[Proposed microbiological indexes for the assessment of air quality in buildings]

An assessment of biological risk in the workplace requires the measurement of microorganisms present in the air. Although it is currently difficult to establish a dose-response relationship on the basis of existing epidemiological data, the number and types of microorganisms present offer a useful index when evaluating the quality of the air. Using the results obtained from a study of 226 different offices located in buildings equipped with forced ventilation we propose an assessment of contamination levels based on a Global Index of Microbial Contamination/m3 (GIMC/m3). This index is calculated as the sum of the values of the total microbial count determined for mesophilic bacteria, psychrophilic bacteria and fungi in all sampled areas. GIMC/m3 fluctuates between a minimum value of 23 and a maximum value of 18.069: the mean value varies depending on the ventilation system mode setting, with a value of 145 when operating in the heating mode, 265 in the air-conditioning mode and 258 under simple forced ventilation. 95.5% of the offices under study had a GIMC/m3 value of below 1000: this value is proposed as a threshold limit above which a more extensive assessment of contamination levels based on the measurement of further microbiological indexes should be carried out. To this aim we propose an Index of Mesophilic bacterial Contamination (IMC), obtained by calculating the ratio between the CFU/m3 value measured for mesophilic and psychrophilic bacteria in the same sampling point. The measurement of the Amplification Index (AI) completes the evaluation and this is determined by calculating the ratio between the GICM/m3 values measured inside the building and those measured outside. When considering these observations as a whole we are able to classify the various work areas under study into groups and categories according to the observed values of the proposed microbial contamination indexes.