Selecting respirators: protection factors, IDLH and oxygen

Selection turns the exposure assessment into a decision. The question is simple to state: which approved respirator will keep the concentration inside the facepiece below the exposure limit, in this atmosphere, for this task and this worker? Answering it requires three tools: the hazard ratio, assigned protection factors and the limits set by oxygen levels and by atmospheres that are immediately dangerous to life or health.

The hazard ratio and required protection

Start by comparing the expected concentration with the exposure limit. The ratio tells you the minimum level of protection needed.

If a worker could face 180 ppm of a solvent with a limit of 20 ppm, the respirator must reduce the concentration by at least a factor of 9. The respirator chosen must have an assigned protection factor equal to or greater than that number. Remember from lesson 3 that the expected concentration should be a reasonable upper estimate, not the average, and should consider short-term peaks against the STEL or ceiling.

Assigned protection factors

An assigned protection factor (APF) is the workplace level of protection that a properly functioning respirator, or class of respirators, is expected to provide to properly fitted and trained users. It is a conservative planning value drawn from workplace protection studies, not the best result from a laboratory test. An APF of 10 means the respirator is expected to reduce the concentration inside the facepiece to one-tenth of the outside concentration or less.

Two conditions apply to every APF. First, it assumes a complete program: the respirator is approved, selected correctly, fit-tested if tight-fitting, worn properly the whole time and maintained. Without a program, real protection can be a small fraction of the APF. Second, the APF covers leakage around the seal and through valves; it does not cover the filter or cartridge being overwhelmed. Cartridges have their own concentration limits and service lives, covered in lesson 7.

Maximum use concentration

The maximum use concentration (MUC) is the highest concentration in which a respirator can be used. It is found by multiplying the APF by the exposure limit.

The MUC is capped in three ways. It must not exceed the IDLH concentration for an air-purifying respirator, because air-purifying respirators are never permitted in IDLH atmospheres. It must not exceed any maximum concentration stated by the cartridge or canister manufacturer. And where a substance-specific regulation sets lower limits for particular respirator types, those apply.

Immediately dangerous to life or health

An atmosphere is immediately dangerous to life or health (IDLH) when it poses an immediate threat to life, would cause irreversible adverse health effects, or would impair a worker's ability to escape. NIOSH publishes IDLH values for several hundred substances. Examples include about 100 ppm for hydrogen sulphide, 1,200 ppm for carbon monoxide and 300 ppm for ammonia. IDLH values are designed for escape planning and respirator selection; they are not safe exposure levels.

Three kinds of atmosphere are treated as IDLH for respirator selection.

For IDLH atmospheres, CSA Z94.4 and Canadian regulations require respirators that supply air independently of the surrounding atmosphere: pressure-demand SCBA with a full facepiece, or a pressure-demand airline respirator with an auxiliary self-contained escape supply. Air-purifying respirators, including PAPRs, are not permitted. A standby person outside the atmosphere, with communication and rescue equipment, is expected, and confined space regulations add specific entry and rescue requirements covered in HSE-216.

Oxygen-deficient atmospheres

Normal air contains about 20.9 per cent oxygen. As the oxygen level falls, judgment and coordination are impaired, and below about 10 per cent unconsciousness can come within seconds, often without any warning sensation. Air-purifying respirators remove contaminants but cannot add oxygen, so they are useless, and dangerously misleading, in an oxygen-deficient atmosphere.

Canadian regulations generally treat an atmosphere with less than 19.5 per cent oxygen as oxygen-deficient for respirator selection; Alberta's OHS Code, for example, requires atmosphere-supplying protection below 19.5 per cent. Confined space rules in Ontario and other provinces set an acceptable range of 19.5 to 23 per cent, the upper limit reflecting the fire hazard of oxygen-enriched air. CSA Z94.4 also discusses the effect of altitude, where the partial pressure of oxygen is lower even at 20.9 per cent.

A selection procedure

Respirator selection can be organized as a series of questions. The figure shows the logic most programs use.

Beyond the numbers

Numbers set the minimum, but the best respirator is the one workers will wear correctly for the whole exposure. Heat, heavy work, long shifts, the need to talk on radios, use of hard hats and welding helmets, and the wearer's comfort all affect wear time. A PAPR with an APF of 25 that a worker wears all shift may protect far better in practice than a full facepiece with an APF of 50 that is lifted every few minutes to wipe sweat. Involve workers in trials when choosing models, and offer more than one model and size.

Selection is also not a one-time decision. Reselect when the exposure assessment changes, when a new process or material is introduced, when a worker cannot pass a fit test or develops a medical problem, or when program evaluation shows that the selected respirator is not being worn correctly.