The lungs and airways are in constant contact with the outside world and are especially vulnerable to toxic substances present in the environment. Within seconds of exposure to an inhaled toxin, pathologic events occur that may cause immediate distress, produce illness lasting days, or even lead to the development of chronic lung disease. In this chapter we discuss the pathology and pathophysiology that can result from various inhaled toxins and highlight the role of several common and medically significant toxic inhalants that are known to cause acute and chronic pathophysiologic responses in the lung. We also describe several systemic syndromes caused by acute toxic inhalations. The scope of this chapter does not include chronic exposure to low levels of toxins.
DETERMINANTS AND MECHANISMS OF IRRITANT-INDUCED PULMONARY INJURY
Inhaled toxins exist in many forms and may be categorized by considering their physical properties. General categories include gases, vapors, fumes, aerosols, and smoke. A variety of factors determine the pathologic results of a toxic inhalation: the size of inhaled particles, the water solubility of the inhaled substance, the concentration of the inhalant in ambient air, the duration of exposure, the presence or absence of ventilation, and relevant host factors (age, smoking status, comorbid diseases, use of respiratory protection, and perhaps even genetic susceptibility). While toxic inhalants provoke a broad range of chemical and biologic activities that contribute to pathogenesis, their physical properties, namely, their particle size and water solubility, are of fundamental importance in determining the site and severity of pulmonary injury. Tables 90-1, 90-2, 90-3 and Fig. 90-1 summarize the physical properties of the discussed inhalants that substantially affect the resulting pathogenesis of these agents.
TABLE 90-1Definitions of Types of Inhaled Substances ||Download (.pdf) TABLE 90-1 Definitions of Types of Inhaled Substances
|Gas: a formless state of matter in which molecules move freely about and completely occupy the space of enclosure
|Aerosol: a relatively stable suspension of liquid droplets or solid particles in a gaseous medium
|Coarse particles: particles between 1 and 10 µm
|Fine particles: particles between 0.1 and 1 µm
|Ultrafine particles: particles less than 0.1 µm
|Vapor: the gaseous form of a substance that normally exists as a liquid or solid and that generally can be changed back to a liquid or solid by either increasing ambient pressure or decreasing the temperature
|Fume: an aerosol of solid particles generally less than 0.1 µm in size that arises from a chemical reaction or condensation of vapors, usually after volatilization from molten materials
|Smoke: the volatilized gaseous and particulate products of combustion; particles generally less than 0.5 µm in size and do not settle readily
TABLE 90-2Water Solubility and Mechanisms of Lung Injury of Gaseous Respiratory Irritants