## 4 Discussion  
In this study, we applied a combination of methods to assess the quality of air in two horse stables in southern Poland (Krakow and Tarnow). The highest values of all PM fractions, as well as inhalable and respirable fraction of particulate matter, were observed during the activities carried out in the stables. Our observations are similar to those made by [PERSON] et al. [19], who also recorded considerable differences in the concentration of both fine and coarse fractions of particulate matter depending on the activities inside the examined horse stable. However, the values observed in our study are much lower than those reported by [PERSON] et al. [19], who observed the concentrations of organic dust within the ranges from 400 to 800 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\), or those by [PERSON] et al. [7], who observed the PM concentrations reaching 117-368 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) for PM\({}_{10}\), 117-230 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) for PM\({}_{2.5}\), and 107-225 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) for PM\({}_{1}\). The PM data presented in this study were obtained from the optical LOKOO2 laser photometer. This type of device has now become very popular due to their lower cost compared to reference monitoring instruments [20]. However, what is important is that low-cost portable monitors can easily supplement existing networks of certified monitor stations and can be applied not only outdoors--their use is cautiously encouraged to monitor indoor air quality [21; 22]. The low-cost sensors have been reported  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline  & & & & & & & & & & & & & & & \\ \multicolumn{11}{c}{**Kraków Stable**} \\ \hline \multicolumn{11}{c}{**Fungal Aerosol**} \\ \hline \multicolumn{11}{c}{**F1**} & \multicolumn{1}{c}{**F2**} & \multicolumn{1}{c}{**F3**} & \multicolumn{1}{c}{**F4**} & \multicolumn{1}{c}{**F5**} & \multicolumn{1}{c}{**F6**} & \multicolumn{1}{c}{**Total**} & \multicolumn{1}{c}{**Respiable**} & \multicolumn{1}{c}{**PM\({}_{10}\)**} & \multicolumn{1}{c}{**PM\({}_{25}\)**} & \multicolumn{1}{c}{**PM\({}_{40}\)**} & \multicolumn{1}{c}{**Respiable**} & \multicolumn{1}{c}{**Temp. **} & \multicolumn{1}{c}{**Relative Humidity**} \\ \hline F1 & - & **0.997** & 1.000 & **0.985** & **0.997** & **0.902** & **0.997** & **0.997** & \(-0.297\) & \(-0.153\) & \(-0.319\) & \(-0.153\) & 0.347 & \(-0.203\) \\ F2 & & - & **0.997** & **0.97  
to have low accuracy, as the OM concentrations estimated by the light scattering sensors can be affected by environmental parameters. Among them, the relative humidity has been mentioned to affect the PM concentration due to varying hygroscopic properties of the particles that form the particulate matter fractions [20; 23]. Even though a site-specific calibration, as well as the calibration under different humidity and temperature conditions, would improve the accuracy of the low-cost sensors [21], there are studies showing inter-model performance agreement in PM readings across 19 different PM sensors with no calibration equipment available or feasible [22]. Thus, the use of low-cost PM sensor devices in research of indoor air quality and comparing the obtained data obtained with the same equipment, calibrated before deployment, should be treated as reliable. The concentration of fungal aerosol changed considerably depending on the time of day and the activities performed indoors in both studies. The highest concentrations of all fractions of fungal aerosol in the Krakow stable were observed during feeding. In the case of the Tarnow stable, the highest concentrations of all fungal aerosol fractions were observed during daily activities performed indoors, and similarly for the particulate matter. For this reason, statistically significant positive correlations between the PM and fungal aerosol levels were observed only in the Tarnow stable. Moreover, the concentrations of most fractions of fungal aerosol varied significantly between the times of day.