containing atmosphere, which helps in the cleaning process, but is also a health hazard. Light pollution considerations For placements where light pollution is of prime importance (for example, an observatory parking lot), low-pressure sodium is preferred. As it emits narrow spectral lines at two very close wavelengths, it is the easiest to filter out. Mercury vapor lamps without any phosphor are second best; they produce only a few distinct mercury lines that need to be filtered out. Bans In the EU the use of low efficiency mercury vapor lamps for lighting purposes was banned in 2015. It does not affect the use of mercury in compact fluorescent lamp, nor the use of mercury lamps for purposes other than lighting. In the US, ballasts for mercury vapor lamps for general illumination, excluding specialty application mercury vapor lamp ballasts, were banned after January 1, 2008. Because of this, several manufacturers have begun selling replacement compact fluorescent (CFL) and light emitting diode (LED) bulbs for mercury vapor fixtures, which do not require modifications to the existing fixture. The US Department of Energy determined in 2015 that regulations proposed in 2010 for the mercury vapor type of HID lamps would not be implemented, because they would not yield substantial savings. Ultraviolet hazards The arctube of mercury lamps produces large amount of short wave UV-C radiation which can cause eye and skin burn. Usually the glass outer jacket of the lamp and in some lamps, also the phosphor coating, block this radiation. However, a care should be taken if the outer jacket of the lamp breaks, because the arctube would continue to operate, presents a safety hazard. There have been documented cases in the United States, of lamps being damaged in gymnasiums by balls striking the lamps, resulting in sun burns and eye inflammation from shortwave ultraviolet radiation. When used in locations like gyms, the fixture should contain a strong outer guard or an outer lens to protect the lamp's outer bulb. As a result of the said documented cases, some American manufacturers made "safety" lamps that will deliberately burn out if the outer glass is broken. This is usually achieved by using a thin tungsten strip, which will burn up in the presence of air, to connect one of the electrodes. Typical mercury vapor lamps with an outer envelope made of soda lime or borosilicate glass still allow a relatively large amount of 365