WASHINGTON — Last warning: Asthma inhalers go "green" on Dec. 31, forcing patients still using the old-fashioned kind to make a pricey and even confusing switch.

The medicine inside these rescue inhalers — the albuterol that quickly opens airways during an asthma attack — isn't changing. But the chemicals used to puff that drug into your lungs are.

No more chlorofluorocarbons, or CFCs, that damage Earth's protective ozone layer. By year's end, all albuterol inhalers must be powered by the more eco-friendly chemical HFA, or hydrofluoroalkane.

The down side: The new inhalers cost more, $30 to $60 compared to as little as $5 or $10 for the disappearing generic CFC inhalers.

And patients face a learning curve. HFA inhalers must be used differently than the old-fashioned kind. The medicine feels and tastes different, alarming new users despite doctors' assurances that it works just as well.

"There's still significant confusion," says Dr. Harvey Leo of the University of Michigan's C.S. Mott Children's Hospital. "Patients will tell you, 'I don't feel the puff anymore."'

Calls from parents unsure how to use the new inhalers, or even what they are, have increased in the past two months as more drugstores run out of CFC-powered inhalers and automatically switch people who'd been expecting a mere refill, he adds.

The change shouldn't be a surprise. The Food and Drug Administration has long warned it was coming, and lung specialists have spent the past year easing many of the nation's 20 million asthma patients — as well as millions of emphysema sufferers who also use albuterol to ease breathing — into it.

But industry figures show that in mid-November, 20 percent of all albuterol prescriptions still were being filled with CFC versions.

Some patients may purposefully be buying up cheaper CFC inhalers before the sales ban. But many patients don't see a lung specialist, or their prescription may not expire until next year so they haven't been seen recently enough to be told.

Reaching the last fraction "is, as you can imagine, a very difficult task," says Dr. Bidrul Chowdhury, FDA's pulmonary drugs chief. "How to get to somebody who is not tuned in?"

The CFC-free options: GlaxoSmithKline's Ventolin HFA, Schering Plough's Proventil HFA and Teva Specialty Pharmaceuticals' ProAir HFA all contain albuterol. Also, Sepracor's Xopenex HFA contains the similar medication levalbuterol.