Acceptable risk: The risk that is considered acceptable and allows work to proceed bearing in mind the expected benefit of the planned activities. Accident: An inadvertent occurrence that results in actual harm such as infection, illness, injury in humans or contamination of the environment. Aerosol: Liquid or solid particles suspended in air and of a size that may allow inhalation into the lower respiratory tract (usually less than 10 micrometres in diameter). Aerosol/airborne transmission: The spread of infection caused by the inhalation of aerosols. Aerosol-generating procedure: Any procedure that intentionally or inadvertently results in the creation of liquid or solid particles, which become suspended in the air (aerosols).