ISO 28439:2011 – Workplace Atmospheres — Characterization of Ultrafine Aerosols/Nanoaerosols — Size Distribution and Number Concentration Using DMA

Introduction to Ultrafine Aerosol Measurement in Workplaces

ISO 28439:2011 provides a standardized methodology for characterizing ultrafine aerosols (particles smaller than 100 nm) and nanoaerosols in workplace atmospheres using differential electrical mobility analysing systems. As engineered nanomaterials have proliferated across industries, the need for reliable, reproducible measurement of airborne nanoparticles has become critical for occupational health risk assessment. The standard addresses the entire measurement chain from aerosol sampling through classification to particle counting.

Measurement Principle and Instrumentation Requirements

The measurement system consists of three core components: a particle charger (typically a bipolar diffusion charger using a radioactive source or corona discharge), a differential mobility analyzer that selects particles within a narrow mobility window, and a condensation particle counter (CPC) for detection. The system must be capable of measuring particle diameters from 5 nm to 100 nm with a size resolution (ΔZ/Z) better than 10%.

Sampling Strategy and Data Processing

ISO 28439 specifies that sampling must be representative of the breathing zone of workers, with the sampling inlet positioned within 300 mm of the nose and mouth. Sampling duration should be at least 8 hours for time-weighted average exposure assessment, with shorter grab samples (15-30 minutes) for peak exposure identification. The standard mandates calculation of number-weighted size distribution parameters including count median diameter (CMD), geometric standard deviation (GSD), and total number concentration.

Data inversion algorithms must account for multiply charged particles — larger particles acquiring multiple charges in the bipolar charger are classified at the same mobility as smaller singly charged particles. The standard recommends using the Twomey-Markowski iterative inversion algorithm for recovering the true size distribution from raw DMA-CPC data.

Instrument Calibration and Quality Assurance

ISO 28439 mandates a comprehensive calibration regimen for all components of the DMA measurement system. The differential mobility analyzer must have its mobility classification calibrated using certified monodisperse polystyrene latex (PSL) spheres across the entire measurement range, with at least five sizes per decade. The condensation particle counter efficiency must be verified using an electrometer reference at particle concentrations spanning the expected measurement range (typically 10³ to 10⁷ particles/cm³). The sheath flow rate must be calibrated using a primary standard (bubble flow meter or dry piston calibrator) with accuracy within ±1% of the setpoint. The standard specifies that calibration verification must be performed before each measurement campaign, with full recalibration required every 6 months or after any instrument maintenance. Quality assurance procedures include daily checks of zero-count rate (should be less than 0.1 counts per second in particle-free air), flow rate verification using calibrated mass flow meters, and weekly checks of the charger neutralization efficiency using mobility-classified particles at known concentrations. These procedures ensure data quality across long-term monitoring campaigns that may extend over months or years, particularly important for workplace exposure assessment programs.

Data Processing and Uncertainty Analysis

The standard provides detailed guidance on data processing algorithms essential for obtaining accurate size distributions. Raw DMA data consists of particle counts at each voltage step, which must be inverted to recover the true size distribution using algorithms that account for multiple charging effects, diffusion broadening, and CPC counting efficiency. The standard recommends the Twomey-Markowski iterative algorithm with at least 15 iterations and a smoothing factor between 0.5 and 2.0 for optimal noise suppression without distorting the size distribution features. Uncertainty analysis must account for: flow rate measurement uncertainty (typically ±2%), voltage accuracy (±0.5%), CPC counting statistics (Poisson counting uncertainty proportional to 1/√N), and charging efficiency uncertainty (approximately ±3% for particles above 20 nm, increasing to ±10% below 10 nm). The combined standard uncertainty in number concentration measurement is typically ±15-25% for the 20-100 nm size range, dominated by flow rate and charging efficiency uncertainties. For workplace compliance measurements, the expanded uncertainty (k=2, 95% confidence) must be reported alongside the measured concentrations to enable proper comparison with occupational exposure limits.