Charge levels and Gram (±) fractions of environmental bacterial aerosols

•Gram+ and Gram− bacterial aerosols are equally abundant in natural environments.•Outdoor viable bioaerosols have smaller average particle size than indoor environment.•Indoor and outdoor bioaerosol charges have skewed and normal distributions, respectively.•A significant fraction of indoor bioaerosol particles are viable-but-not-culturable (VBNC).

Here, we investigated charge levels and Gram (+/–) fractions of environmental bacterial aerosols. The bioaerosols with +/– charges were collected separately over different time periods using an electrostatic sampler. Air samples were cultivated using selective media for G+ and G–, and further confirmed by Gram stain method. In addition, the viable bioaerosol size distributions over a 24 h time period were also obtained using an ultraviolet aerodynamic particle sizer (UV-APS). Finally, the elementary charge units carried by environmental G+ and G– bacterial aerosols were derived based on distribution-weighted particle aerodynamic diameters.Results showed that G+ had equal abundances with G– regardless of the charge polarity and environments. The outdoor viable bioaerosols were observed to have slightly smaller distribution-weighted particle size (1.64 µm) than that (1.79 µm) of the indoor environment. In general, the outdoor culturable bacterial aerosol charge levels turned to be normally-distributed with a peak around 21–29 elementary charge units, while the indoor ones seemed to be skewed toward 46–92 elementary charge units. Results here suggest that a significant fraction of viable-but-not-culturable (VBNC) bioaerosol particles might be present for indoor environments. Results here can not only help design better electrostatic sampling device, but also assist in bioaerosol exposure assessment.