Chemical Contaminants: Classification and Impact
Classification of Chemical Contaminants by Presentation
Aerosol
An aerosol is a dispersion of solid or liquid particles smaller than 100 μm in a gaseous medium. Aerosols exhibit a range of physical states:
- Powder (Dust): Airborne solid particles from physical disintegration processes, ranging from 0.1 to 25 μm in size.
- Mist: Airborne liquid droplets generated by condensation from a gaseous state or by liquid spray disintegration, ranging from 0.01 to 10 μm.
- Fog: Visible airborne liquid droplets caused by condensation from a gaseous state, sized between 2 and 60 μm.
- Smoke: Airborne solid particles from incomplete combustion, smaller than 0.1 μm.
- Metallic Smoke (Fume): Airborne solid metal particles generated by condensation from a gaseous state (sublimation or volatilization), often involving oxidation. Size is similar to smoke.
Gas
The normal physical state of matter above 25°C. Gases are amorphous fluids that occupy their container and change physical state with pressure and temperature. They consist of molecule-sized, distributable particles.
Vapor
The gas phase of a solid or liquid substance under normal conditions.
Classification of Chemical Contaminants by Impact
- Irritants: Cause inflammation through chemical action, primarily affecting the respiratory system’s mucous membranes.
- Pneumoconiotic Agents: Solid chemicals that deposit and accumulate in the lungs, causing fibrosis.
- Systemic Toxins: Compounds that, regardless of entry route, distribute throughout the body, affecting specific organs or systems.
- Anesthetics and Narcotics: Central nervous system depressants.
- Carcinogens: Substances that can cause or promote disordered cell growth.
- Allergens: Substances that affect only sensitized individuals.
- Asphyxiants: Substances that prevent oxygen flow to tissues.
- Dermatosis-Producing Agents: Substances that cause skin changes like primary irritation, allergic sensitization, or photo-sensitization.
Properties of Particles
Particle size is a crucial property of particulate aerosols, determining their ability to reach lung alveoli and cause damage. Size depends mainly on the aerosol formation process.
Dust
Dust refers to solid particles of any size or kind, suspended or capable of suspension in the air.
Features
Dust encompasses all systems of solid particles dispersed in a gaseous medium. These systems are called dispersed systems and consist of the dispersed phase (particles) and the dispersion medium (air). Polydisperse aerosols have a range of particle sizes, while monodisperse aerosols have similar-sized particles. When particles are suspended in air, the mixture is known as an aerosol. When particles deposit and the gas phase proportion is small, it’s called an aerogel.
Aerosols change in composition over time due to gravity, Brownian motion, and particle agglutination. Brownian motion is the erratic movement of fine particles caused by molecular collisions. This can lead to particle coagulation. Other factors influencing coagulation include particle size, homogeneity, thermophoresis, and electrical charges.
Thermophoresis is the effect of temperature differences between a gas and a surface on Brownian motion. Electrically charged particles, often due to friction, can influence coagulation. Like charges repel, hindering coagulation, while opposite charges attract, favoring it.
Dynamic Behavior of Suspended Particles
Suspended dust particles are subject to gravity, causing them to fall. Air resistance opposes this fall, eventually leading to a uniform terminal velocity.
Generation and Dispersion
Dust generation is the process by which particles are suspended in air from a state of rest. Dust clouds form through primary generation (mechanical or pneumatic projection of particles) and secondary dispersion (air currents carrying the dust).
Classification
Industrial dust can be classified by size, shape, composition, and effects.
Size
- Sedimentary: Deposits quickly (10-15 microns).
- Inhalable: Can enter the respiratory system (<10 microns).
- Respirable: Can enter the lungs (<5 microns).
- Visible: Distinguishable to the naked eye (>40 microns).
Shape
- Dust: Solid particles in suspension, other than fibers.
- Fibers: Particles longer than 5 microns, with a cross-sectional diameter smaller than 3 microns and a length-to-width ratio greater than 3.
Composition
- Animal: Feather, fur, leather, bone.
- Plant: Pollen, corn, straw, snuff, hemp.
- Mineral: Metals, asbestos, etc.
Effects
- Pneumoconiotic Dust: Causes irreversible lung changes (e.g., silicosis).
- Toxic Dust: Has a primary toxic effect (e.g., lead oxide poisoning).
- Carcinogenic Dust: Can cause malignant tumors (e.g., asbestos, chromic acid).
- Inert Dust: Causes discomfort and mild respiratory disease.
Pneumoconiosis
“The accumulation of dust in the lungs and tissue reactions to its presence.” Two types exist:
- Collagenous: Fibrous tissue production and permanent alveolar damage (e.g., silicosis, asbestosis).
- Non-Collagenous: Reticulin reaction without alveolar damage (e.g., pneumoconiosis from radiopaque metals).
Silicosis
A disease caused by free silica (SiO2), characterized by nodular fibrosis and impaired lung function.
Agents
Free crystalline silica (SiO2). Distinct from total silica, where silicon is bound to other elements.
Forms
- Crystalline: Quartz (most common), tridymite, cristobalite.
- Cryptocrystalline: Chalcedony, opal, tripoli, flint.
- Amorphous: Diatomaceous earth.
Silicatosis
Natural silicates are minerals composed of SiO2 combined with other elements. They are classified as:
- Fibrous: Asbestos, talc, sericite, sillimanite.
- Non-Fibrous: Kaolin, bentonite, micas.
Asbestosis
A diffuse collagenous pneumoconiosis caused by inhaling asbestos.
Talcosis
A diffuse collagenous pneumoconiosis caused by inhaling talc (hydrated magnesium silicate). Often associated with quartz, magnesium, dolomite, calcite, anthophyllite, and tremolite.