Air filtration 1 Air permeability 1 Alkali treatment 1 Box-Behnken design 1 Efficiency 1 Experimental plans 1 Filtration efficiency 1 Morphology 1 Nano-fibrous 1 Nanofibers 1 Nanofibrous composite 1 Overall filtration efficiency 1 Polyamide 6 1 Polypropylenes 1 Scanning electron microscopy 1 Sodium hydroxide 1 Surface morphology 1 Surface-modified 1 air permeability 1 alkali treatment 1 box- behnken design overall filtration efficiency 1