# Antibiotic Resistance in Filters from Car Air Conditioning Systems
## 1 Introduction  
The spread of bacterial resistance to antibiotics and the associated health risks is a real and growing problem worldwide [1, 2, 3, 4]. This is increasingly the case in a variety of environments, not just medical ones [3, 4]. The possibility of the air-mediated transmission of agents responsible for antibiotic resistance has attracted attention in recent years. This phenomenon involves both resistance genes and resistant microorganisms, as confirmed by numerous research [3, 5, 6]. For example, [PERSON] et al. [6] detected significant amounts of ampicillin-resistant bacteria (accounting for more than half of all the culturable bacteria) and vancomycin-resistant bacteria (almost one-quarter of the culturable bacteria) in bioaerosols. On the other hand, [PERSON] et al. [7] found antibiotic resistance genes such as _me_A, _blaZ_, _ern_A, _ern_B_, _emC_ in strains of _Staphylococcus_ bacteria isolated from air samples. Means of transport are among those places where increased exposure of people to microbial agents of air pollution can occur [8, 9, 10, 11, 12]. A study by [PERSON] and [PERSON] [13] found the presence of bacteria in a car air conditioning system in numbers as high as 2.6 \(\times\) 10\({}^{3}\) CFU/m\({}^{3}\). [PERSON] et al. [12] observed that the fungal concentrations in vehicle air reached 8.4 \(\times\) 10\({}^{3}\) CFU/m\({}^{3}\) while bacteria concentrations accounted for about 1.7 \(\times\) 10\({}^{4}\) CFU/m\({}^{3}\). It should also be noted that more than half of the bacteria and fungi isolated from the vehicle's indoor air representthe respirable fraction, easily penetrating into the respiratory tract [14]. This is of particular importance in view of the fact that, according to various reports, we spend an average of 45-90 min per day inside vehicles [15; 16], and more than 4 years in a lifetime [17]. Nowadays, the vast majority of newly manufactured cars are equipped with an air conditioning system as standard [10; 18]. Despite the undoubted benefits of improved travel comfort, this type of installation can be a site of accumulation and secondary emission into the air of undesirable microflora, including potentially pathogenic microorganisms [10; 18; 19; 20; 21; 22]. The phenomenon of biofilm formation inside the installation can result in an increase in the level of air pollution entering the cabin [20]. On the other hand, the ability of microorganisms to form biofilms significantly facilitates the spread of drug resistance through gene transfer [1]. The space inside a passenger car is a very specific indoor environment, with its relatively small area, separated from the external environment most of the time. The contaminated air conditioning system, which is a source of emissions of various substances and microorganisms, in view of the small distances inside the cabin, increases the intensity of exposure of the driver and vehicle passengers to potentially dangerous agents [12; 18; 20; 21; 23]. The car cabin environment thus becomes a site for the potential spread of pathogenic microorganisms [24; 25]. Contaminants of various natures can accumulate in cabin filters [18], as well as inside ductwork in the system and in its individual components like air coolers [26; 27]. It has been found that with prolonged use of the filter (more than two years), the microbiological quality of the air inside the vehicle drastically decreases [12]. Studies involving analysis of the movement of PM\({}_{2.5}\) particles inside the vehicle have shown that the ventilation system induces themixing of air in the confined space of the cabin, resulting in similar exposure to pollutants, regardless of the place occupied in the vehicle [28]. The indoor air quality in vehicles is an outcome of the air conditioning operational parameters [29], intensity of use of the vehicle [30], the frequency and timeliness of filter replacement and disinfection of the air conditioning unit [10; 12; 19; 22; 31].