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.

"A lot of patients may already have them," said Dr. Nathanael Horne, of Asthma and Allergy PC and an assistant attending at St. Vincent's Hospital, which are both located in New York City. "Many doctors already switched their patients over."

The down side to the switch: The new inhalers cost more, $30 to $50 compared to as little as $5 or $10 for the disappearing generic CFC inhalers, Horne said.

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

"The particles aren't going to feel as large, but the patient is getting an equal amount of medicine," Horne said. "It's equally calibrated, so even though you might feel like you aren't getting enough medication, don't take an extra puff."

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 were still 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.

Albuterol inhalers are for emergencies, for quick relief of wheezing. Patients also need daily medication to control their asthma and prevent flare-ups.

A person who uses their rescue inhaler more than twice a week may not have their asthma under control and may need to speak with their doctor about adding a controller medication to their regimen, Horne said.

Recent research suggests only 1 in 5 children has their asthma under good control; no one knows how many adults do.

The last to go CFC-free will be the poor and uninsured whose asthma is less likely to be controlled, said Dr. Harvey Leo of the University of Michigan's C.S. Mott Children's Hospital, who researched that issue at Michigan's Center for Managing Chronic Disease.

Albuterol manufacturers are providing free samples and posting coupons on their Web sites.

Still, specialists worry that some patients will try to save money with a decades-old nonprescription inhaler that contains a different drug, epinephrine, best known by the brand name Primatene Mist — inhalers that also contain ozone-harming CFCs.

National asthma guidelines argue against such self-treatment as too risky and less effective than albuterol. The government will allow sale of those over-the-counter inhalers until December 2011 as manufacturers reformulate.

Leo has another concern: Only one of the new inhalers counts doses used. He's monitoring emergency-room statistics to see if cost-conscious patients trying to squeeze out last drops wind up using empty inhalers.

What do patients need to know as they switch?

— Expect a softer puff instead of the CFC version's cold blast of air in the back of the throat.

"Often, each of these inhalers will have different instructions, so talk with your doctor," Horne said. "Some you will have to prime or shake, some you will have to clean regularly."

— The new inhalers clog more often because HFA makes the drug stickier. Clean the hole weekly, following the instructions unique to each brand.

— Never get the whole device wet.

The FDA says there's plenty of supply; it gave manufacturers several years to ramp up before the ban.

But don't wait until the last minute. When Eric Stoermer of Ann Arbor, Mich., made the switch in August, he waited a week for a new inhaler for his 11-year-old son Ethan. Their drugstore was temporarily out of stock.

"I ended up having to hunt around on an emergency basis," Stoermer says. "This is a bad thing to run out of."

The Associated Press and FOXNews.com Health Editor Jessica Doyle contributed to this report.