Lethal voltages, ozone.
Tesla coil is a generator of high frequency and high potential. It is an air-core transformer with a primary consisting of a few turns of large-diameter wire and a secondary consisting of a large number of turns of fine wire. The primary of the Tesla coil is part of a tuned circuit consisting of a bank of capacitors and a spark gap. This tuned circuit is in parallel with the secondary of an iron-core high-coltage transformer. The transformer charges the capacitor bank to a sufficiently high potential. A very high potential is induced in the secondary. A spark will jump between the top of the Tesla coil and the sharp point of a wire which is connected to the other end of the coil. Hold one end of a fluoresent light tube, let the other end of the tube close to the top of the Tesla coil, the tube will light up.