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Compact Fluorescent Lights, Energy, and Mercury

We recieved several responses to Dave’s post this week on the bizarre “Save the Light Bulb” movement. A movement that seeks to ban energy efficient compact fluorescent lights (CFL) and return to the old, energy expensive, incandescent bulbs. The primary critique is that CFL’s contain mercury, and thus, any environmental benefit is negated by mercury exposure when the bulbs break or are thrown out.

A bright idea

First, a nod to the validity of the point. Yes, CFL’s do contain mercury. Older designs contain up to 4 mg of mercury, while newer, more advanced bulbs contain 1.4 – 2.5 mg of mercury. The mercury is essential to the CFL’s function, so it’s in the light to stay. If the bulbs break, that mercury will be released into the environment. CFL’s should not be thrown away, but disposed of properly by your local recycling agency. CFL packaging does not have detailed disposal instructions so that people know not to treat CFL’s like normal light bulbs. That is an over site that should be addressed by CFL manufacturers.

The real question is “Does the mercury in CFL’s cause more environmental harm than the energy gains from using CFL’s?” To answer that we need some numbers:

  • Amount of mercury in a CFL (we’ll use the highest figure) – 4 mg
  • Amount of mercury in an incandescent bulb – 0 mg
  • Energy consumption of an average CFL – 20 watts
  • Energy consumption of an equivalent incandescent bulb – 75 watts

You can see immediately from these numbers that incandescent bulbs don’t directly release mercury into the environment, but there is one final number that matters to this equation:

  • Amount of mercury released by a coal burning power plants (2006, most recent available data) – 50.7 tons or 0.0234 mg/kWh

So let’s take the average life of a CFL, 7500 hours. That comes out to 150,000 watts, or 150 kilowatts. At 0.0234 mg Hg per kilowatt, we get 3.51 mg of mercury. Add that to the 4 mg we’ll assume will be released when the bulb breaks, and we get 7.51 mg mercury.

Now compare to an incandescent bulb over the same time frame. At 75 watts over 7500 hours, we end up with 562,500 watts, or 562.5 kilowatts. That works out to 13.16 mg of mercury.

So, even if we buy the least advanced CFL’s and don’t dispose of them properly, they’ll still release only half as much mercury into the environment as an incandescent light.

But wait, coal-fired plants only produce half the United States energy. These equations assume 100% of the power is from coal plants. Ok, lets cut the mercury production in half to account for energy production from energy sources other than coal.

  • Mercury production by CFL’s – 5.76 mg Hg
  • Mercury production by incandescent lights – 6.81 mg Hg

So even with our most conservative estimates – 50% energy from coal, highest mercury content in a CFL, everyone throws away their bulbs – CFL’s still win out.

And, when you factor in low mercury bulbs and proper disposal, that mercury production drops to 1.76 mg Hg over the course of the bulb’s life. Couple that with the huge energy savings (those kilowatts aren’t free) and you have to wonder why anyone would be fighting to save the incandescent light bulb.

~Southern Fried Scientist

Postscript – why is it that we only bust out our calculators for the mercury posts?


Deep-sea biologist, population/conservation geneticist, backyard farm advocate. The deep sea is Earth's last great wilderness.


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