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Presentation on theme: "INDUSTRIAL HYGIENE - SAMPLING AND SIZING OF AIRBORNE PARTICLES"— Presentation transcript:


2 INTRODUCTION Introduce the techniques available for Industrial Hygienists to evaluate exposures to particulates in occupational 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 and time of exposure, and other factors, etc.; so, - health effects – irritation, illness, disease.

3 AEROSOLS Aerosol – described as solid and/or liquid particles dispersed in a 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 Advances in aerosol science and inhalation toxicology have extended understanding and also promoted development of refined techniques for aerosol exposure characterization.

Techniques for sampling and analysis continue to evolve for characterization of aerosols. A single technique is not appropriate; IH must be familiar with properties and assessment techniques. Evolving ultrafine and nano-sized aerosols. Incidentally addresses bioaerosols or aerosols of biological origin.

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

6 DUSTS Particulate aerosols produced by mechanical processes such as breaking, grinding, and pulverizing. Examples: mining; material handing; dry material prep and packaging. Chemically unchanged; smaller size and higher specific surface area may enhance 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.

7 FUMES Fine solid aerosol particles produced from the re-condensation of vaporized material that is normally solid at standard conditions is melted and vaporized; condensation (0.01 um) occurs during cooling of vapor – nucleation; coagulation – agglomerates at 1 um in diameter are nearly spherical. Small enough to penetrate to deep lung areas; chemically quite reactive. Examples: welding/smelting – “metal fume fever”.

8 MISTS Spherical droplet aerosols produced from bulk liquid by mechanical processes such as splashing, bubbling, or spraying. Droplets are chemically unchanged from the parent liquid and range in size from a few microns to over 100 um. Mist aerosol from spray painting. Examples: mist aerosol from spray painting; crop spraying operations designed to produce mists; mist droplet aerosols by coughing or treatment of infected patients in health care settings.

9 FOGS Droplet aerosols produced by physical condensation from the vapor phase. Fog droplets are typically smaller than mechanically generated mist droplets, and are of the order of 1 to 10 um. Whereas mists may visibly settle toward the ground, fogs appear to remain suspended in the air.

10 SMOKES Complex mixtures of solid and liquid aerosol particles, gases, and vapors resulting from incomplete combustion of carbonaceous materials and are formed by complex combinations of physical nucleation-type mechanisms and chemical reactions. Examples: tobacco smokes; smokes from other combustion (i.e. plastics, synthetic fabrics, and petrochemical products – toxic). Primary smoke particles are on the order of 0.01 to 1 um in diameters; but like fumes, agglomerates containing many particles may be much larger.

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

12 AEROSOLS Aerosol concentrations in air are often assessed by mass per unit volume (mg/M3); when using mass, large particles have the most significant impact on total mass. mass = volume x density Other methods to assess aerosols include particle counting (mppcf) and total surface area. Can account for contribution of reactive surfaces and give consideration to smaller particles; good evaluation tools to assess risk of ultrafine and nano-materials.

Aerosol distribution – Figure 14.1 Monodisperse vs. Polydisperse Particle size distribution: log normal !!! Isometric – length dimension independent of particle orientation (e.g. dusts) Spherical – based on diameter Singlet – single discrete particles and remain Aggregate – coagulate or flocculate (i.e. soot); large surface are per unit mass Morphology – 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 by IH. Influenced by physical properties of aerosol (size, shape, density, and hygroscopicity [take up water].

15 TRANSPORT MECHANISMS - Sedimentation - Inertial Motion and Deposition
- Diffusion - Interception - Others: + Centrifugation (large); + Electrical (large); and, + Thermophoretic motions – (small); temperature gradient.

16 SEDIMENTATION Refers to movement of an aerosol particle through a 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 the 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 the diameter of a spherical particle of the same density that would exhibit the same behavior as the particle in question = Stokes diameter (dST). Aerodynamic equivalent diameter, dae, which is the diameter of a unit density sphere (density = 1 g/cm3) that would exhibit the same settling velocity as the particle in question. 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 a change in motion; important for human inhalation/deposition as well as aerosol sampling. Impaction on surface within distance traveled; likelihood increased with the mass and velocity of particle and the sharpness of change in direction. Stokes Number = St. (Equation 14-7) Inefficiency of impaction increases with increasing St.

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

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

23 DEPOSITION For a given exposure situation, the amount of aerosolized material actually inhaled; the fraction of inhaled aerosol depositing in the different regions of the respiratory tract, and the fate of the 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 is most significant. Tracheobronchial (TB): conducting airways distribute the inhaled air quickly and evenly 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 the dominant deposition mechanism. Respirable fraction.

25 SAMPLING THEORY 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 a critical issue regarding performance. Aerosol mass per unit air volume (mass concentration) based on size fractions and respiratory system penetration by inhalation.

26 SAMPLING THEORY 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 and insure that the particle size fraction of interest is properly collected.

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 IH related to sampler selection for application and assess losses and apply correction factors.

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

- Study aerosol mass concentration; number concentration; particle morphology; radioactivity; chemical content; and biohazard potential. - Choice of media depends on the aerosol characteristics and the analytical technique to be used. - Gravimetric analysis: mass/volume; mg/M3. - Open–face vs. Closed-face filter cassettes. - “Total” particulates; under-sample inhalable fraction of larger particle sizes. - Best is IOM sampler for inhalable fraction.

30 FILTRATION MEDIA - Fiber filters: cellulose, glass, or quartz.
- Porous membrane: gels of cellulose ester, PVC; high porous mesh microstructure leaving convoluted flow paths. Efficiency by pore size (i.e. 0.1 to 10 um) but can be misleading. - Capillary membrane: PC or polyester film with straight-through 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 deposition in matrix; 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 to effect particle capture. Cut size indicates aerodynamic diameter of particle for which penetration through the cyclone is 50% (d50 at 4 um for respirable fraction). Efficient for large particle sizes and IH use as pre-separators in respirable aerosol samplers: - Dorr-Oliver nylon – 1.7 lpm - Casella and SKC cyclones – 1.9 lpm. Others: electrostatic or thermal precipitators.

33 IMPACTION TECHNIQUES Among most widely used in aerosol characterization in relation to particle size. Impactor performance by 50% cut point size as d50, which is the particle size captured by the impactor with 50% efficiency. Single stage – DPM or PEM. Multi-stage – used in cascade configuration – cumulative mass distribution; Andersen or Marple. Different analysis – gravimetric; chemical, etc.. Airborne Particulate Matter fractions – PM 2.5 / 10. Liquid impingers for mists; particle counting.

34 OPTICAL TECHNIQUES Measurement employ scattered light to characterize the concentration and/or particle size distribution of aerosols. Examples: count concentration; count particles and individually measure size; estimate aerosol mass concentration of a cloud of aerosol particles. Miniaturized personal samplers vs. real-time aerosol measurements; datalogging, etc.. IH application of single-particle optical counters is the condensation nucleus counter (CNC) – range of 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 related to method; and also with respect to resolution power of microscope. Also a “representative” sample is necessary for analysis.

36 OTHER SAMPLERS Significant interest in ultrafine and nanometer sized 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 in an aerosol are often approximately lognormally distributed; that is , the logarithms of the particles sizes follow a 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. Graphical technique – Table 14.2; Figure 14.15

CMD is taken as the particle size corresponding to the 50% probability of occurrence, and the GSD is calculated as either the ratio of CMD to the 15.75% particle size or the ratio of the 84.13% particle size to the CMD (both will give the same GSD value). If a single straight line cannot be fitted to the data, then the 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.

39 CALCULATIONS Airborne Concentration Air Volume
Unit Conversions – mg/M3 to/from ppm Temperature/Pressure Corrections Time-Weighted Averages Potential Work Shift Adjustments

A range of temperature and pressure changes can be tolerated before corrections are applied to the volume or air sampled during an exposure assessment. All OELs and environmental exposure standards and limits are expressed at 25 degrees C and 1 atmosphere (760 mm Hg), defined as normal temperature and pressure (NTP). Therefore, corrections needed for meaningful comparisons related to published exposure limits.


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