140. Sulphuric, hydrochloric, nitric and phosphoric acids
Sulphuric, hydrochloric, nitric and phosphoric acids are common inorganic or mineral acids. The first three acids are strong, whereas phosphoric acid is weaker. They are all important industrial chemicals used in a variety of applications, e.g. in the manufacture of chemicals and metal or food products. The relatively nonvolatile sulphuric and phosphoric acids will occur in air primarily as aerosols and the more volatile hydrochloric and nitric acids as vapours or aerosols. Following absorption, the toxic effects of the acids will be mainly from protolysis yielding protons in the mucosa. The reaction between sulphuric acid and water generates heat. Except for sulphuric acid, the toxicological database is poor or very poor. The acids are corrosive and will cause chemical burns when in contact with eyes, skin and mucous membranes. Acid vapours and aerosols are respiratory tract irritants and may cause pulmonary impairment, as well as dental erosion, and laryngeal cancer. Sulphuric acid: The critical effects are alterations in bronchial mucociliary clearance, lung function effects and airway and eye irritation. The effects begin to appear at approximately 0.1 mg/m3 in humans. At slightly higher levels, dental erosion and pathological changes of the nasal mucosa have been reported. Cellular changes of the respiratory tract epithelium have been observed in animals after repeated exposures to concentrations in the range 0.125-0.38 mg/m3. An excess risk of laryngeal cancer has been found among workers exposed to strong inorganic acid mists containing H2SO4. The mechanism of laryngeal cancer from acid mist exposure seems to be secondary to the local airway irritation caused by the acid. Hydrochloric acid: The critical effect is airway irritation. No airway irritation at 2.5 mg/m3 was reported in asthmatics but mild irritation, which regressed rapidly, at 5 mg/m3 in workers. Tracheal and laryngeal hyperplasia observed in animals after chronic exposure to 14 mg/m3 is regarded secondary to airway irritation in analogy with sulphuric acid. Nitric acid: There is a general lack of data. In healthy volunteers, no effects on pulmonary function and inflammatory response were noted after a single exposure to 0.5 mg/m3 but defence functions of alveolar macrophages were affected at 0.2 mg/m3. The potency of nitric acid seems to be similar to that of sulphuric acid. Phosphoric acid: As data are lacking, the assessment has to be based on comparison with the stronger irritant phosphorous pentoxide, which is converted to the acid in the airways.
van der Hagen, Marianne
occupational exposure limit
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