CFLs are fluorescent bulbs that have been designed in spiraling shapes that mimic the shape of older incandescent lights so that they can be used in many of the same situations. The lights are designed to be energy efficient, using chemical coatings and gases to product light instead of filaments. The energy savings are offset by several common CFL issues.
CFL bulbs use a small amount of mercury, an extremely toxic element that is very difficult to safely dispose of. The bulbs cannot simply be placed in a dump, because the mercury then leaks out and can contaminate groundwater. Many CFL bulbs break before they reach safer places like landfills. Recycling the gases is an expensive and time-consuming process.
CFL lights tend to be susceptible to conditions around the home. If the air is cold they can take longer to turn on. They take about a minute to reach full brightness in common conditions. If it is too hot, about 77 degrees Fahrenheit, they can fail more easily.
CFL bulbs are designed to take the place of incandescent bulbs, but not all CFLs fit in incandescent fixtures, especially older fixtures. Bulbs in the wrong fixtures can explode or burn out. Few CFLs work with dimmer switches or three-way light fixtures, and finding bulbs that will work can be a challenge, since few large retailers carry a large variety.
CFL bulbs cost several dollars per bulb, compared to the lower cost of incandescent bulbs, which tend to cost between 50 cents and $1. This starting cost is offset by the eventual energy savings of the CFL bulb, but this takes time.
Fluorescent lights have been linked to several medical conditions, including migraines and eye strain. The bulbs also can have worse effect on skin diseases than incandescent light.
CFL lights do not produce natural light like incandescent lights, although the light quality has improved some. The bulbs can also fail more easily than other types of lights, and when turned on and off repeatedly they may break or burn out.