Inhalation of crystalline silica is harmful to the lungs, causing silicosis. Amorphous silica is considered to have low toxicity, but prolonged inhalation causes changes to the lungs. Diatomaceous earth is mostly amorphous silica, but contains some crystalline silica, especially in the saltwater forms. In a study of workers, those exposed to natural D.E. for over 5 years had no significant lung changes, while 40% of those exposed to the calcined form had developed pneumoconiosis.Today's common D.E. formulations are safer to use as they are predominantly made up of amorphous silica and contain little or no crystalline silica. The crystalline silica content of D.E. is regulated in the United States by the Occupational Safety and Health Administration (OSHA), and there are guidelines from the National Institute for Occupational Safety and Health setting maximum amounts allowable in the product (1%) and in the air near the breathing zone of workers, with a recommended exposure limit at 6 mg/m over an 8-hour workday. OSHA has set a permissible exposure limit for diatomaceous earth as 20 mppcf (80 mg/m/%SiO2). At levels of 3000 mg/m, diatomaceous earth is immediately dangerous to life and health. In the 1930s, long-term occupational exposure among workers in the cristobalite D.E. industry who were exposed to high levels of airborne crystalline silica over decades were found to have an increased risk of silicosis. Today, workers are required to use respiratory-protection measures when concentrations of silica exceed allowable levels. Diatomite produced for pool filters is treated with high heat (calcination) and a fluxing agent (soda ash), causing the formerly harmless amorphous silicon dioxide to assume its crystalline form.
