A CFL, or compact fluorescent lamp, is an energy-saving light bulb that produces light by running electricity through a tube filled with mercury vapor and argon gas. It uses about one-third the energy of a traditional incandescent bulb to produce the same amount of light. A 13-watt CFL, for example, puts out roughly the same brightness as a 60-watt incandescent. Though once the go-to replacement for inefficient incandescent bulbs, CFLs are now being phased out in favor of LEDs.
How a CFL Produces Light
Inside every CFL is a curved or spiraled glass tube containing a small amount of mercury (both as vapor and liquid) mixed with argon gas. At each end of the tube sits an electrode. When you flip the switch, a ballast built into the bulb’s base sends a controlled burst of electricity between those two electrodes, creating an arc of electrons that flows through the gas.
As those electrons collide with mercury atoms, the atoms release ultraviolet radiation, which is invisible to the human eye. The inside of the glass tube is coated with three different phosphors that absorb that ultraviolet energy and re-emit it as visible light in the red, blue, and green parts of the spectrum. Blended together, those three colors appear as white light. The specific mix of phosphors determines whether the bulb looks warm and yellowish or cool and bluish.
The Role of the Built-In Ballast
What makes a CFL “compact” is the electronic ballast integrated into its base. Older fluorescent tube lights, the long ones common in offices and garages, needed a separate ballast fixture mounted in the ceiling. CFLs fold the tube into a tight spiral and pack a miniature electronic ballast right into the screw-in base, so they fit standard lamp sockets.
The ballast does three things: it delivers the initial high-voltage spike needed to ignite the gas, it limits current so the bulb doesn’t burn out instantly, and it stabilizes the flow of electricity during normal operation. Electronic ballasts operate at high frequencies, which virtually eliminates the visible flicker and audible hum that older magnetic ballasts were known for.
Energy Use and Lifespan
CFLs were designed to solve two problems with incandescent bulbs: wasted energy and short life. An incandescent bulb converts most of its electricity into heat rather than light. CFLs generate about 70 percent less heat, directing far more energy toward actual illumination.
A standard screw-base CFL lasts between 8,000 and 10,000 hours, while plug-in CFLs used in commercial fixtures can reach 10,000 to 20,000 hours. Compare that to a traditional incandescent at just 750 to 2,000 hours, and a single CFL could outlast five or more incandescent bulbs in the same socket. LEDs, however, have since raised the bar dramatically, with rated lifespans of 40,000 hours or more.
Light Quality and Color
One common complaint about early CFLs was that their light looked flat or slightly greenish. That came down to the phosphor blend. Basic single-phosphor fluorescent lamps scored around 50 to 64 on the Color Rendering Index (CRI), a 0-to-100 scale measuring how accurately a light source reveals the true colors of objects. Under low-CRI lighting, skin tones look washed out and reds appear dull.
Modern tri-phosphor CFLs improved significantly, scoring in the high 70s to low 90s on the CRI scale. A tri-phosphor cool-white fluorescent, for instance, reaches a CRI of about 89. Standard LED bulbs sit around 83, though high-end LEDs now push above 95. In practice, a good-quality CFL renders colors well enough for most home settings, though side by side with a high-CRI LED the difference is noticeable.
Mercury: How Much and Why It Matters
Mercury is essential to how CFLs work, and it’s also the main reason they require careful handling. On average, a CFL contains about four milligrams of mercury sealed inside the glass tube. For perspective, an old-fashioned glass thermometer holds around 500 milligrams, equal to the mercury in more than 100 CFLs. It’s a tiny amount, but it still warrants proper cleanup if a bulb breaks and proper disposal when it burns out.
What to Do If a CFL Breaks
If you drop a CFL and it shatters, the EPA recommends a specific cleanup process to minimize your exposure to mercury vapor. First, get people and pets out of the room. Open a window or exterior door and let the room air out for 5 to 10 minutes. If you have central heating or air conditioning, shut it off so the system doesn’t circulate mercury vapor through the house.
Do not vacuum. A vacuum can spread mercury-containing powder into the air. Instead, gather stiff paper or cardboard, sticky tape (duct tape works well), damp paper towels, and a glass jar with a metal lid or a sealable plastic bag. Scoop up the glass fragments and visible powder with the cardboard, then press sticky tape over the area to pick up any remaining fine particles. Wipe the spot with damp paper towels.
Place all the debris, tape, and paper towels into your sealable container and move it outdoors right away. Check with your local waste agency about disposal rules. Some areas require you to bring broken CFLs to a recycling center. If your jurisdiction allows regular trash disposal, seal everything in a plastic bag and place it in your outdoor trash bin. Continue airing out the room for several hours if possible.
Recycling and Disposal
Even unbroken CFLs shouldn’t go in your regular trash if you can avoid it. The EPA recommends recycling them to keep mercury out of landfills, and several states actually require it by law. California, Maine, Massachusetts, New Hampshire, Vermont, and Washington all prohibit tossing mercury-containing bulbs in household garbage.
You have several recycling options. Many hardware stores offer free in-store CFL recycling drop-off. Local waste collection agencies often run hazardous waste collection days that accept fluorescent bulbs. Some manufacturers sell pre-labeled mail-back kits that let you ship used bulbs to a recycling center. The site search.Earth911.com can help you find the nearest drop-off location. If your local regulations do permit regular trash disposal, seal the intact bulb in a plastic bag before putting it in your outdoor bin.
CFLs Are Being Phased Out
CFLs were the bridge technology between incandescent bulbs and LEDs, and that bridge is closing. LED bulbs now match or exceed CFLs in every meaningful category: they use even less energy, last several times longer, contain no mercury, and reach full brightness instantly. As a result, governments have begun banning CFL sales.
California prohibited the sale of new compact fluorescent lamps starting January 1, 2024, followed by a ban on linear fluorescent tubes on January 1, 2025. The European Union has implemented similar restrictions. If you still have working CFLs in your home, there’s no need to replace them immediately. But when they burn out, an LED replacement is the clear upgrade in efficiency, lifespan, and convenience.