Desktop 3D printers are widely used in the office and home settings; however, very limited information is available on the potential exposures to operators and nearby persons from the emission of ultrafine particle during printing. This study was conducted to measure the concentrations of ultrafine particles (UFP) where 3D printers were operating. UFP were recorded in 10-second intervals before and during 3D printer operation by using a condensation particle counter (CPC). Particle mass concentrations of size-fractionated particulate matter (PM) were also evaluated by a nephelometer at 10 second intervals (DustTrak DRX). Ten sets of data were collected from October 19, 2015 to March 04, 2016 at two different locations: the San Diego State University (SDSU) library 3D printing lab, where Polylactic Acid (PLA) filament in three 3D printers were used in a complete ventilated space and second location, a local 3D printing shop in San Diego used different filaments such as PLA, Acrylonitrile Butadiene Styrene (ABS), nylon, polyethylene terephthalate (PET) and others in nine printers with minimum ventilation. At the local print shop, operating printers with multiple filaments types, the concentration of UFP was consistently higher inside the facility when compared to outside the facility (median 54,337 particles/cm_) versus (median 5,727 particle/cm_, p<0.05). Much lower UFP concentrations were observed at the SDSU library printing lab where fewer printers, operating with PLA filament only, (overall median 4000 particle/cm_), though when more than one printer was operating higher UFP concentrations were measured as compared to only one printer. An equivalent increase in PM mass concentrations was not observed for any particle size cut (PM 1, 2.5, or 10 _m in aerodynamic diameter, or total), indicating that the printers present a UFP exposure hazard not captured by PM mass measurements. Occasional large increases in PM were observed, indicating that other operations such as grinding or painting could contribute to PM mass increases. These results suggest that appropriate ventilation systems should be required, and caution should be used while operating 3D printers. More controlled experiments should be conducted to understand the correlation between the type of 3D printer and exposures to UFP.