Particle concentration inside the facepiece against outside, reported as a fit factor.

The result is a measurement rather than a judgement.

It does not depend on the wearer detecting anything.

That removes wearer sensitivity, honesty and comprehension as variables.

The number can be compared against a required pass level.

Pass criteria differ by respirator type and by market.

For tight-fitting full-face respirators in many regimes, and wherever the required protection factor exceeds what a pass/fail response can demonstrate.

Full-face respirators provide higher protection factors than a qualitative test can prove.

The method has to be capable of demonstrating the factor being claimed.

Some regimes also require it for particular industries or hazards.

Which methods are accepted for which respirators is set locally.

Establishing that before buying equipment avoids buying the wrong capability.

Ambient particle counting and controlled negative pressure, and they suit different situations.

Particle counting uses the particles already in the room air as the challenge.

It needs sufficient ambient particle concentration, which clean environments may not provide.

A particle generator is used where they do not.

Controlled negative pressure seals the facepiece temporarily and measures air leaking in.

That method suits some respirator types and requires specific adapters.

Because a numerical result tied to a person, a specific respirator and a date is documentary evidence.

A pass/fail note is a much weaker record of the same event.

Where fit testing is a legal requirement, the record is what demonstrates compliance.

It also allows trends to be seen if someone's fit deteriorates over time.

The record must identify make, model and size, not just the type.

How long these records must be kept is a jurisdictional question.

Usually - a sampling probe has to be fitted, either permanently or using a test adapter.

Probed test respirators are kept specifically for fit testing.

Adapters allow a wearer's own respirator to be tested without permanent modification.

Testing the respirator the person actually wears is preferable where possible.

A probed test sample of the same make, model and size is the usual compromise.

Any modification must not itself affect the seal being measured.

For organisations testing many people regularly, yes; for occasional testing, a service provider is usually more sensible.

Instruments require periodic calibration and service.

Tests take longer than qualitative ones.

The operator needs training and, in some regimes, demonstrable competence.

Against that, in-house testing removes scheduling delays when someone changes respirator.

The break-even depends on headcount and turnover more than on anything else.

Facial hair at the sealing surface, a different respirator model or size, and the passage of time.

A fit test is specific to a make, model and size, not to respirators generally.

Facial hair in the seal area prevents a seal regardless of the result.

Significant weight change, dental work and facial surgery all change the fit.

Retest intervals are set by the local regime.

A test carried out on a respirator the person does not actually wear demonstrates nothing.