INDUSTRIAL HYGIENE SAMPLING AND SIZING OF AIRBORNE PARTICLES

* INDUSTRIAL HYGIENE SAMPLING AND SIZING OF AIRBORNE PARTICLES UNIVERSITY OF HOUSTON - CLEAR LAKE SPRING 2015

*INTRODUCTION Techniques to evaluate exposures to particulates in occupational workplace settings. Inhaled particles may react with or be absorbed through tissues to cause adverse health effects. Variables include: - size, shape, and density; - chemical properties; - airborne concentration, time of exposure, other factors; - health effects – irritation, illness, disease.

*AEROSOLS Aerosol – described as solid and/or liquid particles dispersed in gaseous medium. Range of > 50 um to microscopic particles invisible to naked eye. For IH, gaseous medium is usually AIR. Occupational aerosol hazards recognized by: Pliny Agricola Ramazzini Aerosol science and inhalation toxicology assist with better understanding and development of exposure characterization.

* Sampling and analysis techniques for aerosol characterization. Single procedure is not appropriate; be familiar with properties of aerosols. Evolving ultrafine and nano-size aerosols, also incidentally address bioaerosols.

* Forms of Aerosols: *Dust (also crystalline materials) *Fumes *Mists *Fogs *Smokes *Fibers (length exceeds diameter)

* Particulate aerosols produced by mechanical processes (i. e. breaking, grinding, pulverizing). Examples: mining; material handing; dry material preparation and packaging. Chemically unchanged; smaller size and higher surface area enhances ability to be airborne, inhaled, penetration, toxicity, solubility or explosion. Less than 1 um up to 1 mm; regular in shape; some crystalline materials; length to width ratio less than 3: 1 with some notable exceptions.

* Fine solid aerosol particles produced from re-condensation of vaporized material normally solid at standard conditions. Condensation (0. 01 um) occurs during cooling of vapor – nucleation; coagulation – agglomerates at 1 um diameter nearly spherical. Small enough for deep lung penetration; chemically quite reactive. Examples: welding – “metal fume fever”.

* Spherical droplet aerosols produced from bulk liquid by mechanical processes (i. e. splash or spray). Droplets are chemically unchanged from the parent liquid and range in size from a few microns to over 100 um. Examples: mist aerosol from spray painting; crop spraying operations designed to produce mists; mist droplet aerosols by coughing or health care treatment.

* Droplet aerosols produced by physical condensation from vapor phase. Fog droplets typically smaller than mechanically generated mist droplets, at 1 to 10 um. Fogs appear to remain suspended in air compared to mists which visibly settle to ground.

* Mixtures of solid and liquid aerosol particles, gases, and vapors resulting from incomplete combustion of carbonaceous materials and formed by complex combinations of physical nucleationtype mechanisms and chemical reactions. Examples: tobacco smokes; smokes from other combustion (i. e. plastics, synthetic fabrics, and petrochemical products – toxic). Primary smoke particles with diameters of 0. 01 to 1 um, but like fumes, agglomerates containing many particles may be much larger.

* Elongated particles with length much greater than width. May be naturally occurring or synthetic. Examples: asbestos with convention to define a “fiber” as particle with ratio of length or width greater than 3: 1; specific diseases; synthetic fibers. Fibrous aerosols display aerodynamic and health effects behaviors that differ in many respects from spherical or near-spherical particles of same material and mass, so aerosol characterization is more complex for fibers than other aerosols.

*AEROSOLS Aerosol concentrations in air are often assessed by mass per unit volume (mg/M 3); when using mass, large particles have the most significant impact on total mass = volume x density Other assessment methods include particle counting (mppcf) and total surface area. Can account for contribution of reactive surfaces and considers smaller particles; evaluation tools to assess risk of ultrafine and nano-materials.

* *Aerosol distribution – Figure 14. 1 Refer to info regarding size ranges and general definition of particle types. *Monodisperse (single particle size) vs. Polydisperse (range of particle sizes) Figure 14. 2 *Particle size distribution: log normal !!! Tail that extends out to larger particles.

* *Isometric – length dimension independent of particle orientation (e. g. dusts) *Spherical – based on diameter (e. g. bioaerosols) *Singlet – single discrete particles *Aggregate – coagulate or flocculate (i. e. soot); large surface are per unit mass *Morphology determination by optical or electron microscopy.

* Dose – drives biological response; result of exposure history; deposition efficiency; target organs; depends on exposure history; pharmacokinetics of clearance process and intrinsic toxicity. Bioaccumulation. Dose rate – rate at which substance arrives (inhaled or deposited) and exposure may be measured. Influenced by physical aerosol properties (size, shape, density, and hygroscopicity [take up water]).

* - Sedimentation - Inertial Motion and Deposition - Diffusion - Interception

* Refers to movement of an aerosol particle through gaseous medium under the influence of gravity. The rate of settling depends on particle size, shape, mass, and orientation (for non-spherical particles) and on air density and viscosity. Gravitational force opposed by gas viscosity. Stokes’ Law and Diameter (Equations 14 -2/14 -4). Aerodynamic equivalent diameter – “normalizes” different aerosols to common basis for comparison.

* Discuss particle size in terms of diameter of a spherical particle of the same density that would exhibit the same behavior as the particle in question = Stokes diameter (d. ST). Aerodynamic equivalent diameter, dae, is diameter of a unit density sphere (density = 1 g/cm 3) that would exhibit the same settling velocity.

* Aerodynamic equivalent diameter “normalizes” different aerosols to a common basis so that behaviors may be directly compared. Particle Stokes and aerodynamic diameters are important for inertial and gravitational deposition, collector design, and data interpretation.

* Inertia - defined as tendency to resist change in motion; important for human inhalation/deposition and aerosol sampling. Impaction on surface within distance traveled; likelihood increased with mass and velocity of particle and sharpness of change in direction. Stokes Number = St. (Equation 14 -7) Inefficiency of impaction increases with increasing St.

* Diffusion - aerosol particles in a gaseous medium collide with individual gas molecules that are in random Brownian motion associated with fundamental microscopic thermal behavior. Diffusion coefficient is inversely proportional to particle geometric size and independent of particle density. Favored by small particle diameter, large concentration differences, and short distances for diffusion.

* Interception – flow of an aerosol past a surface may produce particle deposition. Deposition process does not depend on particle motion across fluid stream lines, as for inertial impaction. Depends on particle coming close enough to a flow boundary (by any means) that it may be collected by virtue of its own physical size. Significant to elongated particles (i. e. fibers).

* Other mechanisms are relevant to aerosols either in terms of deposition in the respiratory tract or sampling: - Centrifugal motion; - Electrical motion; - Thermophoretic particle motion. Centrifugal and electrical forces are more effective for larger particles.

* Action of forces between particle and surfaces: London-van der Waals (by molecular interactions); electrostatic attraction (charge differences); capillary forces (adsorption of water {or other liquid} film between …). Smaller particles are more difficult to dislodge than larger ones.

* For given exposure situation, amount of aerosolized material actually inhaled; fraction of inhaled aerosol depositing in different respiratory tract regions, and fate of deposited material are functions of: - Physical and chemical nature; - Exposure conditions; - Individual characteristics.

* Definition of “Breathing Zone”: *Nasopharyngeal (NP): hygroscopic; absorb water; humid; inertial impaction most significant. *Tracheobronchial (TB): conducting airways distribute inhaled air to deeper portions of lung; therefore, lower velocities and higher residence times favor sedimentation and diffusion. Thoracic fraction of < 10 um. *Pulmonary (P): depending on particle size, either sedimentation or diffusion is dominant mechanism. Respirable fraction.

* Sampling objective is to obtain information about aerosol properties at a given location over a specified length of time. Therefore, nature of air flow and particle motion both inside and outside of sampling device is critical issue about performance. Aerosol mass per unit air volume (mass concentration) based on size fractions and respiratory system penetration by inhalation.

* The intention of “total dust” sampling is that all particles in the air should be collected with equal efficiency without respect to any particular particle size fraction. By contrast, particle size-selective sampling is intended to separate the aerosol into size fractions based on health rationale. Exercise caution regarding sampler choice.

* *Size selective aerosol sampling. *External and internal sampling losses. *Sampler efficiency is a complex function of: sampler geometry; sampling rate; flow external to sampler; and sampler orientation with respect to direction of air flow. *Rudimentary sampler theory, but useful for sampler selection for application and assess losses and apply correction factors.

* Aerosol particle size greatly influences where respiratory tract deposition occurs, and site of deposition determines degree of exposure hazard. Sampling techniques to measure aerosol as: - inhalable, - thoracic, or - respirable fractions.

* - Study aerosol mass concentration; numbers; particle morphology; radioactivity; chemical content; and biohazard potential. Choice of media depends on aerosol characteristics and analytical technique used. Gravimetric analysis: mass/volume; mg/M 3. Open–face vs. Closed-face filter cassettes. “Total” particulates; under-sample inhalable fraction of larger particle sizes. Best is IOM sampler for inhalable fraction.

* - - Fiber filters: cellulose, glass, or quartz. Porous membrane: cellulose ester gels, PVC; high porous mesh microstructure with convoluted flow paths. Efficiency by pore size (i. e. 0. 1 to 10 um). Capillary membrane: PC or polyester film with direct pores of nearly uniform size and distribution. Polyester foam media Also treated filters; sorbent/filter combo.

* - Fiber: low pressure drop at high flow rates; high loading capacity; inexpensive; not adequate for submicron size; water! - Porous: also “depth” filters as above due to matrix deposition; higher flow resistance and lower loading capacity than fiber filters. - Capillary: high pressure drop and low loading capacity; susceptible to static charge build-up that can affect particle capture and retention; microscopy advantage [particles > pores are captured at smooth and flat surface to view].

* Cyclones use centrifugal forces for capture.

* Cut size indicates aerodynamic diameter of particle for which penetration through cyclone is 50% (d 50 at 4 um for respirable fraction). Efficient for large particle sizes and IH use as preseparators in respirable aerosol samplers: - Dorr-Oliver nylon – 1. 7 lpm - Casella and SKC cyclones – 1. 9 lpm/2. 5. Others: electrostatic or thermal precipitators.

* Widely used to characterize aerosols by size. Impactor performance by 50% cut point size as d 50, which is particle size captured by impactor with 50% efficiency. Single stage – DPM or PEM. Multi-stage – used in cascade configuration – cumulative mass distribution. Different analysis – gravimetric; chemical, etc. . Airborne Particulate Matter fractions – PM 2. 5 / 10. Liquid impingers for mists; particle counting.

* Measurement employs scattered light to characterize aerosol concentration and/or particle size distribution. Examples: count concentration; count particles and measure size; estimate aerosol mass conc. Miniaturized personal samplers vs. real-time aerosol measurements; datalogging, etc. . IH application of single-particle optical counters is Condensation Nucleus Counter (CNC) – range of 0. 005 to 1 um; component of respirator quantitative fit-test systems (i. e. TSI Porta-Count). Nephelometer

* Examination of the aerosol particles after deposition on a suitable substrate by microscopy: - Phase Contrast Microscopy (PCM) – fibers; look at count regarding concentration or size distribution; - Polarized Light Microscopy (PLM) – fiber ID; - Scanning Electron Microscopy (SEM); and, - Transmission Electron Microscopy (TEM). Particle size characteristics by method, and resolution power of microscope. “Representative” sample needed!

* Significant interest in ultrafine and nanometer size particulates. Engineered nano-particles (particles with at least one dimension less than 100 nm) may have designed physical, chemical, or biological properties. - Tapered-Element Oscillating Microbalance (TEOM) - Electrical Aerosol Detectors (EAD) - Denuder systems and diffusion batteries – research tools used to collect ultrafine or nano-particles between 1 m and 0. 1 um.

* Particle sizes are often approximately lognormally distributed; e. g. , logarithms of particles sizes follow Gaussian, or normal frequency distribution. Therefore, “statistics” include geometric mean (or median) size and geometric standard deviation (GSD). Distribution expressed using either the Count Median Diameter (CMD) and GSD or the Mass Median Aerodynamic Diameter (MMAD) and GSD, depending on how the measurement data were obtained.

* CMD is taken as the particle size corresponding to the 50% probability of occurrence, and the GSD is calculated as either ratio of CMD to 15. 75% particle size or ratio of 84. 13% particle size to the CMD (both give the same GSD value). If a single straight line cannot fit the data, then distribution is not lognormal, as for mixtures of aerosols from different sources (i. e. multimodal). Cascade impactors are examples of mass-based measurement instruments that characterize the mass fraction rather than count fraction in specified particle size intervals. MMAD>CMD.

* *Airborne Concentration *Air Volume *Unit Conversions – mg/M 3 to/from ppm *Temperature/Pressure Corrections *Statistics and Confidence Limits *Time-Weighted Averages *Potential Work Shift Adjustments NOTE: Corrections needed for comparisons to published occupational exposure limits.