of less than could lead to ultraviolet exposures approaching the current workplace limit set to protect workers from skin and retinal damage. However, industry sources claim the UV radiation received from CFLs is too small to contribute to skin cancer and the use of double-envelope CFLs "largely or entirely" mitigates any other risks. Tests have shown that radiation exposure from CFLs is negligible at 150 centimeter distance from the source. At closer distances, comparisons show that CFLs emit less UVA (long wavelength) radiation than incandescent light bulbs. They do, however, emit higher levels of UVB (short wavelength) radiation. UVB can penetrate deep into the skin while sufficient levels of UVA can burn superficial layers. Closed (double-envelope) CFLs are shielded and emit a lower total UV radiation compared to incandescent or halogen bulbs of a similar wattage. For the average user, UV radiation from indoor lights does not appear to be a concern. For those with skin sensitivity long term indoor exposure may be a concern, in which case they may want to use a bulb with lower UV radiation output. There seems to be more variability within bulb types than between them, but the best option is shielded CFLs. A 2012 study comparing cellular health effects of CFL light and incandescent light found statistically significant cell damage in cultures exposed to CFL light. Spectroscopic analysis confirmed the presence of significant UVA and UVC radiation, which the study's authors conjectured was attributable to damage in the bulbs' internal phosphor coatings. No cellular damage was observed following exposure to incandescent light of equivalent intensity. The study's authors suggest that the ultraviolet exposure could be limited by the use of "double-walled" bulbs manufactured with an additional glass covering surrounding the phosphor-coated layer. When the base of the bulb is not made to be flame-retardant, as required in the voluntary standard for CFLs, overheating of the electrical components in the bulb may create a fire hazard. Mercury content CFLs, like all fluorescent lamps, contain mercury as vapor inside the glass tubing. Most CFLs contain 3–5 mg per bulb, with the bulbs labeled "eco-friendly" containing as little as 1 mg. Because mercury is poisonous, even these small amounts are a concern for landfills and waste incinerators where the mercury from lamps may be released and contribute to air and water pollution. In the U.S., lighting manufacturer members of the National Electrical Manufacturers Association (NEMA) have