Introduction:
Kraków is one of the most beautiful but also one of the most crowded Polish cities with large numbers of cars, pedestrians and cyclists travelling each day. There has been an increasing concern about the human exposure to bioaerosols, which can occur, among others, at the sites characterized by increased dustiness, such crowded streets. The aim of this study was to evaluate the microbiological quality of air in the selected sites situated by the main roads in Kraków.

Material and Methods:
Air samples were collected in 10 sites located by per year. Four microbial groups were enumerated: mesophilic bacteria, fungi, actinomycetes and staphylococci. The results were expressed as colony forming units perm³ of air and compared with Polish Standards concerning microbiological air quality.

Results:
The greatest bacterial and fungal aerosol concentration was observed in autumn, when atmospheric conditions could have promoted abundance of those microorganisms. In general, fungi were the most numerous group of airborne microorganisms, while staphylococci were the least numerous. The number of actinomycetes was alarmingly high in all sites. It was found that seasonal differences in the bioaerosol concentration were statistically significant and the recorded differences could have been affected by atmospheric conditions.

Conclusions:
The conducted studies showed that in terms of airborne bacteria and fungi, the air in the examined locations was microbiologically unpolluted. However, high numbers of airborne actinomycetes were observed in all sites, which can cause adverse health effects in pedestrians or cyclists who are frequently exposed to bioaerosols by the main routes in Kraków. Statistically significant variability was found in the prevalence of the examined microorganisms in different seasons of the year.