Описание
The following terms of measurement are
commonly used in toxicological testing and in
industrial hygiene practice:
ppm: Parts of vapor or gas per million parts
of air by volume
mg/m3
: Milligrams of a substance per cubic
meter of air
mg/l: Milligrams of a substance per liter
of air
TOXICOLOGICAL CONCEPTS
Routes of Entry of Chemicals into
the Body
In the occupational setting, inhalation is the
most important route of entry of chemical
agents into the body, followed by contact with
skin and subsequent cutaneous absorption.
Although the gastrointestinal tract is a potential site of absorption, the ingestion of significant amounts of chemicals is rare in the
occupational setting.
Inhalation
The respiratory tract is exposed to chemicals in the inspired air. The two main factors
that determine the tissue responses to chemicals are the functional anatomy of the respiratory tract and the physicochemical nature of
the material.1–3
The respiratory tract may be divided into
three major regions: the nasopharyngeal (upper
airways), the tracheobronchial tree (lower
airways), and the pulmonary (alveoli).
The nasopharynx begins with the anterior
nares and extends down to the larynx. The
nasal passages are lined with vascular mucous
epithelium composed of ciliated epithelium
and scattered mucous glands. The nasopharynx
filters out large inhaled particles and is where
the relative humidity is increased and the temperature of the air is moderated.
The airways (trachea, bronchi, and bronchioles or tracheobronchial tree) serve as conducting airways between the nasopharynx and
alveoli. They are lined with ciliated epithelium
and coated with a thin layer of mucus secreted
primarily by goblet cells in the upper airways
and primarily by Clara cells at the bronchiolar
level. This mucous covering terminates at the
film covering the alveolar membrane. The
surface of the airways serves as a mucociliary
escalator, moving particles up to the oral cavity,
where they are swallowed and excreted or
expectorated.
The ciliated cells are most vulnerable to
damage. The most frequent degenerative
changes in these cells are loss of cilia, necrosis,
and sloughing of cells into the airway lumen.
Necrosis and desquamation of nonciliated and
secretory cells are less frequently observed.
After acute mild insult the nonciliated cells
proliferate and the epithelium regenerates to
normal. In the airways, nonciliated basal cells
are the main proliferating population. In the
bronchioles, the Clara cell is the main precursor cell for regeneration. Because of the delicate nature of the respiratory tract epithelium
and the close proximity of subepithelial blood
vessels, an inflammatory response occurs to all
but the mildest form of injury. Many lesions are
therefore diagnosed as rhinitis, tracheitis, and
bronchiolitis and qualified as acute, subacute,
and chronic depending on the stage of the
response.
If the insult persists, hyperplasia (cell proliferation) proceeds and leads to an abnormal
epithelium. Injury produced by chronic exposure to irritants such as SO2, NO2, O3,
formaldehyde, and tobacco smoke includes
undifferentiated basal cells (hyperplasia), squamous metaplasia, and goblet cell metaplasia. In
practice, many irritants produce responses
between mild and severe, and various combinations of degeneration, inflammation, and
proliferation may be observed.
The lower respiratory tract (pulmonary
region or alveolar ducts and sacs) is the area
where gas exchange occurs. Alveolar sacs, clusters of two or more alveoli, branch from alveolar ducts. It is generally considered that there
is a total of approximately 300 million alveoli
in the lungs of adult humans.4 The total alveolar surface area in the lungs of adult humans is
4 TOXICOLOGICAL CONCEPTS
Детали
- Год издания
- 2004
- Format