There are many different infectious bioaerosols and infection control and prevention is becoming increasingly important.
A bioaerosol is an assembly of particles of variable biological origin suspended in a gaseous medium (e.g. air) long enough to enable observation and measurement. Typically, a bioaerosol is an aerosol of bacterial, viral, or fungal origin capable of initiating an infectious process in a susceptible host. This aerosol usually consists of a mixture of mono-dispersed and aggregate cells, spores, or viruses, carried by other materials, such as respiratory secretions and/or inert particles. With rapid desiccation, the resultant smaller aerosols can remain airborne longer, while larger aerosols may initially fall out and then become re-suspended after desiccation.
Respiratory disease agents are expelled from the respiratory tract within a matrix of mucus and other secretions that typically begin to desiccate upon expulsion by coughing, sneezing or even talking. The dried residuals of these large aerosols are termed droplet nuclei (0.5-12 μm). For instance, a sneeze can generate as many as 40,000 droplets, most of which can evaporate to droplet nuclei.
Infectious bioaerosols can also be generated from wet environmental sources (such as cooling tower water with Legionella) or from dry sources (such as construction dusts with Aspergillus fumigatus spores, or indoor dusts with Hantavirus).
From a general point of view, the transmission of infectious disease by the airborne route is dependent upon the interplay of several critical aerosol factors:
• The particle size and shape (aerodynamic diameter).
• The formation of droplet nuclei, the effective aerial transport (subject to gravitational, diffusional, thermal, and electrostatic forces) and finally, the landing or deposition in a new host.
• The infective dose, i.e. a sufficient numbers of pathogens must survive to aerosolization, relative humidity, temperature, oxygen toxicity, ultraviolet and other radiation to constitute an appropriate infective dose.
• Microbe virulence.
• Genetically based, disease-promoting factors that enable an agent to overcome normal physical and immunologic defenses (e.g. SARS, smallpox viruses).
• Host susceptibility.
• A slow, weak, or non-existent immune function (due to immunodeficiency disease, or immunosuppression as a result of chemotherapy, transplantation, pregnancy, or lack of appropriate and available vaccination).