Airborne nanoparticles in the size range 15-150 nm present severe health hazards while micrometer size water droplets suspended in diesel fuels reduce fuel economy and promote corrosion of metallic components. Current technologies cannot adequately address both these issues. Glass fiber-based air filters cannot efficiently remove airborne nanoparticles of size below 300 nm. Also, improper tuning of hydrophobicity and hydrophilicity in glass fiber mats limits their performance in separating water droplets from diesel fuels. Our research focuses on the fundamentals of design of polymeric building blocks to address both these problems. First, we exploit reaction-induced phase separation or thermo-reversible gelation for designing polymer aerogel monoliths with appropriate balance of meso- and macro-porosity. The characteristic features of polymer networks – spheres of typical dimension 20 nm or cylindrical strands of typical diameter 50 nm – show strong dependence on temperature, solid concentration, and the nature of the solvent and uniquely determine a balance of meso- and macroporosity. The resultant macropores handle bulk of the air flow while the mesopores remove the nanoparticles with efficiency approaching 99.998%. Second, we exploit evaporation-induced phase separation of immiscible polymer pairs in designing interpenetrating network morphology in polymer nanofibers of diameter 100-300 nm. These nanofibers, coated onto glass fiber mats, provide functionality in separating water droplets suspended in diesel fuel with much higher efficiency than is possible in current technology. This paper discusses the principles of size and shape selection of polymeric building blocks in aerogel monoliths and bi-component polymer nanofibers using several processing variables. In addition, the paper discusses quantitative data on airborne nanoparticle filtration and water droplet separation from diesel fuels and relates them to polymer morphology.