# Bacterial Aerosol in Ambient Air--A Review Study
## 1 Introduction  
Bioaerosols, comprising airborne biological particles, represent a significant category of pollutants. Both indoor and outdoor air quality directly impact human health [1; 2; 3]. Although individuals spend 80-90% of their time indoors [4; 5; 6], indoor air quality (IAQ) is significantly influenced by bioaerosols originating from the outdoor environment [7; 8; 9]. Consequently, comprehensive monitoring of ambient aerosol concentrations is crucial not only for environmental management but also for assessing the health impacts of microbiological air pollution (MAP). Bioaerosols can infiltrate indoor spaces through natural ventilation routes, such as windows and doors, contributing to a range of health issues, including infectious, allergic, and toxic diseases, and even cancer [10; 11; 12; 13]. The health impacts of bioaerosols are influenced not only by their biological characteristics and chemical composition but also by the volume inhaled and the specific regions within the respiratory tract where deposition occurs [14]. Since particle deposition in the respiratory system is closely linked to aerodynamic diameter, the health effects of bioaerosols are largely dependent on their physical properties, particularly size distribution [15; 16; 17]. Bioaerosols refer to airborne particles suspended in a gas that may contain (i) intact living or dead microorganisms (as single units, in homogeneous or heterogeneous groups, or attached to other particles); (ii) microbial spores (resistant reproductive structures produced by many fungi and some bacteria); (iii) fragments of microorganisms and larger organisms (e.g., body parts of arthropods, skin scales from mammals, pollen, and plantdebris); and (iv) other particles from living or dead organisms (e.g., excreta from arthropods such as dust mites and cockroaches, allergens from microorganisms, plants, animals, and microbial toxins) [18]. Humans typically inhale air containing approximately 10\({}^{6}\) airborne microorganisms per day [19; 20]. Bioaerosols constitute 20-30% of the total atmospheric particulate matter larger than 0.2 \(\upmu\)m [21; 22], while other studies report that 5-10% of the total suspended particulate mass is composed of bioaerosols [23; 24]. Among these, bacteria, fungal spores, and pollen are the most prevalent. Airborne bacteria are a major component of microbiological air pollution (MAP), representing over 80% of the total microbial load in the atmosphere [23]. Bacteria have been detected in various atmospheric layers, including the atmospheric boundary layer (up to 1.5 km altitude), the upper troposphere (up to 12 km altitude), and even the stratosphere at altitudes of 20 km and 41 km above sea level [25]. However, they are most abundant in the lower atmosphere, where atmospheric transport affects the distribution and spread of organisms and species. Bioaerosol transport is believed to be analogous to the transport of heat and moisture, influencing the rate of evolution, the formation of new species and microbial communities, and the adaptation to changing environments [11]. Current knowledge of the distribution of bacteria in the atmosphere remains limited, as most bioaerosol studies to date have focused on indoor air quality (IAQ) [26; 27; 28; 29; 30; 31; 32; 33]. In this review, we focus on bacterial aerosols and their interactions with critical pollutants such as particulate matter (PM) and ozone (O\({}_{3}\)), as well as their sensitivity to meteorological parameters such as temperature, relative humidity, wind speed, rainfall, and solar radiation. Understanding bacterial concentrations in the atmospheric boundary layer is essential for assessing potential health impacts related to air quality. Despite its importance, there is a notable shortage of comprehensive data on the correlation between ambient bacterial concentrations and other pollutants and meteorological factors, highlighting the urgent need for more extensive research on bacterial aerosols in outdoor environments.