WASHINGTON — Asthma patients who rely on over-the-counter inhalers will need to switch to prescription-only alternatives as part of the federal government’s latest attempt to protect the Earth’s atmosphere.

The Food and Drug Administration said today patients who use the epinephrine inhalers to treat mild asthma will need to switch by Dec. 31 to other types that do not contain chlorofluorocarbons, an aerosol substance once found in a variety of spray products.

The action is part of an agreement signed by the U.S. and other nations to stop using substances that deplete the ozone layer, a region in the atmosphere that helps block harmful ultraviolet rays from the Sun.

But the switch to a greener inhaler will cost consumers more. Epinephrine inhalers are available via online retailers for around $20, whereas the alternatives, which contain the drug albuterol, range from $30 to $60.

The FDA finalized plans to phase out the products in 2008 and currently only Armstrong Pharmaceutical’s Primatene mist is available in the U.S. Other manufacturers have switched to an environmentally-friendly propellant called hydrofluoroalkane. Both types of inhalers offer quick-relief to symptoms like shortness of breath and chest tightness, but the environmentally-friendly inhalers are only available via prescription.

“If you rely on an over-the-counter inhaler to relieve your asthma symptoms, it is important that you contact a health care professional to talk about switching to a different medicine to treat your asthma,” said Badrul Chowdhury, FDA’s director of pulmonary drug division.

Chowdhury told reporters and doctors via teleconference that “in the worst case scenario we are looking at 1 to 2 million people using” Primatene, adding that most of those patients likely use multiple medications to treat their asthma.