Little attention has been paid to the generated submicron ultrafine and nanoparticles and their exposure levels on construction jobsites. This information is needed because cytotoxicity of nanoparticles is well known now. In addition, the performance of particulate respirators generally used by construction workers has never been evaluated in field conditions against ultrafine and nano-sized particles. We hypothesized that workers on construction jobsites are exposed to high levels of nanoparticles and current NIOSH-recommended N95 respirators may not provide them adequate protection against aerosolized nanoparticles. Our exposure assessments of ultrafine and nanoparticles on several construction sites using SMPS nanoparticle counters showed that particle mass concentrations ranged between 1.41 and 99.96 µg/m3. The real-time filtration efficiency of N95 respirators against nanoparticles greater than 20.5 nm in aerodynamic diameter was often less than 95%. When surface electrostatic charge was removed in N95 respirators by isopropanol treatment, the filtration efficiency of larger nanoparticles dropped compared to smaller nanoparticles of