The aim of this study is to determine the concentration of fungi and bacteria in the air inside the car cabin and, on this basis, to determine the air quality in the passenger car cabin. The aim of the work is also to demonstrate the impact of hygienic maintenance of filtering devices on the quality of indoor air. The subjects of the research are car cabins, as an example of small, enclosed spaces in which people may remain. The tests were carried out in the summer of 2020 in three passenger cars. Based on the conducted research, it was found that the operation time of regularly serviced air conditioning has a positive effect on the air quality in small, confined spaces. The vehicle with the longest-used filter was characterised by the highest concentration of fungi (8369 CFU/m3) and bacteria (16563 CFU/m3) in the environment inside the car cabin, which means that periodic replacement of the filters in the car's ventilation system is very important. In Poland, it is recommended that such a filter be replaced after a year or after driving 10,000–15,000 kilometres. In analysing the state of air quality in the examined confined spaces, it can be concluded that by ensuring regular replacement of cabin filters and air conditioning servicing, we have a very large impact on indoor air quality.