturers.

Nuisance alarms

What these manufacturers have found is that it is very hard to produce an affordable carbon monoxide detector that not only maintains the necessary degree of measurement accuracy over the lifetime of the unit but also reacts solely to carbon monoxide. The limiting factor is the carbon monoxide sensor, for which there are three available options: tin oxide semiconductors; electro-chemical devices; and chemical sensors. Each detector type has specific advantages and disadvantages.

1. Tin-oxide semiconductors currently dominate the marine market for carbon monoxide sensors. These sensors are all made by Figaro, a Japanese company that has been the leader in this business for many years, although there is competition appearing on the horizon. The sensors continuously repeat a 21/2-minute cycle during which they are heated to a high temperature to burn off any contaminants, after which the electrical resistance across the semi-conductor is read. The presence of carbon monoxide changes the resistance from normal, and it is this changing resistance that is recorded by the carbon monoxide detector’s microprocessor.

The good news is that these sensors are very reliable in detecting carbon monoxide and sounding an alarm. The bad news is that false alarms have been a consistent problem. “These problems have not been licked,” reported Tom Hale, the technical director of the ABYC. “The sensors become contaminated and hypersensitive. There have been deaths where an alarm was installed but turned off because of previous alarms.”

Keith Weldy, the president of Fireboy/Xintex (which, along with Marine Technologies, has been in the marine carbon monoxide alarm business since 1986 using the Figaro sensor), says we have to look at the false-alarming issue quite carefully. First of all, he said, “the boat industry has a bigger carbon monoxide problem than boaters realize. In many of these so-called false alarms the carbon monoxide levels are real. If carbon monoxide had a color and was visible, we would hear a lot less about false alarming.”

Fireboy/Xintex sells a pocket-sized, portable carbon monoxide meter that measures from one to 1,000 PPM carbon monoxide, with an accuracy to within five PPM (the cost is $500). With such a meter, he claims, it would be found that a lot of so-called false alarms are, in fact, the real thing.

Tom Wisniewski, president of Marine Technologies, concurs. He cites a recent case in which the three occupants of a boat (trolling, with the canvas up, creating a station-wagon effect) were found unconscious as a result of carbon monoxide poisoning. On the bulkhead was one of his detectors with a wire cut. It had presumably alarmed in the past, and been assumed to be false-alarming.

Having made these points, both Weldy and Wisniewski concede that there are instances in which false-alarming does occur. Unfortunately, in addition to carbon monoxide, the Figaro sensor is sensitive to hydrocarbons, including such products as styrene (used in all fiberglass boats). “You cannot manufacture an affordable carbon monoxide alarm which will discriminate against all these chemicals,” said Weldy. The Figaro sensor has a small charcoal filter that will filter out ambient (i.e. normal) quantities of hydrocarbons and other contaminants. But take a newly constructed boat, with recent varnish and paint and maybe a carpet giving off formaldehyde, and close this up, and it will produce an atmosphere loaded with chemicals to potentially trigger the alarm. This is known as out-gassing.

The answer, Weldy says, is not to omit the alarm but to find ways to deal with the situation. Fireboy now ships its alarms to boatbuilders with the circuit board in a separate, sealed bag. The circuit board is fitted by the dealer (not the manufacturer) after the boat is sold, which gives the hull and interior longer to stabilize chemically and air out. Marine Technologies has taken a somewhat different tack, going through 15 iterations of the logic for its microprocessor over the past few years, making it increasingly sophisticated and able to filter out potential nuisance alarms. The microprocessor now uses a combination of specific set points and time-weighted averaging. “We don’t maintain a long-term time-weighted average in memory,” said Wisniewski. “If we find the atmosphere has cleared, we re-set the timers. As a result, there has been a dramatic reduction in nuisance alarm complaints in the last 12 months.”

But even with these measures there are still reported instances of false alarms on older boats that are no longer outgassing. This seems to be a problem more or less unique to the marine environment, and no one is sure of the cause, although one variable may be the fact that the Figaro sensors become progressively more sensitive over the first three years of use (about 30% more sensitive) and then stabilize. They are then good for at least five to seven years; Fireboy has had some in the field for 10 years.

2. Chemical sensors. The chemical sensors may solve these problems in as much as they are reportedly far less sensitive to out-gassing. These sensors change color in the presence of carbon monoxide, with the rate of change approximating the carboxyhemoglobin curve. A light beam is shone through the sensor. As the sensor darkens, a device measures the reduction in the light passing through. A microprocessor converts this to a carbon monoxide reading, and sounds an alarm at the appropriate time-weighted levels.

This type of alarm, however, got a bad rap a few years ago when the fire department in Chicago had to deal with 8,000 nuisance alarms in one month! Subsequently, it was determined that a temperature inversion in the atmosphere over the city, coupled with very low trip points in the carbon monoxide detectors, had raised the carbon monoxide levels to the point at which the devices alarmed.

Newer versions of chemical sensors and their associated microprocessors have higher thresholds and faster response times that seem to have eliminated these problems in the residential market, where these sensors are currently widely used (First Alert, for example). They are just now entering the marine market in Europe; we may soon see them in this market in the U.S. It will be interesting to see how they work out.

3. Electro-chemical sensors. The third type of sensor, the electro-chemical sensor, avoids many of these problems but introduces new ones. These sensors contain a kind of battery (an electro-chemical cell) whose output voltage changes in the presence of carbon monoxide, triggering the alarm. They are very accurate, and can be made specific to just carbon monoxide; therefore, they are not subject to nuisance alarms. Unfortunately, they are expensive (typically costing from hundreds of dollars on upthe $500 handheld unit from Fireboy/Xintex has an electro-chemical sensor), and the sensor consumes itself in operation, so the life expectancy in continuous use is quite short. This rules them out for anything other than test purposes.

Alarm installation

Beyond the manufacturing issues with carbon monoxide detectors, there are installation issues. ABYC requires that, if installed, a detector shall be located to monitor the atmosphere in each accommodation area equipped with sleeping provisions. Detectors need to be located away from corners and other dead-air areas that do not experience the natural circulation of air through the boat but, on the other hand, should not be in the air stream from ventilators or air-conditioning ducts, which may dilute any concentrations of carbon monoxide in a cabin. In other words, a detector needs to be located somewhere where it receives a representative sample of air. This location will need to be protected from spray and out of the way of likely physical abuse; to minimize nuisance alarms, it should also be at least five feet from any galley stove (particularly alcohol stoves, since alcohol is another substance that will trigger most alarms). The detector should be at eye level to make it easy to monitor.

The ABYC standard requires any carbon monoxide detector that is wired into a boat’s DC system to be wired with overcurrent protection in its circuit (i.e., a fuse or circuit breaker). If a circuit breaker is used, it must either be a non-switchable, manual-reset-only type, or must include a block or other multi-step means to prevent it from being inadvertently turned off. In other words, any time the boat’s batteries are in service (the battery isolation switch is on), the detector should be in service, and the boat’s operator should not be able to easily disconnect it. Some boat manufacturers go a step further and wire carbon monoxide detectors to the battery side of the isolation switch so that they are in continuous service. The one disadvantage to this is the fact that existing detectors draw 90 to 240 milliamps at 12 volts, which amounts to a two- to six-amp-hour battery drain per day, which may be enough to kill a typical group 27 battery stone dead over a period of a month if the boat is not in use and not plugged into shorepower or other charging sources. (Note that soon-to-be-implemented improvements in detector design should reduce the current draw to as little as 60 milliamps, which amounts to less than 1.5 amp-hours a day in continuous use.)

Safety first

The bottom line in all this is that many pieces of equipment found on boats, especially gasoline engines, produce carbon monoxide, and this in turn creates a substantial problem for the boating industry.

What is needed is a cooperative effort on the part of boat designers and builders, and appliance manufacturers and installers, to ensure that boats are designed, built, and equipped in a manner that minimizes the potential for carbon monoxide formation and that keeps out of accommodation spaces any carbon monoxide that does form.

But regardless of the measures taken by the boating industry, a broad-based program of education is needed to alert the public to the dangers of carbon monoxide.

Contributing editor Nigel Calder is the author of several books, including Boatowner’s Mechanical and Electrical Manual, published by International Marine.

By Ocean Navigator