PRINCIPLES OF TOXICOLOGY

170 PULMONOTOXICITY: TOXIC EFFECTS IN THE LUNG
Inhaled air is highly “ conditioned” before it leaves the upper airway system. Relatively cold air,
for instance, will be warmed to body temperature (37 °C) before it reaches the lung. In like manner,
air that is at an elevated temperature will be cooled to body temperature within the nasopharyngeal
system. The lung and the portion of the respiratory system below the upper airway is a very moist
physiological system and is quite sensitive to humidity. The inhaled air is therefore highly humidified
during its passage from the nares to the lung, and the air that enters the nares is cleared of the larger
particles. The nose hairs function to some extent in this process, and the turbulent nature of the air
passages in the nares also contributes to the deposition of the larger particles, preventing them from
being inhaled into the lower passages of the respiratory system.
The lining of the nasal wall is known as the mucosa and is highly inundated with blood capillaries.
Therefore, air that is inhaled through the nose comes immediately into contact with mucosal surfaces,
which only thinly separate the air from these blood vessels. Deposition of toxic chemicals in the upper
airway system can therefore result in both toxicity to the mucosal tissue and absorption of the agent
into the systemic circulation by way of these capillaries.
Sinus Cavities
There are four pairs of hollow cavities within the skull that are lined with a mucosal lining that is
similar to the lining of the nasopharyngeal region. In order to view these sinuses from different angles,
Figure 9.1 shows a frontal view of the skull, while Figure 9.2 represents a sagittal view. Since the
sinuses are connected to the nasopharyngeal airways through a number of small openings, inhaled air
also enters the sinuses. Acute sinusitis can occur when inhaled airborne toxins irritate the surfaces of
sinus mucosa. As in other parts of the respiratory system, irritation of the mucosal lining leads to an
inflammatory response in these tissues. As a result of the inflammation, there is an accumulation of
Figure 9.1 Frontal view of the skull, showing frontal, maxillary, and ethmoid sinuses. (Reproduced with
permission from W. O. Fenn and H. Rahn, Handbook of Physiology. American Physiology Society, Washington,
DC, 1964.)