Incandescent light bulb Small variations in resistivity along the filament cause "hot spots" to form at points of higher resistivity; a variation of diameter of only 1% will cause a 25% reduction in service life. These hot spots evaporate faster than the rest of the filament, which increases the resistance at that point—this creates a positive feedback that ends in the familiar tiny gap in an otherwise healthy-looking filament. Irving Langmuir found that an inert gas, instead of vacuum, would retard evaporation. General service incandescent light bulbs over about 25 watts in rating are now filled with a mixture of mostly argon and
Phase-out of incandescent light bulbs significant health risks to adults, including pregnant women, or to children. If all electricity was generated by a coal power plant (which produce about half the electricity the U.S. consumes) and fluorescent light bulbs were all recycled with no mercury being lost, nearly 75% less mercury could be released in power plant emissions if incandescent bulbs were replaced by fluorescents, and with significantly less total mercury release even if no recycling occurred. However, a concern is that broken bulbs will introduce the mercury directly into a populated indoor area. Though more recent analysis indicates that the concerns about mercury release