# Bacterial Aerosol in Ambient Air--A Review Study
## 3 Discussion
### Interactions with Air Pollutants
Generally, most studies indicate a positive correlation, meaning that as PM\({}_{10}\) levels increase, so do bacterial aerosol concentrations. This relationship is particularly strong and significant in studies by [PERSON] et al. [49] and [PERSON] et al. [48], and under specific conditions such as dust days [40]. Seasonal variations noted by [PERSON] et al. [4] suggest that this relationship can reverse in winter, potentially due to different sources or behaviors of PM\({}_{10}\) and bacteria during colder months. Data from the pandemic period, as reported by [PERSON] et al. [65], show that significant public changes also affect this relationship but maintain a strong positive correlation overall. Inconclusive relationships between PM and bacterial aerosols may support the thesis of [PERSON] et al. [8], which posits that meteorological factors have a greater influence on shaping airborne bacterial communities than air pollutants. Conversely, some studies reviewed indicate a negative correlation between PM and bacterial aerosols during the winter season, which might highlight the significant role of toxic substances in PM. [PERSON] et al. [73] noted that inverse trends between bioaerosol concentration and PM are related to the air quality index. During low-haze conditions, the growth-promoting effect of sulfate and nitrate in suspended particles predominates, while the concentrations of toxic and hazardous chemicals adhering to these particles do not reach levels sufficient to exert a significant poisonous effect. However, as haze pollution intensifies, PM\({}_{2.5}\) concentrations increase, amplifying the toxic effects. Research by [PERSON] et al. [74] demonstrated that bacterial community diversity in PM\({}_{2.5}\) was higher than in PM\({}_{10}\), but high PM concentrations had a certain inhibitory effect on the bacterial community richness of PM\({}_{2.5}\). This may be due to the larger specific surface area of PM\({}_{2.5}\), which facilitates the absorption of a greater quantity of chemical substances detrimental to microbial survival. Toxic and hazardous substances in PM, including crustal elements, heavy metals, and inorganic ions, play a particular role. For example, heavy metals released from airborne PM with redox potential act as catalysts in the generation of reactive oxygen species (ROS), especially hydroxyl radicals (HO\({}^{\bullet}\)) which damage cellular biological molecules [75,76]. Additionally, some inorganic ions cannot be neglected. SO\({}_{4}\)\({}^{2-}\) has a distinct ability to influence the existence and growth of microbes, thereby affecting their relative abundance [77]. [PERSON] et al. [78] pointed out that secondary inorganic aerosols, including SO\({}_{4}\)\({}^{2-}\) and NO\({}_{3}\)\({}^{-}\), formed by aqueous or photochemical transformation from SO\({}_{2}\) and NO\({}_{2}\), are recognized as an important source of nutrients for microorganism growth. Ca\({}^{2+}\) affects various bacterial cellular processes, including the cell cycle, cell division, competence, pathogenesis, motility, and chemotaxis [79]. Cl\({}^{-}\) is involved in water disinfection processes and can cause bacterial stress and injury [80], while high concentrations of K\({}^{+}\) and Na\({}^{+}\) may disrupt bacterial structure and function, potentially leading to bacterial death [80]. Consequently, lower bioaerosol concentrations may be observed during periods of extreme pollution. Based on this premise, it can be hypothesized that the interplay between growth-promoting effects and toxic effects from chemical pollutants adhering to bioaerosols may influence bioaerosol concentration levels during winter. However, the effects of toxic chemical components in airborne particulate matter on microorganismsremain largely unexplored. [PERSON] et al. [63] highlighted the significant role of the size distribution of viable bioaerosols. Their results showed that airborne bacteria of different particle sizes varied distinctly with PM\({}_{2.5}\) concentrations.