【Reference】 Compared with water and soil, air is an unfavorable environment for microorganisms to live in. Air lacks nutrients; dry air in particular causes microorganisms to lose moisture, and the ultraviolet rays of the sun also have a bactericidal effect. Only in enclosed spaces such as rooms do factors such as poor indoor ventilation and the activities of humans and other organisms raise the indoor microbial concentration, and pathogenic microorganisms may appear.

【Reference】 Because different methods of collecting bacteria yield different bacterial concentrations, there are usually the following three methods:

(1) For example, if a culture dish with a diameter of 90mm is left standing in the room for 30min, the airborne bacteria in the air will settle into the dish from time to time; the number of settled colonies per dish obtained after culturing is called the bacterial concentration by the depositing method, or simply the depositing bacterial concentration, with the unit of count/(φ9cm dish·30min) or CFU/(φ9cm dish·30min).

(2) For example, if a slit-type bacterial sampler is used for suction sampling in the room, a certain amount of air is drawn through the slit and impacts the culture medium at high speed, so that the airborne bacteria in the air are captured; the number of settled colonies per dish is then obtained after culturing. The per unit volume derived from this The number of planktonic bacteria in the air is called the bacterial concentration by the airborne method, or simply the airborne bacterial concentration, in units of count/m³ or CFU/m³.

(3) If a cotton swab immersed in a sterile physiological saline sampling solution, or another specified object, is used to wipe, by a specific method, the surface to be tested over a specified surface area, the number of colonies obtained after culturing as required is called the surface bacterial contamination density, in units of count/cm² or CFU/cm².