Nitrogen dioxide is a common atmospheric pollutant, produced by the burning of nitrogen containing fossil fuels, especially diesel.
It is extremely irritating to the lungs (especially for asthmatics) and should normally be handled only in a fume cupboard. In this experiment, however, the very small amounts produced are contained within the petri dish, thus limiting exposure.
Reactions of Nitrogen Dioxide – Pupil
Reactions of Nitrogen Dioxide – Teacher
Reactions of Nitrogen Dioxide – Risk Assessment
Image By Eframgoldberg – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=28364789