Supplied air, SCBA and emergencies

Atmosphere-supplying respirators give the highest protection, and they are the only respirators permitted in atmospheres that are immediately dangerous to life or health. They also introduce new hazards: breathing air that can itself be contaminated, hoses that can fail, cylinders that run out, and high-pressure equipment that needs specialist maintenance. This lesson covers breathing air quality, supplied-air systems, SCBA duration and air management, and the emergency planning that goes with IDLH work.

Breathing air quality

A supplied-air respirator or SCBA is only as safe as the air it delivers. Compressed breathing air can be contaminated by carbon monoxide from engine exhaust drawn into the compressor intake or from an overheating oil-lubricated compressor, by oil mist and vapor, by water, which can freeze in valves and regulators, and by excess carbon dioxide. Carbon monoxide is the most dangerous because it has no smell and can cause collapse while the wearer believes they are breathing clean air.

OSHA also requires that compressors be located to prevent contaminated air from entering the system; that moisture be minimized so the dew point at 1 atmosphere is at least 10 °F (5.56 °C) below the ambient temperature; that sorbent beds and filters be maintained and tagged with the date of change; that non-oil-lubricated compressors keep carbon monoxide at or below 10 ppm; and that oil-lubricated compressors have a high-temperature alarm, a carbon monoxide alarm or both, with frequent testing for carbon monoxide if only a high-temperature alarm is used (1910.134(i)). Cylinder air must come with a certificate of analysis showing it meets Grade D.

Supplied-air systems

A supplied-air system includes the air source (compressor or cylinder bank), purification (filters, separators, sorbent beds), pressure regulation, hoses and couplings, and the respirator. Each part must be compatible and, for the respirator and hose, part of the NIOSH-approved assembly. Approval specifies the hose length and the pressure range at the point of attachment; using a longer hose or a different pressure can starve the respirator of air.

Couplings deserve special attention. Breathing air couplings must be incompatible with outlets for other gases, such as nitrogen, argon or plant air. Deaths have occurred when a worker connected an airline hood to a nitrogen line that used the same type of quick-connect fitting. Plants that have both breathing air and other gases should use distinctive breathing air couplings and label every outlet.

SCBA duration and air management

SCBA cylinders are rated for nominal durations such as 30, 45 or 60 minutes. Those ratings come from approval tests at a moderate breathing rate of about 40 liters per minute. Real breathing rates vary widely: a worker at rest may breathe 15 to 20 L/min, while a firefighter or rescuer working hard and under stress can breathe 60 to 100 L/min. A 30-minute cylinder may last only 12 to 15 minutes during strenuous work.

The air available in a cylinder can be estimated from its water volume and pressure.

This simple version ignores the fact that air at very high pressure compresses less than an ideal gas, so the real volume is somewhat smaller, and it ignores the reserve that triggers the low-air alarm. Use it to plan conservatively, not to push limits.

NIOSH-approved SCBA must have a remaining service-life indicator, usually an audible alarm plus a gauge or heads-up display. For fire service SCBA, the NFPA 1981 standard sets the low-air alarm at 33 percent of capacity. Many industrial teams use an air management rule such as planning to begin the exit when one-third of the air has been used for entry, leaving the rest for exit and reserve, and they track entry times and pressures at a control point.

Inspection and maintenance of SCBA

SCBA requires a more demanding inspection regime than other respirators. Before each use and at the monthly inspection, check that the cylinder is full, the gauge readings at the cylinder and regulator agree within the manufacturer's tolerance, the low-air alarm sounds when tested, the facepiece and harness are in good condition, the regulator delivers air and maintains positive pressure, and the cylinder's hydrostatic test date is current.

Compressed air cylinders must be requalified by hydrostatic testing at intervals set by Department of Transportation rules (49 CFR Parts 173 and 180) and the cylinder's specification or special permit, commonly every five years for many composite SCBA cylinders, and many composite cylinders have a limited service life, often 15 years. Annual flow testing and servicing of SCBA by a manufacturer-trained technician is standard practice.

Emergency planning for IDLH work

Any task that requires IDLH-capable respirators also requires an emergency plan. Respirators alone do not make IDLH work safe; what matters is what happens when something goes wrong.

The single most important rule in respiratory emergencies is that no one enters to rescue without proper respiratory protection. Many deaths in sewers, tanks and pits have been would-be rescuers who rushed in to help a collapsed co-worker and were overcome by the same atmosphere. Training must make this rule automatic: raise the alarm, use retrieval equipment from outside if possible, and enter only with SCBA as part of a planned rescue.

Escape planning

Workers in areas where a sudden release is possible, such as near ammonia refrigeration, chlorine handling or process units with toxic gases, should carry or have immediate access to escape respirators approved for the hazard. They need to know where escape respirators are, how to don them quickly, how long they last and where to go. Escape respirators are inspected before being carried into the workplace and as the manufacturer specifies, and they are never used to re-enter or continue work.

Training for SCBA users goes well beyond the training for air-purifying respirators. Workers must practice donning quickly and correctly, reading gauges, responding to the low-air alarm, managing hoses and lines, communicating through the facepiece, and escaping with limited visibility. Many teams require periodic practical drills in realistic conditions so that these actions remain automatic under stress.