AIR FILTRATION · CERTIFICATION
NAFA CAFS Test 2026/2027 (This Quizlet is based on questions that were in the
NAFA CAFS test, April 2024. The NAFA CAFS test has 70 multiple choice
questions, and 30 true/false.) Graded A+ 2026/2027
A+
Complete Blueprint Coverage · NAFA Guide to Air Filtration Domains
A+ 5 100%
QUESTIONS VERIFIED EXAM DOMAINS COVERED RATIONALES INCLUDED
CATEGORIES
Principles of Air Filtration and Particle Capture Mechanisms
Filter Types, Media, and Construction
Testing Standards and Efficiency Ratings
Applications: IAQ, Healthcare, Cleanrooms, and Industrial
Installation, Operation, Maintenance, and Selection
STUVIAACTUALEXAM
, PRINCIPLES OF AIR FILTRATION AND PARTICLE CAPTURE MECHANISMS
Q1
An HVAC engineer is evaluating particle capture in a commercial office building filter bank. The system operates at a
face velocity of 500 fpm and the dominant particles are 0.3 µm. Which primary capture mechanism is most responsible
for removing these fine particles in a high-efficiency mechanical filter?
A. Inertial impaction due to high particle mass and momentum
B. Diffusion (Brownian motion) that increases the probability of fiber contact
C. Straining that physically blocks particles larger than the inter-fiber spacing
D. Gravitational settling that causes particles to fall out of the airstream
Correct Answer: B
Rationale: Particles around 0.3 µm are in the most penetrating particle size range and are captured primarily by diffusion. Brownian
motion causes random movement that brings them into contact with fibers. Impaction dominates for larger particles; straining is
limited for fine aerosols; gravity is negligible at these sizes and velocities.
Q2
A facility manager measures a pressure drop of 0.45 in. w.g. across a clean MERV 13 filter installed in a 24×24 in. frame.
After three months the pressure drop has risen to 0.95 in. w.g. while airflow remains within design limits. What does the
increase in pressure drop primarily indicate?
A. The filter media has lost electrostatic charge and is no longer efficient
B. Particulate loading has increased the resistance of the media to airflow
C. The filter frame has warped, creating a major bypass leak
D. The fan has slowed, reducing system static pressure
Correct Answer: B
Rationale: As dust accumulates on the media, the open flow paths become restricted, raising differential pressure. This is the normal
loading curve for a mechanical filter. Loss of electrostatic charge would mainly affect efficiency, not necessarily produce a large
pressure-drop rise; frame warp or fan slowdown would typically reduce measured airflow or create other symptoms.
Q3
During a training session a technician asks why a filter can show high efficiency on large particles yet allow a measurable
fraction of 0.3 µm particles to pass. Which explanation correctly describes the underlying physics?
A. Large particles are captured by straining while 0.3 µm particles are smaller than every pore and therefore cannot be
captured
B. Capture efficiency is lowest near the most penetrating particle size where neither diffusion nor impaction is optimally
effective
C. All filters are designed to pass 0.3 µm particles so that downstream sensors can detect them
D. 0.3 µm particles carry an electrostatic charge that repels them from the media fibers
Correct Answer: B
Rationale: The most penetrating particle size (typically 0.1–0.4 µm) is the region where diffusion is less effective and inertial
impaction has not yet become dominant, producing a minimum in the efficiency curve. This is a fundamental characteristic of fibrous
media, not a design flaw or intentional pass-through.
Q4
A design engineer is selecting a prefilter for a system that will later use a final MERV 14 filter. The goal is to extend the
life of the final filter. Which particle-size range should the prefilter primarily target?
A. Sub-micron particles below 0.3 µm that would otherwise penetrate the final filter
B. Larger particles greater than approximately 3–5 µm that would rapidly load the final filter surface
, C. Only gaseous contaminants that the final filter cannot remove
D. Biological aerosols exclusively, ignoring non-viable dust
Correct Answer: B
Rationale: Prefilters are intended to remove the bulk of larger, high-mass particles that would otherwise cake on the final filter and
cause rapid pressure-drop rise. Sub-micron particles are better handled by the higher-efficiency final stage; gases require molecular
filtration; biologicals are a subset of the particulate load.
Q5
In a laboratory clean-air hood the airflow is laminar and the face velocity is low. A filter manufacturer claims that
interception is a significant capture mechanism under these conditions. Which statement best supports that claim?
A. Particles follow streamlines and come within one particle radius of a fiber, allowing contact without inertial deviation
B. Particles are forced to change direction abruptly and collide with fibers because of high momentum
C. Particles diffuse randomly until they strike a fiber
D. Particles are larger than the gaps between fibers and are simply sieved out
Correct Answer: A
Rationale: Interception occurs when a particle following a streamline approaches a fiber closer than its own radius and touches the
fiber surface. It is independent of particle mass or Brownian motion and is therefore important for mid-size particles under low-velocity
laminar flow. Impaction requires inertia; diffusion requires Brownian motion; straining requires particles larger than inter-fiber spacing.
Q6
A hospital infection-control officer is reviewing filter performance data. The officer notes that efficiency for 1 µm particles
is substantially higher than for 0.3 µm particles on the same filter. Which capture mechanism accounts for the improved
performance on the larger particles?
A. Diffusion becomes stronger as particle size increases
B. Inertial impaction becomes more effective as particle mass and Stokes number increase
C. Gravitational settling becomes the dominant mechanism above 0.5 µm
D. Electrostatic attraction increases linearly with particle diameter
Correct Answer: B
Rationale: Larger particles possess greater inertia and cannot follow the curved streamlines around fibers as readily, so they impact
the fiber. Diffusion weakens with increasing size; gravity is still minor at typical HVAC velocities; electrostatic effects depend on
charge state, not solely on diameter.
Q7
An air-filter salesperson is explaining the difference between face velocity and media velocity to a new technician. In a
pleated filter with a high media-to-face area ratio, which statement is accurate?
A. Media velocity is always equal to face velocity because the air has nowhere else to go
B. Media velocity is lower than face velocity, reducing pressure drop and improving fine-particle capture by diffusion
C. Media velocity is higher than face velocity, which improves inertial impaction of large particles
D. Face velocity and media velocity are interchangeable terms used only for marketing
Correct Answer: B
Rationale: Pleating multiplies the media area relative to the face area, so the actual speed of air through the media is lower than the
face velocity. Lower media velocity favors diffusion capture of fine particles and reduces resistance. Higher media velocity would
increase pressure drop and reduce diffusion effectiveness.
NAFA CAFS Test 2026/2027 (This Quizlet is based on questions that were in the
NAFA CAFS test, April 2024. The NAFA CAFS test has 70 multiple choice
questions, and 30 true/false.) Graded A+ 2026/2027
A+
Complete Blueprint Coverage · NAFA Guide to Air Filtration Domains
A+ 5 100%
QUESTIONS VERIFIED EXAM DOMAINS COVERED RATIONALES INCLUDED
CATEGORIES
Principles of Air Filtration and Particle Capture Mechanisms
Filter Types, Media, and Construction
Testing Standards and Efficiency Ratings
Applications: IAQ, Healthcare, Cleanrooms, and Industrial
Installation, Operation, Maintenance, and Selection
STUVIAACTUALEXAM
, PRINCIPLES OF AIR FILTRATION AND PARTICLE CAPTURE MECHANISMS
Q1
An HVAC engineer is evaluating particle capture in a commercial office building filter bank. The system operates at a
face velocity of 500 fpm and the dominant particles are 0.3 µm. Which primary capture mechanism is most responsible
for removing these fine particles in a high-efficiency mechanical filter?
A. Inertial impaction due to high particle mass and momentum
B. Diffusion (Brownian motion) that increases the probability of fiber contact
C. Straining that physically blocks particles larger than the inter-fiber spacing
D. Gravitational settling that causes particles to fall out of the airstream
Correct Answer: B
Rationale: Particles around 0.3 µm are in the most penetrating particle size range and are captured primarily by diffusion. Brownian
motion causes random movement that brings them into contact with fibers. Impaction dominates for larger particles; straining is
limited for fine aerosols; gravity is negligible at these sizes and velocities.
Q2
A facility manager measures a pressure drop of 0.45 in. w.g. across a clean MERV 13 filter installed in a 24×24 in. frame.
After three months the pressure drop has risen to 0.95 in. w.g. while airflow remains within design limits. What does the
increase in pressure drop primarily indicate?
A. The filter media has lost electrostatic charge and is no longer efficient
B. Particulate loading has increased the resistance of the media to airflow
C. The filter frame has warped, creating a major bypass leak
D. The fan has slowed, reducing system static pressure
Correct Answer: B
Rationale: As dust accumulates on the media, the open flow paths become restricted, raising differential pressure. This is the normal
loading curve for a mechanical filter. Loss of electrostatic charge would mainly affect efficiency, not necessarily produce a large
pressure-drop rise; frame warp or fan slowdown would typically reduce measured airflow or create other symptoms.
Q3
During a training session a technician asks why a filter can show high efficiency on large particles yet allow a measurable
fraction of 0.3 µm particles to pass. Which explanation correctly describes the underlying physics?
A. Large particles are captured by straining while 0.3 µm particles are smaller than every pore and therefore cannot be
captured
B. Capture efficiency is lowest near the most penetrating particle size where neither diffusion nor impaction is optimally
effective
C. All filters are designed to pass 0.3 µm particles so that downstream sensors can detect them
D. 0.3 µm particles carry an electrostatic charge that repels them from the media fibers
Correct Answer: B
Rationale: The most penetrating particle size (typically 0.1–0.4 µm) is the region where diffusion is less effective and inertial
impaction has not yet become dominant, producing a minimum in the efficiency curve. This is a fundamental characteristic of fibrous
media, not a design flaw or intentional pass-through.
Q4
A design engineer is selecting a prefilter for a system that will later use a final MERV 14 filter. The goal is to extend the
life of the final filter. Which particle-size range should the prefilter primarily target?
A. Sub-micron particles below 0.3 µm that would otherwise penetrate the final filter
B. Larger particles greater than approximately 3–5 µm that would rapidly load the final filter surface
, C. Only gaseous contaminants that the final filter cannot remove
D. Biological aerosols exclusively, ignoring non-viable dust
Correct Answer: B
Rationale: Prefilters are intended to remove the bulk of larger, high-mass particles that would otherwise cake on the final filter and
cause rapid pressure-drop rise. Sub-micron particles are better handled by the higher-efficiency final stage; gases require molecular
filtration; biologicals are a subset of the particulate load.
Q5
In a laboratory clean-air hood the airflow is laminar and the face velocity is low. A filter manufacturer claims that
interception is a significant capture mechanism under these conditions. Which statement best supports that claim?
A. Particles follow streamlines and come within one particle radius of a fiber, allowing contact without inertial deviation
B. Particles are forced to change direction abruptly and collide with fibers because of high momentum
C. Particles diffuse randomly until they strike a fiber
D. Particles are larger than the gaps between fibers and are simply sieved out
Correct Answer: A
Rationale: Interception occurs when a particle following a streamline approaches a fiber closer than its own radius and touches the
fiber surface. It is independent of particle mass or Brownian motion and is therefore important for mid-size particles under low-velocity
laminar flow. Impaction requires inertia; diffusion requires Brownian motion; straining requires particles larger than inter-fiber spacing.
Q6
A hospital infection-control officer is reviewing filter performance data. The officer notes that efficiency for 1 µm particles
is substantially higher than for 0.3 µm particles on the same filter. Which capture mechanism accounts for the improved
performance on the larger particles?
A. Diffusion becomes stronger as particle size increases
B. Inertial impaction becomes more effective as particle mass and Stokes number increase
C. Gravitational settling becomes the dominant mechanism above 0.5 µm
D. Electrostatic attraction increases linearly with particle diameter
Correct Answer: B
Rationale: Larger particles possess greater inertia and cannot follow the curved streamlines around fibers as readily, so they impact
the fiber. Diffusion weakens with increasing size; gravity is still minor at typical HVAC velocities; electrostatic effects depend on
charge state, not solely on diameter.
Q7
An air-filter salesperson is explaining the difference between face velocity and media velocity to a new technician. In a
pleated filter with a high media-to-face area ratio, which statement is accurate?
A. Media velocity is always equal to face velocity because the air has nowhere else to go
B. Media velocity is lower than face velocity, reducing pressure drop and improving fine-particle capture by diffusion
C. Media velocity is higher than face velocity, which improves inertial impaction of large particles
D. Face velocity and media velocity are interchangeable terms used only for marketing
Correct Answer: B
Rationale: Pleating multiplies the media area relative to the face area, so the actual speed of air through the media is lower than the
face velocity. Lower media velocity favors diffusion capture of fine particles and reduces resistance. Higher media velocity would
increase pressure drop and reduce diffusion effectiveness.
