Flash duration can be carefully controlled with the use of an inductor. The flash that emanates from a xenon flashtube may be so intense that it can ignite flammable materials within a short distance of the tube. Carbon nanotubes are particularly susceptible to this spontaneous ignition when exposed to the light from a flashtube. Similar effects may be exploited for use in aesthetic or medical procedures known as intense pulsed light (IPL) treatments. IPL can be used for treatments such as hair removal and destroying lesions or moles. The lifetime of a flashtube depends on both the energy level used for the lamp in proportion to its explosion energy, and on the pulse duration of the lamp. Failures can be catastrophic, causing the lamp to shatter, or they can be gradual, reducing the performance of the lamp below a usable rating. Catastrophic failure can occur from two separate mechanisms: energy and heat. When too much energy is used for the pulse duration, structural failure of the glass envelope can occur. Flashtubes produce an electrical arc flash contained in a glass tube. As the arc develops, a supersonic shock-wave forms, traveling radially from the center of the arc and impacting the inner wall of the tube. If the energy level is low enough, a tap against the glass is all that will be heard. However, if the energy level used equals the "explosion energy" rating of the lamp, the impacting shock wave will fracture the glass, rupturing the tube. The resulting explosion creates a loud, sonic shock-wave, and may throw shattered glass several feet. The explosion energy is calculated by multiplying the internal surface-area of the lamp, between the electrodes, with the power-loading capacity of the glass. Power loading is determined by the type and thickness of the glass, and the cooling method that is used. Power loading is measured in watts per centimeter squared. However, because the pulsed-power level increases as the flash duration decreases, the explosion energy must then be decreased in direct proportion to the square root of discharge time. Failure from heat is usually caused by excessively long pulse-durations, high average-power levels, or inadequate electrode-size. The longer the pulse; the more of its intense heat will be transferred to the glass. When the inner wall of the tube gets too hot while the outer wall is still cold, this temperature gradient can cause the lamp to crack. Similarly, if the electrodes are not of a sufficient diameter to handle the peak currents they may produce too much resistance, rapidly heating up and thermally expanding.