NAFA CERTIFIED AIR FILTER SPECIALIST (CAFS)
EXAM STUDY SET — 2026/2027
75 Multiple-Choice & Scenario-Based Questions
Complete Questions, Correct Answers & Verified Rationales
Aligned with NAFA CAFS Body of Knowledge, ASHRAE 52.2, ISO 16890
,1. According to ASHRAE Standard 52.2, which particle size range is used to
determine the MERV rating for filters in the E1 efficiency category?
A. 0.1–0.3 µm
B. 0.3–1.0 µm
C. 1.0–3.0 µm
D. 3.0–10.0 µm
Rationale: ASHRAE Standard 52.2 defines three composite particle size ranges for MERV
testing: E1 (0.3–1.0 µm), E2 (1.0–3.0 µm), and E3 (3.0–10.0 µm). The E1 range evaluates filter
efficiency against fine particles including smoke, bacteria, and virus carriers. MERV ratings
13–16 require high composite efficiency in the E1 range, making this category critical for
healthcare and cleanroom applications.
2. Which filtration mechanism is PRIMARILY responsible for capturing particles
smaller than 0.1 µm?
A. Inertial impaction
B. Interception
C. Diffusion (Brownian motion)
D. Electrostatic attraction
Rationale: Diffusion (Brownian motion) dominates capture of ultrafine particles smaller than
0.1 µm because random molecular collisions cause these tiny particles to deviate from airflow
streamlines and contact filter fibers. Inertial impaction captures larger particles (>1 µm) with
high momentum that cannot follow curved streamlines. Interception captures mid-size particles
that follow streamlines but contact fibers due to their physical size. Electrostatic attraction
enhances capture across sizes but is not the primary mechanism for ultrafine particles.
3. A filter's efficiency curve plotted against particle diameter typically shows a 'most
penetrating particle size' (MPPS). For most fibrous media, where does the MPPS
occur?
A. 0.01–0.05 µm
B. 0.1–0.3 µm
C. 1.0–3.0 µm
D. 5.0–10.0 µm
Rationale: The most penetrating particle size (MPPS) for most fibrous filter media occurs in the
0.1–0.3 µm range, typically around 0.15–0.25 µm. Particles smaller than the MPPS are
captured efficiently by diffusion (Brownian motion), while particles larger than the MPPS are
captured by interception and impaction. At the MPPS, neither mechanism dominates, resulting
in minimum filtration efficiency. HEPA filters are tested at 0.3 µm, which approximates the
MPPS for typical glass fiber media.
4. What is the relationship between filtration efficiency and airflow resistance
(pressure drop) in a mechanical filter?
A. Efficiency and resistance are independent of each other
B. Higher efficiency always produces proportionally lower resistance
C. Higher efficiency generally results in higher resistance
D. Higher efficiency always produces proportionally higher resistance
Rationale: Higher filtration efficiency generally results in higher airflow resistance (pressure
drop) because capturing smaller or more particles requires tighter fiber packing, thicker media,
or smaller pore sizes, all of which impede airflow. However, the relationship is not strictly
proportional—advances in media technology (e.g., gradient density, electrostatic enhancement)
can achieve higher efficiency without a proportional resistance increase. This trade-off is
central to filter selection and energy optimization in HVAC system design.
5. Which particle size range constitutes PM2.5 as defined by the U.S. Environmental
Protection Agency (EPA)?
, A. 0.3–2.5 µm
B. ≤2.5 µm
C. 2.5–10 µm
D. ≤10 µm
Rationale: PM2.5 (fine particulate matter) is defined as particles with an aerodynamic
diameter of 2.5 µm or less. These fine particles are respirable and can penetrate deep into the
lungs and even enter the bloodstream. PM10 refers to particles 10 µm or less. ASHRAE MERV
ratings provide guidance on a filter's ability to remove PM2.5 and PM10 fractions, with MERV
11 and above capturing increasing percentages of PM2.5 particles.
6. When describing the four primary filtration mechanisms, which mechanism is
most effective for particles in the 1–10 µm range?
A. Diffusion
B. Inertial impaction
C. Interception
D. Gravity settling
Rationale: Inertial impaction is the dominant filtration mechanism for particles in the 1–10 µm
range. These particles have sufficient mass and inertia that they cannot follow the curving
airflow streamlines around filter fibers. Instead, they continue in a straight line and impact
(collide with) the fiber surface. This mechanism becomes more effective as particle size and
airflow velocity increase. Filters in HVAC systems rely heavily on inertial impaction for
capturing dust, pollen, and larger airborne particles.
7. What does 'arrestance' measure in filter testing, as defined by ASHRAE Standard
52.1?
A. Efficiency against fine particles (0.3–1.0 µm)
B. Weight percentage of synthetic dust captured
C. Pressure drop across the filter at rated airflow
D. Resistance to microbial growth on the filter media
Rationale: Arrestance, as defined in ASHRAE Standard 52.1, measures the weight percentage of
a standardized synthetic dust (ASHRAE dust) captured by a filter. It primarily reflects a filter's
ability to capture large, heavy particles. Arrestance values are often very high (80–95%) even
for low-efficiency filters because the test measures mass removal, not particle count efficiency.
ASHRAE 52.2 MERV ratings, which measure particle count efficiency by size, have largely
superseded arrestance for meaningful filter comparison.
8. Electrostatic enhancement in filter media improves initial efficiency but presents
which significant operational concern?
A. Increased risk of fire
B. Efficiency degradation as electrostatic charge dissipates over time
C. Incompatibility with HEPA-grade filtration
D. Higher initial pressure drop than mechanical media of equivalent efficiency
Rationale: Electrostatically enhanced (electret) media achieves higher initial filtration efficiency
through inherent fiber charge rather than relying solely on mechanical capture mechanisms.
However, the electrostatic charge can dissipate over time due to humidity, particle loading, and
airflow exposure, causing the filter's efficiency to decline toward its baseline mechanical
efficiency. This efficiency degradation means the filter's real-world performance may differ
significantly from its initial rating, which is a key consideration for maintenance scheduling
and filter selection per NAFA guidelines.
9. What is 'loading' in the context of air filtration?
A. The process of installing a new filter into the housing
B. The accumulation of captured particles within the filter media over time
C. The static pressure applied to the filter frame during installation
EXAM STUDY SET — 2026/2027
75 Multiple-Choice & Scenario-Based Questions
Complete Questions, Correct Answers & Verified Rationales
Aligned with NAFA CAFS Body of Knowledge, ASHRAE 52.2, ISO 16890
,1. According to ASHRAE Standard 52.2, which particle size range is used to
determine the MERV rating for filters in the E1 efficiency category?
A. 0.1–0.3 µm
B. 0.3–1.0 µm
C. 1.0–3.0 µm
D. 3.0–10.0 µm
Rationale: ASHRAE Standard 52.2 defines three composite particle size ranges for MERV
testing: E1 (0.3–1.0 µm), E2 (1.0–3.0 µm), and E3 (3.0–10.0 µm). The E1 range evaluates filter
efficiency against fine particles including smoke, bacteria, and virus carriers. MERV ratings
13–16 require high composite efficiency in the E1 range, making this category critical for
healthcare and cleanroom applications.
2. Which filtration mechanism is PRIMARILY responsible for capturing particles
smaller than 0.1 µm?
A. Inertial impaction
B. Interception
C. Diffusion (Brownian motion)
D. Electrostatic attraction
Rationale: Diffusion (Brownian motion) dominates capture of ultrafine particles smaller than
0.1 µm because random molecular collisions cause these tiny particles to deviate from airflow
streamlines and contact filter fibers. Inertial impaction captures larger particles (>1 µm) with
high momentum that cannot follow curved streamlines. Interception captures mid-size particles
that follow streamlines but contact fibers due to their physical size. Electrostatic attraction
enhances capture across sizes but is not the primary mechanism for ultrafine particles.
3. A filter's efficiency curve plotted against particle diameter typically shows a 'most
penetrating particle size' (MPPS). For most fibrous media, where does the MPPS
occur?
A. 0.01–0.05 µm
B. 0.1–0.3 µm
C. 1.0–3.0 µm
D. 5.0–10.0 µm
Rationale: The most penetrating particle size (MPPS) for most fibrous filter media occurs in the
0.1–0.3 µm range, typically around 0.15–0.25 µm. Particles smaller than the MPPS are
captured efficiently by diffusion (Brownian motion), while particles larger than the MPPS are
captured by interception and impaction. At the MPPS, neither mechanism dominates, resulting
in minimum filtration efficiency. HEPA filters are tested at 0.3 µm, which approximates the
MPPS for typical glass fiber media.
4. What is the relationship between filtration efficiency and airflow resistance
(pressure drop) in a mechanical filter?
A. Efficiency and resistance are independent of each other
B. Higher efficiency always produces proportionally lower resistance
C. Higher efficiency generally results in higher resistance
D. Higher efficiency always produces proportionally higher resistance
Rationale: Higher filtration efficiency generally results in higher airflow resistance (pressure
drop) because capturing smaller or more particles requires tighter fiber packing, thicker media,
or smaller pore sizes, all of which impede airflow. However, the relationship is not strictly
proportional—advances in media technology (e.g., gradient density, electrostatic enhancement)
can achieve higher efficiency without a proportional resistance increase. This trade-off is
central to filter selection and energy optimization in HVAC system design.
5. Which particle size range constitutes PM2.5 as defined by the U.S. Environmental
Protection Agency (EPA)?
, A. 0.3–2.5 µm
B. ≤2.5 µm
C. 2.5–10 µm
D. ≤10 µm
Rationale: PM2.5 (fine particulate matter) is defined as particles with an aerodynamic
diameter of 2.5 µm or less. These fine particles are respirable and can penetrate deep into the
lungs and even enter the bloodstream. PM10 refers to particles 10 µm or less. ASHRAE MERV
ratings provide guidance on a filter's ability to remove PM2.5 and PM10 fractions, with MERV
11 and above capturing increasing percentages of PM2.5 particles.
6. When describing the four primary filtration mechanisms, which mechanism is
most effective for particles in the 1–10 µm range?
A. Diffusion
B. Inertial impaction
C. Interception
D. Gravity settling
Rationale: Inertial impaction is the dominant filtration mechanism for particles in the 1–10 µm
range. These particles have sufficient mass and inertia that they cannot follow the curving
airflow streamlines around filter fibers. Instead, they continue in a straight line and impact
(collide with) the fiber surface. This mechanism becomes more effective as particle size and
airflow velocity increase. Filters in HVAC systems rely heavily on inertial impaction for
capturing dust, pollen, and larger airborne particles.
7. What does 'arrestance' measure in filter testing, as defined by ASHRAE Standard
52.1?
A. Efficiency against fine particles (0.3–1.0 µm)
B. Weight percentage of synthetic dust captured
C. Pressure drop across the filter at rated airflow
D. Resistance to microbial growth on the filter media
Rationale: Arrestance, as defined in ASHRAE Standard 52.1, measures the weight percentage of
a standardized synthetic dust (ASHRAE dust) captured by a filter. It primarily reflects a filter's
ability to capture large, heavy particles. Arrestance values are often very high (80–95%) even
for low-efficiency filters because the test measures mass removal, not particle count efficiency.
ASHRAE 52.2 MERV ratings, which measure particle count efficiency by size, have largely
superseded arrestance for meaningful filter comparison.
8. Electrostatic enhancement in filter media improves initial efficiency but presents
which significant operational concern?
A. Increased risk of fire
B. Efficiency degradation as electrostatic charge dissipates over time
C. Incompatibility with HEPA-grade filtration
D. Higher initial pressure drop than mechanical media of equivalent efficiency
Rationale: Electrostatically enhanced (electret) media achieves higher initial filtration efficiency
through inherent fiber charge rather than relying solely on mechanical capture mechanisms.
However, the electrostatic charge can dissipate over time due to humidity, particle loading, and
airflow exposure, causing the filter's efficiency to decline toward its baseline mechanical
efficiency. This efficiency degradation means the filter's real-world performance may differ
significantly from its initial rating, which is a key consideration for maintenance scheduling
and filter selection per NAFA guidelines.
9. What is 'loading' in the context of air filtration?
A. The process of installing a new filter into the housing
B. The accumulation of captured particles within the filter media over time
C. The static pressure applied to the filter frame during installation
