Environmental Engineering Reference
In-Depth Information
from person to person. Humans release particles from the larynx, mouth, and throat during normal
breathing, speaking, singing, and expectorating, but sneezing and coughing expel droplets with
greater eficiency. Most of the droplets from a sneeze originate in the mouth and do not carry infec-
tious agents. However, an increase of nasal secretions due to an upper respiratory tract infection
can trigger sneezing and explosive expulsion of infectious particles from that region. Coughing is
an eficient means of transmitting viruses and bacteria if an infection causes an increase of mucus
secretions that, in turn, induce the cough relex. A sneeze has been estimated to generate millions
of aerosol particles and a cough many thousands. 42 The fate of these particles depends on ambient
environmental conditions (e.g., humidity and air movement) and particle size. Inluenza viral RNA
has been detected in exhaled human breath (generation rate: <3.2-20 particles/min; 87% of par-
ticles <1 μm) in some but not all studies. 43,44
Droplet contact involves large particles (>5 μm) that humans release from the nasopharyngeal
region or that contaminated bodies of water generate through sprays or splashes. 45 Laboratory,
surgical, and dental procedures can also produce aerosols of blood, saliva, enamel, dentin, bone,
and tissue, some of which contain infectious agents. 46 Particles ranging from 0.3 to 20 μm have
been measured from surgical procedures, oral operations, laser smoke, and dermabrasions, their
concentration and size being dependent on the aerosol generation method. 47 After water evapo-
rates, the dried residue of a droplet is called a droplet nucleus. Infectious aerosols are readily
transmitted where people congregate, for example, in transportation vehicles and day care cen-
ters, schools, health clinics, correctional facilities, dormitories, military barracks, and shelters for
the homeless.
12.2.1.2  Infectious Dose
Respiratory tract infections are not equally communicable. 48 The outcome of a human exposure to
an infectious agent depends on the number of microorganisms encountered, the virulence of the
agent, the strength of the body's defenses, and the ability of the pathogen to avoid or interfere with
these defenses. 49 Infectivity and virulence can be expressed in quantitative terms. The 50% infec-
tious dose (ID 50 ) is estimated from the number of microorganisms needed to initiate infection in
half of the exposed subjects. The virulence of an infectious agent can be measured as the 50% lethal
dose (LD 50 ) (the number of microorganisms or amount of toxin required to kill half of the infected
animals). Endpoints other than half also may be considered. In an outbreak investigation, the infec-
tivity of a particular strain of an agent can be determined from the observed attack rate, that is,
the number of cases of clinically apparent disease divided by the number of susceptible persons.
Host defenses that determine the severity of an encounter with an infectious agent include muco-
sal immune factors, which may block adherence and local proliferation, and humoral or cellular
immune responses, which may contain proliferation and cell invasion, resulting in an asymptomatic
infection rather than a more serious illness. 48
An early study demonstrated the relationship between infectious dose, particle size, and pulmo-
nary deposition site. 50 This group showed that the number of 2-3 μm droplet nuclei of M. tubercu-
losis to which rabbits were exposed approximately equaled the number of infectious foci (tubercles)
that developed in their lungs. In contrast, only 6% of 13-μm particles caused tubercles because few
particles of this size reach the deep lung and the bacterium is innocuous in the upper respiratory
tract.
It is dificult to measure the number of infectious particles to which people are exposed and
determine the number of viable organisms in each infectious particle; therefore, Wells introduced
the idea of quantal infection. 51 He noted that the number of persons who become infected bears a
Poisson relationship to the number of infective particles that they breathe. Approximately 63% of
persons homogeneously exposed to an airborne agent will be infected when, on average, each of
them has breathed one infective particle or “quantum.” The infectivity of the organism in combina-
tion with the strength of the host's defenses determines the number of organisms required to induce
a quantal response.
Search WWH ::




Custom Search