The drift contains Respirable suspended particulate matter. Cutting these materials produces airborne dust containing very fine respirable crystalline silica (RCS) particles. The respirable fraction of lunar dusts may be toxic to humans. Silicosis is an often-fatal lung disease caused by the exposure to respirable silica dust. Exposure to higher concentrations of respirable flock particles is associated with more severe disease. Water spray application is also an effective method for reducing respirable dust concentrations. Venturi scrubbers achieve very high collection efficiencies for respirable dust. Owing to their size, UFPs are considered to be respirable particles. The gaseous oxide of azote (the laughing gas) is perfectly respirable when pure. A second type of respirator protects users by providing clean, respirable air from another source. Smaller fractions, penetrating into the non-cartilage respiratory tract, are defined as respirable dust. These scrubbers have a good collection efficiency for respirable dust. Respirable dust can cause long-term lung problems. The respirable fraction is what is deposited in the gas exchange regions (alveoli). He concluded in his 1778 paper "General Considerations on Acids" that respirable air constitutes acidity.