January 1, 2007
Safety equipment selection is often a matter of choosing from a variety of options that result in a balanced, effective solution. When it comes to selecting breathing support systems, there can be no compromise. The correct breathing system for any given situation must come from a complete and up-to-date understanding of the relative strengths and limitations of each type of system.
Air Purifying Respirator (APR)The simplest type of breathing system provides non-power assisted filtered air. Lungs provide the power to force the air through a filter.
This approach is quite simple, relatively inexpensive, and generally operates for a long duration. On the negative side, the work of breathing can be strenuous, and you must address the type of airborne contaminate expected in order to choose the proper filter before actual use. Additionally, the breathing system is, of necessity, in a negative pressure environment during inhalation, and there may be types of contaminates that are unable to be effectively filtered out. This approach is not effective in environments with low or no levels of oxygen.
Powered Air Purifying Respirators (PAPR)Two of the limitations of the non-power assisted filtration system may be overcome with the addition of a mechanical blower to assist in passing breathing gas through the filter. Power assisted filtration systems create a positive pressure breathing environment and provide more effective use of filters. Weight and bulk increase with the addition of batteries, blower motor, user status information and possibly a breathing bag.
Duration relative to APR systems generally decreases due to the addition of battery life considerations.
Open Circuit Self-Contained Breathing Apparatus (SCBA)The compressed air open circuit system can be effective where, for various reasons, filtration systems are not. A positive pressure breathing environment is easily achieved. The purity and consistency of the breathing gas is assured.
Maintenance costs and training requirements are relatively minimal, straightforward and well-known.
Gas capacity is limited as defined by the size and weight of the cylinders required to contain that gas. Further, team operations are limited by the team member with the largest consumption. The amount of use time available is directly related to the users’ lung exchange volume and how rapidly he or she breathes. A typical system is rated between 20 and 60 minutes.
Training, skill, cost, size, bulk and maintenance are greater than are required for a filtered air system but remain within the bounds of practicality.
Pure Oxygen Closed Circuit Rebreather (CCR)The closed circuit rebreather (CCR) provides significant improvements in duration over open circuit versions. With a pure oxygen closed circuit rebreather it is practical to expect duration ratings from two to four or more hours within acceptable bulk and weight limits. This system uses the oxygen supply to simply replace the oxygen metabolized by the user. Variance in consumption of oxygen is dependent on workload. This is the most efficient use of the bottled gas supply.
A major limitation is that the user breathes pure oxygen. This is particularly true where repeated uses come into play for work that could go on for days or weeks. Examples are large hazardous material cleanup or long-term rescues. Another concern is the extreme fire danger connected with maintaining and using pure oxygen. Not all environments have the same level of concern, but it can be a significantly undesirable situation for most applications.
Maintenance and use costs are higher relative to compressed air systems. These are not as limiting concerns as the biological and fire concerns.
Pure oxygen CCRs depend upon mechanics and sufficient training for the maintenance of the proper amounts of oxygen. User controls and system information are approximately the same as for compressed gas systems.
Bailout considerations must be addressed for CCR-type systems since it is possible that the breathing loop integrity can be compromised. One must either carry another cylinder of air for breathing during bailout or breathe pure oxygen through a demand regulator. If the former option is chosen, it increases the system weight. If the latter is chosen, operating costs will increase because of the pure oxygen equipment requirement.
Semi-Closed Mixed Gas Rebreather (SCR)The SCR has little practical land-based value. This device is a type of mixed gas rebreather where the gas in the breathing loop is not pure oxygen but a range of oxygen and nitrogen.
In a semi-closed breathing system, the goal is to provide an amount of oxygen greater than the user might consume. This is accomplished via constant gas flow. The lower the target oxygen in the breathing mix, the greater the gas flow must be to maintain a desired oxygen consumption rate. With a breathing mix maintained with an oxygen level close to air, there is no advantage in duration over compressed air cylinders. The significant duration improvement that is possible with semi-closed rebreathers only comes with relatively high levels of oxygen in the breathing supply. Scrubber duration remains the same as in the pure oxygen CCR and is not a concern. There is also the higher maintenance complexity of maintaining and refilling mixed gas cylinders.
Electronically Controlled Mixed Gas Closed Circuit Rebreather (ECCR)The ECCR has a major advantage in its ability to maintain a much lower breathing gas oxygen level such as that found in the compressed air SCBA systems. The electronically controlled mixed gas closed circuit rebreather has the same significant duration advantages of a pure oxygen CCR (up to four hours within practical weight and bulk limitations). In the past, these systems have not been practical to use in non-water applications. The lack of practical land-based ECCR systems is attributed to companies simply applying water-based design principles to the significantly different environment and market requirements of a land-based system.
New approaches in both technology and design consideration now enable users to manage an ECCR with no direct controls other than those that are found in existing breathing systems. With the ECCR controller, the user is freed of the concern and tasks associated with failure management. This allows safe operation of the system and reduces the information load of the user to that of simple duration-related information.
Make the right choiceThere is a wide range of land-based rebreathers available commercially. Inexpensive air filtration rebreathers to electronically advanced mixed gas closed circuit rebreathers can now provide users with options. Careful analysis of environmental requirements, training and operating costs must be made to confirm the best equipment choice.
Sidebar: OSHA saysâ€¦According to OSHA’s respiratory protection eTool (http://www.osha.gov/SLTC/etools/respiratory/index.html), in order to select an appropriate respirator you must:
- Conduct an exposure assessment to determine the type and amount of hazardous exposure: Employers must characterize the nature and magnitude of employee exposures to respiratory hazards before selecting respiratory protection equipment.
- Take into account the factors that can influence respirator selection, such as the physical configuration of the job site and worker characteristics, including medical condition and comfort.
- Understand the assigned protection factors, i.e. the level of protection that a properly functioning respirator would be expected to provide to a population of properly fitted and trained users.
- Know the various kinds of respirators and their relevant characteristics.