X-ray produced by laboratory sparks in air at atmospheric pressure for rod-rod and rod-plane configurations were observed. A total of 585 sparks were applied with both polarities. The paper shows the effects of the voltage rise time and the peak voltage in the generation of x-rays. It is found here that shorter rise times and high peak voltages tend to produce more x-rays emissions with higher energies than longer front waveforms or lower peak voltages. In a similar way, higher voltage variations produce more energetic emissions. This finding suggests that the variation of the electric field before the breakdown can play a fundamental role in the x-ray production. The results are similar with the observations of x-rays produced in natural lightning where detections have been associated to leader steps before the return stroke.