CNC fit testing works by comparing particles inside the respirator to particles in the test environment. Without enough ambient particles, the instrument can’t detect small leaks. 

• Particles act as the test agent in a CNC quantitative fit test. 

• Low particle counts make leaks harder to detect. 

To properly challenge a respirator there needs to be sufficient particle concentration in the ambient air. AeroFit requires 30 p/cc for respirators with <99% filter efficiency and 1,000 p/cc for respirators with >99% filter efficiency. These are the minimum ambient concentrations needed for CNC leak detection. 

• CNC fit testing uses ambient aerosol as the challenge agent. 

• Below the minimum, the system can’t reliably compare in-mask particles to the room air concentration. 

• OHD also sets maximum limits to avoid overly “rich” environments: 

• <99% filters: max 1,000 p/cc 

• >99% filters: max 30,000 p/cc 

Why it matters 
Correct particle levels prevent inconclusive or invalid fit tests. Without a minimum particle concentration, it becomes difficult to determine if in-mask particles are present due to leaks in the respirator, or random occurrence. 

Room size and airflow are the biggest factors. Large rooms and high HVAC flow dilute particles quickly. 

Key Variables 

• Room size: Bigger rooms require more particles to reach minimum p/cc levels. 

• HVAC vents: Fresh air vents dilute particle concentration. 

• HVAC returns: Pull particles out too quickly. 

• Air movement: Fans, open doors, and drafts disrupt consistency. 

• Generator placement: Direction and distance from the breathing zone impact build-up time. 

Remember 

It is important to identify where the vent and return are in the room and set up your fit test system. If the system is set up under a fresh air vent, the particle concentration will be easily diluted with the incoming air. If set up directly under the return, particles will be evacuated from the room prior to creating a proper concentration for fit testing. In either case, minimum particle concentrations may be hard to achieve and maintain. 

PGM automatically controls particle release using a timed power-cycling sequence between the AeroFit instrument and the particle generator.

Details 

• AeroFit connects to the generator via a dedicated PGM cable. 

• The fit test administrator lets the generator run ~5 minutes prior to fit testing. 

• During a fit test or daily verification, the generator runs continuously. 

• Between tests, PGM cycles the generator on/off to maintain stable concentration. 

• The sequence reduces manual adjustment and keeps levels inside the required p/cc range. 

Why it’s useful 
PGM makes fit testing more efficient and creates more consistent test environments. 

PGM coordinates the timing of particle release based on what the AeroFit instrument is doing, which is something a standalone generator can’t do.

Differentiators 

Automated power control through the PGM cable 

• More consistent particle levels in the fit test environment 

• Reduced guesswork for the fit test administrator 

Connect the generator, set its dial to maximum, run it for 5+ minutes, and check that AeroFit is reading at least the required 30 p/cc or 1,000 p/cc depending on filter type. 

Steps 

1. Connect the particle generator to the AeroFit via the PGM cable. 

2. Turn the generator dial fully on. 

3. Let it run for ~5 minutes before the first test. 

4. Aim the generator so particles flow into the breathing zone of the test subject. 

Verify readings on the AeroFit display. 

Short Answer 
Increase particle output and reduce dilution sources. 

Fixes 

• Aim the generator directly toward the breathing zone. 

• Move the setup away from HVAC vents and returns. 

• Use a smaller room or reduce airflow where possible. 

• Allow extra warm-up time before the first test. 

Confirm the generator dial is fully open. 

Ventilate the room or reduce generator output. 

Fixes 

• Open a door or window briefly to introduce fresh air. 

• Turn the generator dial down. 

• Let PGM cycle the generator off. 

Move the generator farther from the test subject. 

Short Answer 
Place the setup away from HVAC vents and returns and aim the generator toward the test subject’s breathing zone. 

Placement Tips 

• Avoid positioning directly under a fresh-air vent (dilution). 

• Avoid positioning directly under a return (removal). 

• Keep fans and drafts away from the testing area. 

• Use the smallest practical room. 

A running particle generator doesn’t guarantee the room will reach the minimum particle concentration. Room size, HVAC airflow, vent/return placement, and insufficient warm-up time can dilute particles faster than the generator can add them even with AeroFit’s PGM controlling the sequence. 
 
Details 
PGM is not a “set-and-forget” system. It automates particle release, but it can’t overcome poor room conditions. 
Large rooms require more time and output to reach minimum p/cc levels. These rooms may never be suitable for fit testing.  
HVAC supply vents add clean air that lowers concentration, while returns pull particles out before they accumulate. 
Drafts, open doors, and fan airflow can disperse particles. 
Insufficient run-time before the first test prevents the room from building a stable baseline. 
 
Why it matters 
Particle generation management enhances control and consistency, but operators still need to choose the right room, avoid HVAC disruptions, and allow enough time for particle build-up. It supports the process. It isn’t a magic fix for an unsuitable environment.