Submicron ultrafine particles and nanoparticles found in construction dust can be inhaled deeply into the lungs and retained in the lower airways. However, we lack field test data establishing nanoparticle exposure levels on construction sites, and the filtration efficiency of N95 respirators commonly worn by construction workers on the job. Researchers positioned a manikin wearing an N95 respirator on multiple construction sites and used a commercially available nanoparticle counter to measure exposures and respirator efficiency by collecting samples from inside and outside the mask. The team tested two respirators (one pleated and one foldable) on construction sites near concrete drilling and grinding activities, earthmoving operations, and wood cutting during frame carpentry.