🌿 Size fractions of airborne particles — definitions according to PN-EN 481

Environment PN-EN 481

Overview

WHY SUCH A STANDARD EXISTS. Limit values for dusts are set separately for total dust and for the respirable fraction, and the difference between them can be severalfold. If there is no single, shared definition of a fraction, two laboratories measuring "the same" respirable dust obtain incomparable results — not because either is wrong, but because their separators collect different parts of the size distribution. PN-EN 481 removes that latitude: it defines fractions as sampling efficiency curves referred to the aerodynamic diameter of a particle, rather than as thresholds of "everything below X micrometres".

THE SCOPE FROM THE PKN CATALOGUE CARD, IN FULL: "Definitions are given relating to sampling principles for the inhalable, thoracic, extrathoracic, tracheobronchial and respirable fractions of particles. The basis of the adopted principles is described, together with the determination of the particle size distribution for all fractions." Five fraction names in a single sentence — and that is the proper measure of how many misunderstandings this standard settles. Polish practice usually circulates two of them: inhalable and respirable; the remaining three appear less often and are sometimes confused with the first two.

WHY AERODYNAMIC AND NOT GEOMETRIC DIAMETER. A dust particle is not a sphere of known density — it may be a plate, a fibre, an agglomerate. How deeply it penetrates the respiratory tract is decided not by its geometric dimension but by its behaviour in an air stream. The aerodynamic diameter is the diameter of a sphere of a conventional reference density settling in air at the same velocity as the particle in question. We do not quote this definition from the standard — we do not hold its text; we give it as a concept accepted in aerosol science, to explain what purpose it serves. Defining fractions through this quantity makes the definition independent of the shape and density of the dust — which is why the same curves apply to wood dust, silica and fibres.

WHAT FOLLOWS FOR EQUIPMENT. Defining a fraction by an efficiency curve rather than by a product means that the suitability of a separator is decided by its characteristic, not by its trade name. We note at the same time that we do not hold the text of the standard and make no claim as to what exactly it says about equipment. The manufacturer declares the conformity of a cyclone or impactor with the convention curve for a given fraction; the laboratory accepts and documents that declaration. Replacing a separator with a "similar" one without checking which convention it was built to is a change of method, even if its appearance and mounting are identical.

RELATION TO OTHER DOCUMENTS IN THIS DATABASE. PN-EN 481 defines WHAT is collected; PN-Z-04008-7 specifies where, when and how many air samples to take; PN-EN 482 sets requirements for the performance of the whole procedure; PN-EN 689 describes the strategy of exposure assessment. Method entries concerning dusts — among them respirable dust by the gravimetric method — cite 481 precisely so that the name of the fraction has an unambiguous meaning.

THE AGE OF THE STANDARD AND ITS CURRENCY — SOMETHING WORTH STATING PLAINLY. The edition dates from 1998 and adopts a European document from 1993. That makes it an old standard, but not a withdrawn one: the catalogue card retrieved on 15 September 2026 contains neither the heading "Withdrawn" nor a "Date of withdrawal" row. Age in itself is not a defect — currency is decided by the status in the catalogue, not by the year of publication. We note this because with standards from the 1990s the first reflex is to assume they are out of date. We make no claim, however, about the relation of these conventions to those of ISO 7708: the PN-EN 481 catalogue card does not mention that document, and we hold the text of neither standard.

Method principle

A fraction is not a set of particles smaller than a certain limit, but the result of MULTIPLYING the actual particle size distribution of the dust by the sampling efficiency curve proper to that fraction. The curve describes what proportion of particles of a given aerodynamic diameter is to be collected: for the inhalable fraction it is the proportion entering through the nose and mouth, for the respirable fraction the proportion reaching the alveoli. The separator in a measuring instrument is a physical realisation of such a curve; the measurement result therefore depends both on the dust and on the convention to which the separator was built.

Applications

• Dust measurements at workplaces — deciding which fraction to determine and against which limit value to compare the result
• Selection and qualification of separators (cyclones, impactors) for sampling the respirable and inhalable fractions
• Assessing the comparability of results from two laboratories measuring "the same" dust — the most frequent cause of discrepancy lies in the fraction convention, not in the weighing
• Describing the method in accreditation documentation: the name of a fraction without a reference to its definition is not unambiguous

Key parameters

Standard

Standard number

PN-EN 481:1998 (wersja polska; wprowadza EN 481:1993 [IDT])

Title (PL)

Atmosfera miejsca pracy -- Określenie składu ziarnowego dla pomiaru cząstek zawieszonych w powietrzu

Title (EN)

Workplace atmospheres — Size fraction definitions for measurement of airborne particles

Step-by-step procedure

1. Establish which fraction is being assessed

   Check for which fraction a limit value has been set for the dust in question. The same dust may have different values for the inhalable and respirable fractions.

2. Match the separator to the convention

   Check in the manufacturer's documentation which convention curve the separator was built to and at what volumetric flow it retains that characteristic.

3. Maintain the volumetric flow

   The separator characteristic holds only at the design flow. A departure changes the efficiency curve — that is, it effectively changes the fraction being measured.

4. Record the fraction in the report

   Give the name of the fraction together with a reference to its definition. The words "respirable dust" alone, without naming the defining document, do not allow the recipient to verify comparability.

5. Verify after any change of equipment

   Replacing a separator with a different model is a change of method until it has been confirmed that the new one corresponds to the same convention.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Industrial dusts may be carcinogenic (crystalline silica, hardwood dust, asbestos) — handle samples according to the material safety data sheet, not according to this standard
• TECHNICAL NOTE: "below 4 µm" is not equivalent to the respirable fraction — a fraction is a curve, not a threshold; substituting one for the other changes the result
• The standard contains no safety requirements — it is a definitional document

Frequently asked questions