"The air quality in many cities has been deteriorating and affecting population’s health. Airborne particles can block the defense mechanisms of respiratory system and may lead biological elements such as pollen, bacteria, fungi and viruses into the lungs. This paper presents the study of biologically contaminated air in the city of Hermosillo, analyzing filters containing particulate matter with a diameter equal or less than 10 micrometers (PM10) and Total Suspended Particles (TSP). Three sampling periods were analyzed during 2011. Samples taken from the filters were placed in tubes containing nutritive broth BHI (Brain Heart Infusion) and incubated at 37 ˚C for 24 hours. The identification of the isolated microorganisms was done by a biochemical test recommended in Bergey’s Manual. Results showed a high bacteria concentration in the presence of microorganisms such as Klebsiella pneumoniae sub pneumoniae, Citrobacter, Enterococcus faecalis, Proteus penneri, Butiauxellaagrestis, Salmonella paratyphi A and Enterobacter sacazakii. When bacterial concentration was analyzed separately, the lowest number of bacteria was observed during February for the south location and in August for the north location. The highest degree of bacterial air pollution was found in the central area of the city."