Save Your Breath...
Published: 10th Oct 2006
Half a lifetime ago, I spent the interminable hours between the landing of “Eagle” in the Sea of Tranquillity and Neil Armstrong making that “one small step” in the early hours of our morning, following hedgehogs around the garden. At the same age, I was enthralled weekly by the “Undersea World of Jacques Cousteau” on TV, and resolved to become a marine biologist - yet another of youth’s unrequited dreams.
What linked these two situations was that the environments in which my heroes were operating were so hostile and so alien to humans that their presence there was only possible because they were using complex life-support systems incorporating breathing apparatus. But you don’t have to go to the ends of the earth or beyond to need to use this type of equipment. There are a multitude of uses much closer to home.
When might you need BA
Breathing apparatus (BA) is effectively any respiratory protective device (RPD) which provides breathable gas to the wearer from a source which is independent of the atmosphere or environment which immediately surrounds them. This sets it apart from respirators, which can only remove specific contaminants from the air surrounding the user, by means of filters. In the case of respirators, the air surrounding the wearer must contain enough oxygen to be safe to breathe, and the filter must be matched to the type and concentration of contaminants which may be present, and be within its useful lifetime. In virtually any situation, there will be a form of BA which can substitute for a respirator. The converse situation does not hold true; there are many environments where respirators cannot protect the wearer like BA does. So in some situations you might choose to use BA in preference to other types of respiratory protective equipment; in others use of BA is imperative.
Outside recreational use of diving equipment, there are few if any non-occupational situations where breathing apparatus might be needed or used. Its applications are almost invariably work-related in some form, either for protection during a working activity, or emergency escape from hazardous situations which may arise as a result of working processes. This provides yet another distinction between BA and respirators, which are commonly sold into the domestic market for do-it-yourself activities, which are debatably a form of leisure (but only if you are a masochist). The reason for this is probably that relative to a simple disposable respirator, BA is more technically complex and expensive to buy and operate, even if it might provide better protection and comfort.
Whatever RPD you choose, national legislation (in the UK, the COSHH Regulations 2002) will require it to be:
- Adequate - capable of providing the level of protection you need
- Suitable - matched to the working environment, the type of work you are doing, and to the individual wearer
Some of the factors influencing this choice are expanded below. For more detail, see HSG53 (HSE 2005) or EN 529 (CEN 2005).
Swings and roundabouts
Use of BA instead of respirators eliminates both the difficulty of deciding which filters might be appropriate for the various substances which could be present in the atmosphere (particularly if you are dealing with mixtures of substances, either simultaneously or sequentially), and of deciding when these filters should be changed to maintain the required level of protection. In some situations, there may in fact be no suitable filter for the contaminants in the air, or you simply do not know what might be present and emergency intervention is needed; there is no safe alternative to BA in these cases.
Oxygen deficient atmospheres always need BA. There is no filter which will protect against this hazard.
A worker entered a reactor vessel to retrieve something he had dropped in. He was wearing a respirator fitted with a new filter, appropriate for the reactants in the vessel. Within a few seconds of entering the vessel he collapsed, and was dead when recovered by the emergency services some 40 minutes later. The vessel had been purged with nitrogen, and contained little or no oxygen.
Remember that oxygen deficiency might occur in some situations (mainly in confined spaces) as a result of working activities or unexpected releases, and BA might be required as a precaution.
Most types of BA provide the user with an effortless supply of air, removing the distraction and discomfort of the breathing resistance felt with conventional simple respirators. This makes it easier to work harder and/or longer while wearing the equipment. Designs which are supplied with a continuous flow of air create a cooling effect for the user, which can also increase comfort and working duration in hot environments.
Some forms of BA also provide the highest levels of respiratory protection that are available. There are of course trade-offs for this relative ease of selection and the availability of higher levels of protection.
The need to supply breathing gas (which is usually although not always air) to the apparatus invariably places more constraint on a person wearing BA than does a respirator. Either they must be tethered to a remote source by some form of flexible supply hose or tube, or they must carry the supply with them, usually in the form of a cylinder of compressed gas. In the first case, wearer mobility is limited by the length and dragging /snagging of the supply tube; in the second case the wearer has to bear the additional weight and bulk of the cylinder, and the possible working duration is strictly limited by the quantity of gas that can be carried.
Any BA system comprises three basic components:
- A source of breathing gas
- A means of delivering the gas to the user
- A facepiece from which the user breathes
Table 1 describes the different forms that combinations of these components usually take, and gives examples of when they might be used. For technical and practical reasons, not all possible combinations are viable. This is particularly the case for combinations of some types of facepiece and means of delivery. Where a given combination of the three components indicates both working and escape applications, there will often be different forms of the apparatus which are intended for these separate uses.
Supply of gas
Fresh air as a source of breathing gas is about as simple as BA gets. Systems of this type (known as “fresh air hose BA”) are to BA what a snorkel is to diving equipment; a tube, relying on the lung power of the wearer to pull in air, with the addition of a non-return valve in the supply hose. Yet even this simple system can go disastrously wrong if the inlet is not securely anchored in a clean air region. Instances where the inlet end of the hose has been pulled into the contaminated area by movements of the wearer, and they have been overcome, are not unknown.
|Gas source||Delivery means||Facepiece||Typical application|
|Clean fresh air||Wearer’s lung power||Mask||Short duration low mobility work close to clean air, no electricity available|
|Clean fresh air||Powered blower||Mask, hood, blouse, helmet||Long duration, limited mobility work, close to clean air, electricity available|
|Remote compressor||Constant flow valve||Mask, hood, blouse, helmet, suit||Long duration, limited mobility work|
|Remote compressor||Demand valve||Mask||Long duration, limited mobility work|
|Local cylinder bank||Constant flow valve||Mask, hood, helmet, blouse, suit||Intermediate duration, limited mobility work|
|Local cylinder bank||Demand valve||Mask||Intermediate duration, limited mobility work|
|Carried cylinder||Constant flow valve||Mask, hood||Emergency escape|
|Carried cylinder||Demand valve||Mask||Short duration work requiring high mobility. Emergency escape|
|Carried cylinder||Recirculating system||Mask||Intermediate duration work requiring high mobility. Emergency escape|
|Carried chemical oxygen generator||Recirculating system||Mask, mouthpiece, hood||Emergency escape|
Continuous supply of compressed air, either from a live compressor and air receiver, or from a bank of high pressure cylinders gives the widest choice of possible facepieces, although not all of these may be suitable for any given application. The quantity of air supplied to each user must exceed the maximum rate that they need at any point in their breathing cycle. This rate will range from ~50 litres per minute for light work up to 400 litres per minute or more for heavy manual work.
If enough air is not provided, the consequences depend on the type of facepiece being used; for masks the wearer will experience the suffocating feeling of being starved of air, and the pressure inside the mask will fall well below that outside, increasing the chance of contaminants leaking in; for hoods, helmets and the like, reduced levels of protection will result as contaminated air is pulled in from outside.
Most equipment of this type incorporates pre-set valves and warning systems to ensure that supply exceeds a nominal minimum value defined by the manufacturer. However, this does not guarantee protection because work rate might cause wearers to demand more air than this nominal value; the harder they work, the lower the level of protection that will be provided.
Continuously providing air at these rates can be expensive, as it imposes large demands on the supply system, which must be designed and sized according to the maximum demand that each user (and any air-driven equipment they are using, like sprayguns) might require. Most of this air (at least 50%) will be simply vented from the BA as none is needed during the exhalation part of the breathing cycle.
Demand valve systems offer more economical use of compressed air, because they only supply air to the user when they inhale. In consequence, they are cheaper to run, but are more expensive to buy and more technically complex to maintain. They can only be used with a mask because they rely on a balance between the internal and external pressure to control the opening and closing of the supply valve.
The pressure at which the supply valve operates can be set to maintain a slightly positive value inside the facepiece (a so called “positive pressure demand” system), with the intention of preventing external contaminants being drawn into the mask. Devices of this type are thought to provide the highest levels of protection of any conventional BA system, although as described later even they are not infallible.
BA systems where the gas source is carried by the user are termed “self-contained” (leading to the abbreviation “SCBA”). The few systems incorporating recirculation are sometimes called “closed circuit” to distinguish them from those which simply dump the exhaled or surplus gas to the surrounding atmosphere (“open circuit”). Such closed circuit systems must contain special absorbers to remove exhaled carbon dioxide from the rebreathed gas, and top up used oxygen, either from a cylinder of the compressed pure gas, or from a canister of reacting chemicals.
All self-contained systems have a definite limit to the length of time they will operate, which is determined by the size of the gas reserve carried, and how hard the wearer is working. To economise on use of air and so maximise working duration, SCBA almost invariably uses demand valve systems (except for some short duration escape devices). Typical working open circuit SCBA sets (containing a volume of air equivalent to the size of a large double wardrobe) will operate for about 30 minutes, while special closed circuit sets might work for up to 4 hours. Remember that this duration has to include entry into the hazardous situation, the work carried out there, and the time it takes to return to a place of safety, including any decontamination procedures which may be needed before the equipment can be removed. Working practices will usually require that all these operations can be completed with a reserve of approximately 25% of the air remaining in the equipment in case of unforeseen problems.
You can always use a mask-based device with any form of source and means of delivery, if this suits the task and the wearer.
In common with other applications of tight fitting facepieces, suitability for the individual should include demonstration that they can achieve an adequate fit, usually by passing an appropriate fit test (HSE 2003). All mask-based systems rely on this good fit to achieve the intended level of protection or performance. This applies to all types of mask, even if air is delivered by a positive pressure demand system. If the mask doesn’t fit properly there is increased potential for leakage. Even positive pressure demand systems do not guarantee that pressure inside the mask will be maintained at a higher level than the surrounding environment at all times, and contaminants can still leak in if fit is poor. In addition, if the fit of the mask is poor and a positive pressure demand system is being used, outward leakage of air will reduce the working duration of the apparatus, or increase demands on the supply system.
Because people are inherently awkward (inconvenient size or shape, hairy, wrinkly, warty etc - just look around you and tell me if I’m wrong), there may be no mask which fits them properly. Even if masks do fit properly, they can become uncomfortable if they have to be worn for long periods. Alternative forms of facepiece (loose fitting visors, hoods, helmets, blouses or suits) may be just as effective as masks for some applications, while being easier to wear and more comfortable. However, as already mentioned not all means of delivery can be used with these facepieces; there must be a continuous forced supply of some kind. Loose fitting facepieces do not rely on a good seal to the face for their performance, so fit testing is inappropriate and unnecessary. Instead, performance depends on there being enough air being fed into the facepiece to prevent any contaminants leaking inside.
The question frequently arises as to what the appropriate type of facepiece is for people with facial hair. Whether a mask is suitable for such people will depend on whether the hair encroaches into the region where the mask is intended to seal onto the face; if it does, then a mask should not be used. Loose fitting facepieces do not rely on effective sealing to the face for protection, and recent research has confirmed that facial hair has no significant effect on their performance (Clayton and Frost, 2006), so these offer bearded wearers an alternative for some applications. However, loose fitting devices can’t provide the high levels of protection that are obtainable from positive demand masks. If this level of protection is needed, there is no safe alternative but to shave.
Emergency escape (sometimes called self-rescue) requires the apparatus to be as small and light as possible, so that it doesn’t hamper the user in their efforts to reach safety. Typically it will have a working life of only a few minutes, so it must be chosen so that it is possible to reach safety within this rated duration. Emergency escape does NOT include the rescue of others, which is a working activity, and equipment for use during working activities has to satisfy different, more stringent, performance requirements from that intended solely for escape.
Escape equipment can either be located in strategic positions in the workplace, or carried by the wearer in readiness for use. It has to be quick and easy to put on, preferably in less than the time for which you can hold your breath.
Two men were working in a tight space on a gas valve when it failed, and irrespirable gas rushed out. They had put down their escape BA sets, intended to be carried at all times, a few paces away to make access to the work easier. Neither survived.
Wearers are totally dependent on the air supplied to BA. Not only must this be of the required quantity, it must also be of the right quality to breathe safely. Component gases must be correct within usual concentration limits, and there must be no contaminants present at levels which might be harmful, either to the user or the equipment. In Europe, the required quality of air supplied to breathing apparatus is laid down in EN 12021 (CEN 1998). Listed “contaminants” include water, which should never be present in quantities such that it can condense out in the supply system or breathing apparatus, potentially causing corrosion, or freezing in vital components during cooling on reduction of pressure.
Such contaminants may be present in the air drawn into the compressor, or generated by the compressor itself.
A new staff member was instructed to shot blast paint from skips while wearing an air-fed helmet BA (see the accompanying picture). In addition to the helmet being in appalling condition, it was supplied with air from a diesel-driven compressor, but the compressor air intake was sited adjacent to the exhaust from the diesel engine. He did not return to work the next day.
Through bad maintenance, a compressor leaked oil into the air being produced. The heat of compression caused the oil vapour to partially combust; a process known as “dieseling”. This resulted in dangerously high levels of carbon monoxide in the air being supplied to users.
Properly designed and maintained compressor systems can engineer out many of these problems, by including filters, catalytic convertors and dryers. Whatever systems are used, the quality of the air supplied must be confirmed on a regular basis, and quality records kept for at least 5 years. Although this is a well-established legal requirement (since at least 1988 in the UK), it is still often ignored or overlooked. If you were asked for this information by the authorities, could you provide it?
Work in progress
There are two areas of current development relevant to BA that are worthy of mention here. Both relate to today’s global situation, but from diametrically opposite directions.
The globalisation of markets affects just about every aspect of our lives. The safety equipment field is no exception, and in response to this trend the International Standards Organisation (ISO) is developing suites of product standards which will include all forms of RPD, including BA, to remove barriers to free trade across all national boundaries. Within a decade, these will replace existing national and regional standards like those in America or Europe. The developing standards are rethinking the performance of RPD, making characteristics like maximum use time and limiting work rate part of the overt classification scheme, so assisting users to ensure they have the best equipment for their application. There is an educational challenge to be met here, the world over. Old dogs will have to learn new tricks, but the prize of better and more appropriate protection for those who must wear this equipment will be worth the effort.
The other side of today’s global coin is the threat posed by extremism and terrorism, and the real possibility that unconventional weapons might be used in a strike against society, such as the 1995 Sarin attack in Tokyo. Precautionary development of equipment to protect those non-military personnel whose role is to respond to such an event is underway. To assist in this process, British Standards Institution has recently published BS 8468 specifically covering BA for this application (BSI 2006). It is based as far as possible on existing standard test methods, but extends the performance requirements to address the very particular additional potential hazards that wearers might face in these situations. This standard will be offered into European and International standards to assist wider availability of this form of protection against extremist threats.
The last word
Breathing apparatus can offer alternative or improved levels of protection and comfort to users, but at the expense of simplicity. In some cases, no other form of protective equipment will do. Because BA is often used in the most dangerous situations, levels of user training, equipment maintenance and emergency planning have to be better than for other forms of respiratory protection. In these situations, the consequences for the wearer of equipment failure or having to take it off in an emergency are likely to be just as severe as they would be in the vacuum of space or the depths of the sea. The Scandinavians have got it just about right when they refer to firefighters who enter burning buildings as “smoke divers”.
COSHH (2002) Control of Substances Hazardous to Health Regulations, Approved Code of Practice and Guidance (L5), HSE Books, ISBN 0 7176 2534 6.
HSE (2003) Health and Safety Executive Operational Circular OC 282/28. http://www.hse.gov.uk/foi/internalops/fod/oc/200-299/282_28.pdf
Clayton and Frost (2006) , hoods and beards, in press
CEN (1998) EN 12021 Respiratory protective devices - Compressed air for breathing apparatus. CEN, Brussels.
CEN (2005) EN 529 Respiratory protective devices - Recommendations for selection, use, care and maintenance - Guidance document. CEN, Brussels.
HSE (2005) Respiratory Protective Equipment at Work - A practical guide (HSG53), HSE Books, ISBN 0 7176 2904 X
BSI (2006) BS 8468-1 Respiratory protective devices for use against chemical, biological, radiological and nuclear (CBRN) agents. Part 1: Positive pressure, self-contained, open-circuit breathing apparatus - Specification. BSI, London.
Published: 10th Oct 2006 in Health and Safety International