Silicosis

Excerpt

Silica, or silicon dioxide (SiO₂), comprises a silicon atom and 2 oxygen atoms. Silica is the most plentiful mineral on Earth, distributed throughout the Earth's crust as silicate minerals. Silica is the main constituent of more than 95% of rocks and stones.

Occupational exposure to silica dust is one of the oldest known forms of lung disease, first documented by Hippocrates in 400 BC describing breathing disorders in metal diggers. In the 1600s, van Diemerbrock noted the harmful effects of inhaling granite dust. Ramazzini provided a detailed description in the 1700s of the lung scarring observed in stone and coal workers. During the industrial revolution, advancing mechanization of production techniques increased workers' risk of contact with higher concentrations of silica dust. In 1832, it was noted that sandstone workers had significantly shorter life expectancies compared to limestone and brick workers, often dying by the age of 40. The Hawks Nest Tunnel disaster of the 1930s was one of the worst industrial disasters in American history. To create a large tunnel that would redirect a river to power a hydroelectric plant in West Virginia, workers drilled through rock in a confined space without ventilation or respiratory protection. Many developed disabling lung damage within a few months due to exposure to high silica concentrations.

The burden of silica-associated disease remains high, and silicosis is currently the world's most prevalent chronic occupational disease. Many industries place workers at risk of inhaling silica-containing dust, including abrasive blasting, denim jean manufacture, artificial stone production, brickwork, pottery, porcelain work, tunneling, mining, and structural construction and demolition.

Silica exists in amorphous and crystalline forms that have unique physiochemical properties. Amorphic silica is found in biological matter such as plants, sponges, algae, fungi, and bacteria and nonvegetative matter such as volcanic glass, colloidal sols, and powders. Amorphic silica is not considered toxic. Crystalline silica is a mineral commonly known as quartz, present in granite and sandstone. In contrast to the random orientation of molecules in amorphous silica, crystalline silica exhibits a fixed, repeating, polymerized silicon-oxygen tetrahedral framework. Crystalline silica exists in various forms, including α-quartz, β-quartz, α-tridymite, β-tridymite, α-cristobalite, β-cristobalite, keatite, coesite, stishovite, and moganite. α-Quartz is the most common crystalline silica and the most biologically toxic form.

Particles of crystalline silica are considered biologically active when small enough in diameter to reach the distal bronchioles and alveoli of the lungs. Such particles are denoted as "respirable crystalline silica" (RCS) and are less than 5 μm in diameter. RCS is released into the atmosphere by crushing, grinding, or cutting silica-containing matter. RCS can easily go unnoticed as it is odorless, colorless, and nonirritating. In 1997, the International Agency for Research on Cancer labeled RCS as a human carcinogen.

Silica also exists as "silica nanoparticles" (SiNPs), particles of nanoform silicon dioxide that measure less than 100 nm in diameter. SiNPs have physiochemical characteristics distinct from the bulk form of silica. SiNPs are colloidal metal oxides comprising a siloxane (Si-O-Si-O) core with abundant silanol (Si-OH) surface groups. SiNPs are either mesoporous or solid. The silica matrix of mesoporous SiNPs has many channels and pores, whereas solid SiNPs have no pores. Mesoporous SiNPs have a controllable, uniform pore size, large volume-to-surface ratio, easily modifiable surface chemistry, and exceptional biocompatibility. The silica framework of SiNPs yields many advantageous qualities, such as its stability in acidic conditions, temperature fluctuations, and organic solvents. The pores of mesoporous SiNPs can be filled with various biomolecules.

Silicosis, a type of pneumoconiosis, occurs secondary to the inhalation of RCS and causes progressive, irreversible, and fatal lung inflammation and fibrosis. While the condition is preventable, no treatment exists. Silicosis increases susceptibility to Mycobacterial diseases, autoimmune diseases, and bronchogenic carcinoma. This activity reviews the etiology, epidemiology, pathophysiology, clinical findings, evaluation, and management of patients with silicosis.