A respiratory hazard is any airborne substance that, when inhaled, can cause an adverse health effect. These substances can damage the respiratory system directly or enter the bloodstream through the lungs and affect other organs. Hazards are classified based on their physical state and the way they are created, which determines the potential for harm and the necessary protective measures.
Gaseous and Vapor Hazards
Respiratory hazards existing at the molecular level are categorized as either gases or vapors. A true gas, such as carbon monoxide or nitrogen dioxide, remains in the gaseous state under normal temperature and pressure. These invisible hazards rapidly diffuse throughout an area, often making them undetectable by human senses alone. Vapors are the gaseous form of a substance that is typically a liquid or a solid at room temperature. Examples include fumes released by organic solvents or the gaseous phase of elemental mercury. The distinction is based on the substance’s physical state under ambient conditions. The potential for harm relates to their concentration and their ability to displace oxygen or directly poison cells. Lacking particulate mass, these hazards penetrate deep into the lungs. Many hazardous gases and vapors are colorless and odorless, providing no warning to an exposed individual.
Solid Particulate Hazards
Solid particulate hazards consist of fine solid matter suspended in the air, with their generation method defining their specific form.
Dusts and Fumes
Dusts are mechanically generated when bulk materials are broken down through processes like crushing, grinding, or sanding, producing substances such as silica dust or wood fibers. The potential for these hazards to cause lung damage often depends on their size, specifically whether they fall into the inhalable or the smaller respirable fraction. Fumes are created when a solid material is first vaporized, typically by high heat, and then rapidly cooled, causing the vapor to condense back into extremely fine solid particles. Welding fumes are a common example, where metal is heated and the resulting metallic oxide particles become airborne. These condensed solid particles are often much smaller than mechanically generated dusts.
Fibers and Particle Size
Fibers represent a distinct geometric form of solid particulate, characterized by an elongated shape with a specific length-to-width ratio. Asbestos is the most well-known example of a hazardous fiber, but fiberglass and certain synthetic mineral fibers also fall into this category. The physical shape of these fibers allows them to lodge in the lung tissue, where they can cause long-term irritation and scarring. The danger of solid particulates is fundamentally tied to particle size. Only very small particles, generally less than 10 micrometers in diameter, can reach the deeper, gas-exchange regions of the lungs. Larger particles are usually trapped in the upper respiratory tract and cleared by the body’s natural defense mechanisms.
Liquid and Aerosol Hazards
Liquid respiratory hazards exist as fine droplets suspended in the air, commonly categorized as “aerosol.”
Mists
Mists are generated either by the condensation of a liquid from a vapor phase or by the mechanical atomization of a bulk liquid. Examples include acid mists during metal plating or oil mists generated by high-speed machinery.
Sprays
Sprays are created by the mechanical action of breaking a liquid into droplets, often during processes like paint spraying or pesticide application.
The size of the droplets in both mists and sprays dictates how long they remain airborne and how deeply they can be inhaled. The toxicity of a liquid aerosol depends on the droplet size and the chemical composition of the liquid itself. An inhaled liquid droplet can deliver a concentrated dose of a corrosive or toxic substance directly to the sensitive lung tissue.
Biological Agents
Biological agents are a distinct class of respiratory hazard derived from living organisms, posing risks through infection or immune response. These agents include viruses, bacteria, fungi, and their byproducts, which become airborne and can be inhaled.
Transmission and Allergens
Viruses and bacteria are often transmitted via respiratory droplets or droplet nuclei expelled by an infected person. Fungi and mold spores are common biological particulates found in both indoor and outdoor environments. Inhaling these spores can trigger allergic reactions or cause fungal infections within the lungs. The presence of pet dander, which consists of tiny skin flakes, is another common biological hazard that acts as an allergen when inhaled.
Although biological hazards often present as particulates, they are classified separately due to their mechanism of action. Unlike inert dusts that cause physical damage, biological agents can replicate, infect the host, or provoke an immune system reaction. This distinction makes their assessment and control different from those for non-living chemical hazards.