Here, the EBC samples collected by the developed device from seven human subjects recruited from a respiratory unit of Peking University Third Hospital in Beijing were studied using culturing, DNA stain, SEM and molecular methods. In this study, the particle size distributions trends in a typical exhaled breath were also measured and are shown in Figure 4 . As observed in the figure, the number concentration decreased with increasing particle diameter. For bacterial size ranges (0.65-2.2 mm), a concentration level of 329 to 25819 particles/L was observed, while for larger particles of 2.2-4 mm a concentration level of 60 to 400 particles/L was obtained. In previous studies, similar particle size distribution trend in exhaled breath was also found using the OPC, although the droplet concentrations for respective size ranges were slightly different [21, 39] . Nonetheless, due to its rapid evaporation water droplet itself or those adsorbing on bacterial particles in the exhale breath will certainly affect the results obtained here. The results from OPC indicated that particles of larger than 2.5 mm only accounted for 0.4% of the total particles exhaled. According to ICRP (1994), the total lung deposition efficiency for particles larger than 2 mm is more than 80%, while for smaller particles of less than 1 mm, the deposition efficiency is less than 40%, i.e., 60% exhaled out [44] . In addition, larger particles could stick to the straw wall. Therefore, in the exhaled breath as well as those collected into DI water droplet smaller particles would dominate. Figure 5 shows the concentrations of culturable bacterial aerosols in EBC samples collected from seven human subjects. As shown in the figure, bacterial concentration levels ranged from 693 to 6,293 CFU/m 3 . ANOVA tests indicated that there were statistically significant differences in culturable bacterial aerosol concentrations for EBC samples collected from different subjects (p-value = .0001). In a recent study, human occupants are also identified as the significant contributors for indoor bacteria, i.e., the emission rate is about 37 million gene copies per person per hour, and a distinct indoor air signature of bacteria was demonstrated to be associated with human skin, hair, and nostrils [40] . During human breathing, the bacterial particles from environmental air are continuously inhaled, some of which, i.e., smaller ones, can be exhaled out again by the lung and reside with nostrils. Here, bacterial species Sphingomonas paucimobilis and Kocuria rosea were detected using Vitek2 in six EBC samples as shown in Table S2 . Because of limitation of Vitek 2, certain bacterial species were not identified in our study. Among the subjects, subject #6 had substantially higher culturable bacterial concentrations than other subjects. From his medical conditions shown in Table S2 , it was likely that his fever was caused by the bacterial infections. In his EBC sample, we found Kocuria variants which were thought to cause catheter-related bacteremia [41] . For other human subjects, the culturable bacterial aerosol concentra- tion levels ranged from 700 to 3000 CFU/m 3 and Sphingomonas paucimobilis, a non-fermenting Gram-negative bacillus, were detected. In a previous study, S. paucimobilis was found to cause nosocomia bacteremia outbreak [42] . For negative control samples, we did not observe the bacterial growth, indicating no contamination during the EBC collection. Ideally, bacterial particles in EBC should be collected using a suitable size-selective sampling tool to investigate the bacterial counts for different size range. However, such device is currently not available yet. Compared to the environmental culturable bioaerosol concentrations, those in EBC samples collected had relatively higher levels, thus representing an important source of bioaerosols particularly in a high human occupancy environment. In addition to viruses, Rhodococcus equi, a bacterium causing pyogranulomatous bronchopneumonia, were detected in the exhaled air from foals in a recent study [43] . When pathogenic bacteria are breathed out, they could pose a serious public health threat. Figure 6 shows the qPCR amplification plot from EBC samples collected from seven human subjects in a respiratory clinic. As observed from the figure, bacterial samples were successfully amplified (Ct values were [16] [17] [18] [19] , while the positive sample (B. subtilis) had a Ct value of 15 and the negative control had a value of 28. Based on the DNA standards used, the concentrations of bacterial DNA in the EBC samples (Sample 1-7) were in the range of 0.32 mg/mL-3.15 mg/mL. Detection of the bacterial DNA in EBC samples was also confirmed by the melting curve of qPCR amplification as shown in Figure 7 . As observed in the figure, most EBC samples had a peak at 68uC, the same as that of the positive control B. subtilis. For a few different peaks observed, they might be the possible primer dimer (PD) from the PCR non-specific amplification process. In addition to the qPCR amplification of bacteria in EBC samples collected, DNA stain (AO method) was also performed and the results are shown in Figure S2 . As observed in the figure, both viable (green) and dead (yellow) were found in the EBC samples collected and the positive control B. subtilis samples, while no cells were detected in the negative control. SEM images with different resolutions and agar plate culturing shown in Figure 8 also indicated that EBC samples (cultured) had various types of bacteria based on their morphologies and colony color. From SEM images, it can be estimated that most bacteria are in the range of 0.5-1.0 mm. According to total particle deposition curve developed by ICRP (1994) [44] , more than 60% of bacterial particles of below 1 mm could be exhaled out. These smaller bacterial particles could remain airborne for a prolonged time period, thus playing an important role in airborne transmission of potential diseases. Results shown in Figures 5, 6 , 7, and 8 indicate that high levels of bacterial aerosols were detected in the EBC samples collected, and the results on the other hand also implied that the developed device was efficient in collecting bacterial particles in the exhaled breath. These experimental data further confirm that exhaled breath is an important source of bacterial aerosols in the built environments.