In addition to these concerns, HMI lamps have been known to shatter at the end of their lifetime or if stressed enough. While not as violent as the explosion of a xenon short-arc bulb, they still require caution. As a result, HMI lamps should not be used past half their rated lifetime, and care should be taken with larger lamps when striking (turning on the lamp), as a lamp is most likely to explode within the first five minutes of striking. For this reason, each HMI lamp will usually carry with it a detailed log of the number of strikes and number of hours it has been used. Care should also be taken transporting the lamp and replacing lamps. The gases in an HMI lamp are under a very small pressure, but it does increase with temperature. As with quartz-halogen bulbs, care should be taken not to touch the glass directly as skin oils left behind on the glass can actually heat up past the working temperature of the glass and cause bubbling and/or a weak point on the bulb. For this reason, anytime a bulb is handled it should be cleaned with an isopropyl alcohol wipe. Most lamp housing designs are inherently tougher and thicker than traditional tungsten units so that in the event of a bulb explosion, those nearby are protected from flying debris. There is the possibility of the front lens element on the lamp head cracking from thermal shock (though not completely blowing out or shattering). Proper safety procedures should always be followed when using HMI units, as they can be quite dangerous if misused.