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Which Is Better?Switching to a light bulb (left) that contains mercury can often cut overall releases of that toxic element into the environment.iStockphoto-morpheusdog

See also: What to do if your fluorescent bulb breaks

Fluorescent lighting uses less energy than comparably bright incandescent bulbs do. So switching to fluorescents will shave your energy bill. The big surprise: Relying on fluorescent lights may also cut how much mercury — that toxic metallic element — is released into the environment each year.

That finding was not intuitively obvious, since fluorescent lights contain mercury and the bulbs they’re replacing do not. But new calculations by a team of Yale University scientists now indicate that when the electricity used to power lighting comes primarily from coal-fired generating plants, the energy savings associated with fluorescent bulbs will translate into reductions in coal burning. And since most coal contains small but substantial quantities of mercury, burning less coal will reduce the electric industry’s release of mercury into the environment. In fact, most mercury emissions in the United States today trace to coal use.

Now keep in mind that producing — and ultimately discarding — fluorescent lights will release some mercury. When fluorescents accounted for only a small fraction of indoor lighting, they also contributed only minimal amounts of mercury.

However, as government reports and public-service ad campaigns have been touting the “green” benefits of compact fluorescent lights — those strangely shaped alternatives to the conventional incandescent light bulb — a renaissance in lighting has taken place. Within a few short years, CFLs have been sweeping the market. The Energy Department has endorsed this, noting that every home in America swapping out just one 60-watt incandescent for a comparably bright compact fluorescent light, or CFL, would

conserve energy equivalent to what’s needed to light more than 3 million homes for a year

avoid more than $600 million in annual energy costs, and

prevent greenhouse-gas releases equivalent to what’s emitted by more than 800,000 cars.

But the new and growing flood of CFLs into the marketplace has raised concerns about their potential to release a similar flood of mercury into the environment. Environmental engineer Julie B. Zimmerman led her Yale team to calculate whether the extra use of mercury for the expanded production of fluorescent lighting outweighs the drop in mercury releases from the electric plants that will power them.

It wasn’t an easy tally, she notes. Not only did her group have to establish what share of a region’s electricity was produced by coal, but also how efficient that coal burning was and whether the plants used controls to trap emitted mercury. The scientists ultimately tracked down numbers for most of the U.S. states and for 130 other countries.

They also had to calculate what share of electric energy is saved by a CFL’s long life: If it lasts eight times longer than the incandescent bulb it replaces, the industry will need to manufacture eight times fewer light bulbs (of some type). That will translate into less energy associated with shipping the bulbs to market and ultimately disposing of them. Finally, the researchers estimated what share of mercury will likely be released following a CFL’s disposal, such as in a landfill or in breakage on the way to its designated burial site.

For states like North Dakota, West Virginia and New Mexico — big coal burning regions — swapping out CFLs for incandescent lights should result in a net drop in local mercury releases, the Yale scientists report in Environmental Science & Technology. The same is true for many countries, especially China, where electricity production relies on high-mercury coal burned in plants with few or no controls on mercury emissions. (Their paper was posted online October 1and will appear in an upcoming issue of the journal.)

The opposite would be true — overall mercury emissions to the environment would rise — as CFLs replace incandescent bulbs where the local power primarily comes from hydro, nuclear or other virtually mercury-free electricity sources (even some low-mercury coal). Think Alaska, California, Oregon, Idaho, Vermont, New Hampshire, Maine and Rhode Island. Indeed, Zimmerman says, from a mercury perspective, swapping out incandescent bulbs in these states “really doesn’t make any sense.”

Her team has quantified — and mapped — how much of a mercury advantage or disadvantage CFLs will have on a state-by-state and country-by-country basis.

They also note that fluorescent lighting’s mercury advantage gets even better where recycling — to recapture the lamp’s mercury — is high. Such as in the United States, where some 20 percent of fluorescents (mainly those long tubes used in commercial buildings) get recycled. However, because CFLs typically are used in homes and fluorescent recycling is primarily done by businesses, most CFLs today escape recycling.

But that may change soon — especially as a federal law passed last year requires a phasing out of the sale of those energy-hogging incandescent light bulbs by 2012.

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