Samples of settled dust were collected from schools, factories and hospitals during twelve successive months for measuring the concentrations of viable microorganisms. High concentrations of aerobic, anaerobic bacteria and fungi were present in schools, mean number 216.06×103, 153.8×103and 122.56×103/g ofdust respectively followed by factories i.e. 186.55×103, 200.23×103and 97.76×103/g and then hospitals, i.e. 141.02×105, 74.*44×103and 85.24x103/g. The concentration of microorganisms was directly proportional to the atmospheric temperature and wind speed, but inversely proportional to the rainfall and relative humidity.