Sample Post: Do compact fluorescent lights do more harm than good?

When the Energy Independence and Security Act of 2007 was signed into law, setting minimum energy efficiency standards for general purpose lighting, many worried about being forced to rely on the most widely available alternative, compact fluorescent bulbs (CFLs). Fluorescent lamps emit light from an ionized mercury vapor; a typical CFL contains about 4 milligrams of mercury. In this post we’ll look at compact fluorescent lighting from the perspective of mercury emissions. We’ll see that on balance using compact fluorescent lighting in place of traditional incandescent bulbs provides a net benefit, once we account for mercury emissions from coal-fired power plants. Further, the concern about mercury in CFLs sheds light on a more general problem: the safe recycling or disposal of many of the products we rely on every day.

In 2011 US coal plants generated 1.7 million Gigawatt-hours (GWh) of electrical energy and were responsible for 27 metric tons of mercury emitted into the atmosphere. To put this in terms that can be easily compared with the use of a single light bulb, we’ll convert this to milligrams of mercury emitted per kilowatt-hour of energy produced.
Let’s first make a conversion to kilograms of mercury emissions per Gigawatt-hour of energy produced. These are the units used in reporting emissions from individual coal plants.
\[
27\text{ metric tons} \times \frac{1}{1,700,000 \text{
GWH}} \times \frac{1,000 \text{ kg}}{1 \text{ metric ton}} \approx
0.016\: \frac{\text{kg}}{\text{GWH}}.
\]

Since \(1 \text{ GWh } = 1,000,000 \text{ kWh}\) and \(1 \text{ kg } = 1,000,000 \text{milligrams}\),

\[
0.016 \: \frac{\text{kg}}{\text{GWH}} \times \frac{1,000,000 \text{
mg}}{\text{ 1 kg}} \times \frac{1 \text{ GWH}}{1,000,000 \text{
kWh}} = 0.016 \: \frac{\text{mg}}{\text{kWh}}.
\]

As we’ve shown, coal plants emit 0.016 milligrams (mg)  of mercury for each kilowatt-hour (kwh) of energy produced, on average. What quantity of mercury emissions should we attribute to a compact fluorescent or an incandescent bulb?

A 13 watt CFL has a light output similar to that of a 60 watt incandescent bulb (measured in lumens) and has an expected lifespan of 8,000 hours. This translates to about 100 kWh of energy used over the life of the bulb:
\[
13 \text{ watts} \times 8,000 \text{ hours} = 104,000 \text{
watt-hours} \approx 100 \text{ kWh}.
\]
A 60 watt bulb would use 480 kWh over the same time frame (you’d
actually need 8-11 bulbs, because incandescent lights last only 750-1000 hours):
\[
60 \text{ watts} \times 8,000 \text{ hours} = 480 \text{ kWh}.
\]
We need one more piece of information: 40\% of electricity produced in the US
is generated with coal (the share of energy produced by coal has been
declining, supplanted by natural gas).
\[
\begin{aligned}
\text{CFL: }& 100 \text{ kWh}\times 0.016
\frac{\text{mg}}{\text{kWh}}\times 40\% \approx 0.6 \text{ mg}\\
\text{Incandescent: } & 480 \text{ kWh} \times 0.016
\frac{\text{mg}}{\text{kWh}}\times 40\% \approx 3.1 \text{ mg}
\end{aligned}
\]
Replacing an incandescent bulb with a CFL avoids about 2.5 mg of mercury
emissions over the life of the bulb (as compared to using an
incandescent).

Wait! Didn’t we say that the CFL bulb contains 4 mg of mercury? We did. But what happens to that mercury? According to a substance flow analysis by Alexis Cain et al, for CFLs that are not recycled, 11% of the mercury in the bulbs is released to the environment as they make their way through the municipal waste system. That’s about 0.4 mg from our example bulb.   Adding this in to the results above, we still come out about 2 mg ahead with the CFL as compared to an incandescent,  even when we throw the CFL in the trash (but don’t do this). On the other hand when CFLs are recycled, over 99% of the mercury can be recovered.

What about mercury exposure from a broken bulb?

If you  are concerned about CFLs you may be more worried about mercury exposure from a single broken bulb then you are about mercury emissions from coal plants. This concern is misplaced for two reasons. First, the dose is small. When a CFL bulb is broken only a portion of the mercury is released. According to an article published in Environmental Engineering Science,  researchers found that between 0.04 and 0.7 mg of mercury was released during the first 24 hours after the bulb is broken.  For comparison 0.7 mg  is the amount of mercury you’d expect to ingest from eating about 10 cans of tuna fish.   Second, mercury inhaled or ingested in its elemental form, as it is in a CFL or an old thermometer, is processed by the kidneys and excreted by the body whereas the mercury found in fish is in a more harmful form— the organic compound methyl mercury– which bioaccumulates.  The most significant human-caused source of that methyl mercury is emissions from coal fired power plants. In short,  mercury emissions from coal plants are of far greater concern than the risks posed from the mercury in CFLs.

Concern over hazardous substances isn’t specific to fluorescent lighting. A product can be harmless or even beneficial in its day-to-day use, but become harmful if its constituent parts end up in the environment after it’s discarded.  We can work to increase recycling rates and improve the recovery of hazardous (and valuable) materials for reuse in the manufacturing process,  keeping such materials out of the biosphere to the greatest extent possible.

Works Cited:

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