[cdheaven]

i dunno. its a gas. its gotta burn. it needs air to burn... im no rocket science. here is some info from wikeipedia
The xenon gas allows the lamps to produce minimally adequate amounts of light upon startup and speed the warmup time. If argon were used instead, as is commonly done in street and other stationary HID lamps, it would take several minutes for the lamps to reach their full output. HID headlamps use a small, purpose-designed metal halide lamp and produce more light than ordinary incandescent light bulbs (including quartz halogen lamps). The light from HID headlamps has a distinct bluish tint when compared with normal headlamps. The high intensity of the arc comes from metallic salts that are vaporized within the arc chamber.

HID headlamp bulbs produce between 2,800 and 3,000 lumens from 42 watts of electrical power, while halogen filament headlamp bulbs produce between 700 and 2,100 lumens from between 40 and 65 watts. Because of the increased amounts of light available from HID bulbs, HID headlamps producing a given beam pattern can be made smaller than halogen headlamps producing a comparable beam pattern. Alternatively, the larger size can be retained, in which case the Xenon headlamp can produce a more robust beam pattern.

An HID headlamp requires a ballast. The ballast converts the 12 volts used in automotive electrical systems to the several thousand volts required to strike and maintain the arc.

Despite marketing claims to the contrary, HID headlamps' light output is not similar to daylight. The spectral power distribution (SPD) of an automotive HID headlamp is discontinuous, while the SPD of a filament lamp, like that of the sun, is a continuous curve.

The arc within an HID headlamp bulb generates considerable short-wave ultraviolet (UV) light, but none of it escapes the bulb. A UV-absorbing hard glass shield is incorporated around the bulb's arc tube. This is important to prevent degradation of UV-sensitive components and materials in headlamps, such as polycarbonate lenses and reflector hardcoats. The lamps do emit considerable near-UV light).

Vehicles equipped with HID headlamps are required by ECE regulation 48 also to be equipped with headlamp lens cleaning systems and automatic beam levelling control. Both of these measures are intended to reduce the tendency for high-output headlamps to cause high levels of glare to other road users.

The arc light source in an HID headlamp is fundamentally different from the filament light source used in tungsten/halogen headlamps. For that reason, HID-specific optics are used to collect and distribute the light. Installing HID bulbs in headlamps designed to take filament bulbs results in improperly-focused beam patterns and excessive glare, and is therefore illegal in almost all countries.