Background: Switching-on a microwave results in the creation of nano-sized particles, which can spread through the air of a given premise, e.g., room. Material and Methods: The study was carried out to determine the number and surface concentrations as well as the mean particles size using a DiscMini measurement device distributed in 6 measuring points to track changes of particle parameters, primarily at the source of particle creation and in the area of local ventilation (fume cupboard), and also in other places, e.g., near the window or in the middle of the room. Results: Where the fume cupboard was switched-off, i.e., normal pressure ventilation in the room, switching-on the microwave caused a 9.42–14.14-fold increase in the number concentration of nano-sized particles relative to the background and a 3.51–4.81-fold increase in the surface concentration. Where the fume cupboard was switched-on, i.e., negative pressure ventilation in the room switching-on the microwave caused a 3.20–4.43-fold increase in the number concentration of nano-size particles relative to the background and a 1.61–1.89-fold increase in the surface concentration. Conclusions: The analysis of the data shows that switching-on a microwave for 5 min results in the creation of nano-objects already after about 3 min with the maximum concentration values after 12 min since switching-on the microwave in all 6 measurement points distributed in the test room. This applies to both situations, i.e., when the fume cupboard was switchedoff or switched-on. Med Pr 2016;67(3):353–363