An investigation into the types and numbers of airborne microorganisms was carried out in three varying types of residential houses in Al-Ain City, UAE, to establish future reference standards and to determine if they have affected, and to what degree they may affect, human health. Samples were collected using a microbial mechanical air sampler for the enumeration of bacterial and fungal colony forming units (CFU). Nine groups of bacteria and fungi either of human or environmental origin were detected. Environmental agents generally predominated while significantly higher counts were detected as the level of hygiene or standard of housing dropped. Significantly higher human associated microorganisms were detected in bedrooms while environmentally-related microorganisms were more commonly found in living rooms. Only small numbers of potential human pathogens were detected, Five genera of fungi, mainly members of the genus Aspergillus, were isolated from all houses across the board. Microbial occurrence and indoor air quality in the high social strata houses were similar to that reported in clean hospital environment rooms. It was concluded that, although the numbers and types of microbial population in domestic homes were high, they have little adverse effects on human health. (C) 1997 Elsevier Science Ltd.