An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be generated from natural or human causes. The scientific term aerosol refers to the mixture of particulates in gas, and not to the particulate matter alone. The liquid or solid particles in an aerosol have diameters typically less than 1 μm. Larger particles with a significant settling speed make the mixture a suspension, although the distinction is not clear.