Inhaled particles settle in the respiratory tract according to their diameters:

 Inhaled particles settle in the respiratory tract according to their diameters: • 5-30 micron particles are deposited in the nasopharyngeal region. • 1-5 micron particles are deposited in the tracheobronchial region. • Less than 1 micron particles are deposited in the alveolar region by diffusion and Brownian motion.

In general, most particles 5-10 microns in diameter are removed. However, certain small inorganic particles, settle into smaller regions of the lung and kill the cells which attempt to remove them. The result is fibrous lesions of the lung. 

Many chemicals used or produced in industry can produce acute or chronic diseases of the respiratory tract when they are inhaled (Table 5). The toxicants can be classified according to how they affect the respiratory tract. • Asphyxiants: gases that deprive the body tissues of oxygen • Simple asphyxiants are physiologically inert gases that at high concentrations displace air leading to suffocation. Examples: nitrogen, helium, methane, neon, argon. • Chemical asphyxiants are gases that prevent the tissues from getting enough oxygen. Examples: carbon monoxide and cyanide. Carbon monoxide binds to hemoglobin 200 times more readily than oxygen. Cyanide prevents the transfer of oxygen from blood to tissues by inhibiting the necessary transfer enzymes. • Irritants: chemicals that irritate the air passages. Constriction of the airways occurs and may lead to edema (liquid in the lungs) and infection. Examples: hydrogen fluoride, chlorine, hydrogen chloride, and ammonia. • Necrosis producers: Chemicals that result in cell death and edema. Examples: ozone and nitrogen dioxide. • Fibrosis producers: Chemicals that produce fibrotic tissue which, if massive, blocks airways and decreases lung capacity. Examples: silicates, asbestos, and beryllium. • Allergens: Chemicals that induce an allergic response characterized by bronchoconstriction and pulmonary disease. Examples: isocyanates and sulfur dioxide. • Carcinogens: Chemicals that are associated with lung cancer. Examples: cigarette smoke, coke oven emissions, asbestos, and arsenic. 

The skin consists of the epidermis (outer layer) and the dermis (inner layer). In the dermis are sweat glands and ducts, sebaceous glands, connective tissue, fat, hair follicles, and blood vessels. Hair follicles and sweat glands penetrate both the epidermis and dermis. Chemicals can penetrate through the sweat glands, sebaceous glands, or hair follicles.