A method for sampling biological particles from a fluid flow, said method comprising the steps of: providing a impactor comprising: a sampling head comprising one or more intake apertures for sampling a fluid flow containing said biological particles; an impactor base operationally connected to receive at least a portion of said fluid flow from said sampling head; said impactor base comprising an impact surface positioned to receive at least a portion of said biological particles in said fluid flow and an outlet for exhausting said fluid flow, and said impactor base comprising a growth medium positioned to receive said particles in said fluid flow, wherein said impact surface is a receiving surface of said growth medium, and a transparent protective cover provided on said sampling head and covering said intake apertures; wherein said sampling head and said impactor base are integrated components that engage to enclose said impact surface, wherein at least a portion of said impactor base, and sampling head, each are optically transparent for enabling optically counting and characterizing particles received in the growth medium; and sterilizing said impactor in a fully assembled configuration by exposing the fully assembled impactor to radiation, wherein said impact surface remains enclosed by said sampling head and impactor base during sterilization, and wherein said growth medium is present within the impactor base and is sterilized by the sterilizing step, and said protective cover on said sampling head maintains a sterile environment for said growth medium prior to sampling said fluid flow containing particles; removing said protective cover on said sampling head; sampling said fluid flow with said impactor; repositioning said protective cover on said sampling head for covering said intake apertures, thereby sealing said growth medium after said sampling step; wherein particles in said fluid are received by said impactor surface; growing at least a portion of said biological particles received by said growth medium until said biological particles are visible by eye or detectable using an optical detector or imaging device; optically counting said growth particles using an optical particle counter; and optically characterizing at least a portion of said grown particles without disengaging said sampling head and said impactor base and wherein said optically characterization is by visualizing, optically detecting or imaging said particles; wherein said sterilizing step, growing step, counting step, and characterizing step are carried out without disengaging said sampling head and said impactor base. a sampling head comprising one or more intake apertures for sampling a fluid flow containing said biological particles; an impactor base operationally connected to receive at least a portion of said fluid flow from said sampling head; said impactor base comprising an impact surface positioned to receive at least a portion of said biological particles in said fluid flow and an outlet for exhausting said fluid flow, and said impactor base comprising a growth medium positioned to receive said particles in said fluid flow, wherein said impact surface is a receiving surface of said growth medium, and a transparent protective cover provided on said sampling head and covering said intake apertures; wherein said sampling head and said impactor base are integrated components that engage to enclose said impact surface, wherein at least a portion of said impactor base, and sampling head, each are optically transparent for enabling optically counting and characterizing particles received in the growth medium; and sterilizing said impactor in a fully assembled configuration by exposing the fully assembled impactor to radiation, wherein said impact surface remains enclosed by said sampling head and impactor base during sterilization, and wherein said growth medium is present within the impactor base and is sterilized by the sterilizing step, and said protective cover on said sampling head maintains a sterile environment for said growth medium prior to sampling said fluid flow containing particles; removing said protective cover on said sampling head; sampling said fluid flow with said impactor; repositioning said protective cover on said sampling head for covering said intake apertures, thereby sealing said growth medium after said sampling step; wherein particles in said fluid are received by said impactor surface; growing at least a portion of said biological particles received by said growth medium until said biological particles are visible by eye or detectable using an optical detector or imaging device; optically counting said growth particles using an optical particle counter; and optically characterizing at least a portion of said grown particles without disengaging said sampling head and said impactor base and wherein said optically characterization is by visualizing, optically detecting or imaging said particles; wherein said sterilizing step, growing step, counting step, and characterizing step are carried out without disengaging said sampling head and said impactor base.