Understanding PFE Testing: PSL vs. NaCl

Filtration performance is often summarized using a single number, such as “≥98% filtration.” However, the method used to generate that number can significantly influence the result.

Two of the most commonly used particle challenge methods are:

• Polystyrene Latex (PSL) spheres (ASTM F2299)

• Sodium Chloride (NaCl) aerosols (NIOSH 42 CFR Part 84 / TEB-APR-STP-0059)

While both are used to evaluate filtration performance, they behave differently and can produce different results for the same material.

Understanding these differences is important when interpreting mask performance data.

What is PSL Testing?

Polystyrene Latex (PSL) testing uses solid, spherical plastic particles that are:

• Uniform in size (often ~100 nanometers for PFE testing)

• Non-neutralized (can carry charge)

• Relatively stable and consistent

Because PSL particles are uniform and predictable, they are commonly used in:

• ASTM Particulate Filtration Efficiency (PFE) testing (ASTM F2299)

• Medical mask evaluation

Key Characteristics:

• Smooth, spherical particles of a consistent 100 nanometer diameter

• Airflow velocity mimics normal breathing (28.3 Liters / minute)

• Evaluates filtration performance of the material itself, not overall mask fit or leakage

What is NaCl Testing?

Sodium chloride (NaCl) testing uses aerosolized salt particles that are:

• Irregular in shape

• Charge-neutralized (to simulate worst-case conditions)

• More dynamic in airflow

NaCl is used in:

• NIOSH testing (N95 certification)

• Respirator evaluation

Key Characteristics:

• Polydisperse (range of particle sizes)

• Neutralized charge reduces electrostatic capture advantage

• Represents a more challenging filtration condition

Why Do Results Differ?

Although both methods evaluate filtration, they do not test identical conditions.

Key Differences:

FeaturePSL TestingNaCl TestingParticle shapeUniform spheresIrregularChargeOften chargedNeutralizedSize distributionMonodispersePolydisperseUse caseASTM medical masksNIOSH respirators

Electrostatic Effects Matter

Many mask filters rely not only on physical interception, but also on electrostatic attraction.

• Charged particles (like PSL) can be more easily captured

• Neutralized particles (like NaCl) remove this advantage

This means:

NaCl testing often represents a more conservative or “worst-case” filtration scenario

Particle Behavior in Airflow

• PSL particles behave more predictably

• NaCl particles better simulate real aerosol variability

This can influence how particles:

• move through fiber networks

• interact with fibers

• are ultimately captured

What This Means for Interpreting Filtration Ratings

A filtration percentage is not just a property of the mask — it is also a function of:

• the test method used

• the particle type

• the test conditions

Because of this:

Two masks reporting similar filtration percentages may have been tested under very different conditions.

ASTM vs NIOSH Context

• ASTM PFE (medical masks)

  • Often uses PSL particles (~100 nm)

  • Focused on material filtration performance

• NIOSH (N95 respirators)

  • Uses NaCl aerosol (~75 nm median aerodynamic diameter)

  • Includes charge neutralization

  • Designed to represent a more stringent test condition

Why This Matters in Practice

Understanding test methods helps explain why:

• Filtration numbers are not always directly comparable

• Materials may perform differently under different conditions

• Standards reflect different priorities (clinical vs occupational)

It also reinforces a broader point:

Filtration performance is not a single number — it is a function of material, design, and testing conditions.

Key Takeaways

• PSL and NaCl testing evaluate filtration differently

• NaCl testing is generally more stringent due to charge neutralization

• ASTM and NIOSH standards use different methods for different purposes

• Filtration percentages should be interpreted in the context of how they were measured