Those swirly compact fluorescent light bulbs are supposed to be the greenest lighting around for general household use, using 75 percent less energy than their incandescent counterparts.

But they also contain the toxin mercury, and if you break one, special precautions are needed to clean it up.

Now, we learn that all CFL bulbs aren't equal, according to a new report by the Environmental Working Group, a nonprofit public health watchdog. Perhaps not surprisingly, some have lower mercury content than others, and some last much longer.

But unfortunately, the EWG researchers found, "you can't tell the best of the best by their labels - or the U.S. government Energy Star logo. Some Energy Star labeled bulbs could not be legally sold in Europe due to excessive mercury content."

The group has come up with its own green lighting guide, listing CFLs that contain a fraction of the toxic mercury allowed by Energy Star. All last 8-15,000 hours — compared to the Energy Star standard of 6,000 hours, and also offer high efficiency.

"Energy Star's 2008 standards, unveiled last March as a replacement for 2003 specifications, proposed to ratchet up energy efficiency requirements, tighten other performance measures and, for the first time, set a cap on bulb mercury content," according to the EWG.

"The 2008 specs were scheduled to take effect December  2nd. But in mid-October, DOE officials made a concession to industry officials because of the souring economy: they postponed the 2008 standards for a 6-month 'grace period' to July 1, 2009, so that the U.S. inventory of about 100 million bulbs fabricated under 2003 specs could be sold off. EWG research shows that 1 in 3 can't meet 2008 standards for efficiency and life span."

See the EWG report and guide here.

The website for the Energy Star program is here.

Meanwhile, there's more trouble in Energy Star land.

According to a new audit of the EPA by the Inspector General, many of the alleged benefits of the Energy Star labeling program cannot be verified. "We found the Energy Star program's reported savings claims were inaccurate and the reported annual savings unreliable….Deficiencies included the lack of a quality review of the data collected; reliance on estimates, forecasting, and unverified third party reporting; and the potential inclusion of exported items," the report stated.

The Energy Star program has been praised because of its voluntary, market-based approach to reducing greenhouse gas emissions in the U.S. But according to Jeff Ruch, executive director of the watchdog group, Public Employees for Environmental Responsibility, the report underlines "how hard it is to re-orient a massive economy away from dependence on carbon fuels. The danger for the Obama administration is that it, too, will embrace ineffectual market-based approaches as the least politically painful path."

The full report is here.