Abstract The airborne transmissions of pathogens, non-pathogenic organisms, fragments of microbial cells, and byproducts of microbial metabolism, collectively referred as “bioaerosols” represent a potential risk to human health upon inhalation in both outdoor and indoor environments. Outdoors, microbes may be derived from a variety of sources including soils, vegetation and animals, whilst indoors sources include infected individuals who emit pathogenic microbes via coughing, sneezing and perspiration. Normal activities of healthy individuals can also contribute to the airborne microbe level via exhalation and skin cell degeneration. Furniture, carpets, food debris and pets are also important sources of microbes. The indoor levels of airborne microbes is a function of occupant density, duration of room occupation, presence of any infected individuals, and the degree of ventilation and air-exchange with the outdoor environment. The ventilation system itself may also serve as a source of airborne microbes in the case if it is not properly maintained.
The aim of this study was to investigate the quantity, size distribution and species of airborne microbes inside a school classroom, as well as in a hot, desert outdoor environment, in Doha, Qatar. Air samples were collected simultaneously indoors and outdoors during several sampling periods i.e. summer, autumn, winter and spring. An Anderson six-stage impactor was used to collect airborne microbes on nutrient agar (NA) plates by 30 l/min flowrate, over a 5 minutes period, i.e. total 150 liters of air collected per sample. Air samples were collected in triplicate both indoors and outdoors prior to commencement of class at 6:30 AM, and immediately after end of classes at 2:30 PM. After sampling, NA plates were then incubated for 24 hours at 37oC. After incubation the number of colony forming units (CFU) of microbes on the NA plates were counted, and the average of triplicate samples for the morning and afternoon samples were calculated as the mean number of airborne microbes per cubic meter of air (CFU/m3).
The prevalence and concentration of microbes varied spatially (indoors versus outdoors), temporally (morning versus afternoon), and seasonally (winter versus summer). The concentrations of airborne bacteria in classroom air were lowest in the morning (a maximum 204 CFU/m3 in summer, and a maximum 35 CFU/m3 in winter) than in the afternoon after class (a maximum 348 CFU/m3 in summer, and a maximum 543 CFU/m3 in winter). As the classroom was continuously occupied between the morning and afternoon, then human influence is the most likely factor affecting prevailing CFU levels. Other factors likely affecting CFU levels may include: airflow dynamics and exchange between the indoor and outdoor environment; the influence of the classroom air-conditioning and ventilation system (positive or negative) and; the presence of non-human bacterial sources in the classroom (food debris, carpeting, soft furnishings etc.).
Outdoor air samples showed the opposite profile to the classroom, where morning samples had higher numbers of airborne microbes in the morning (a maximum 640 CFU/m3 in summer, and a maximum 111 CFU/m3 in winter) compared to the afternoon samples (a maximum 299 CFU/m3 in summer, and a maximum 82 CFU/m3 in winter). This contrast supports the indication that airborne bacteria in the classroom are derived from internal sources rather than as a result of infiltration of bacteria from outside.