The United States is showing that it is tightening its leash on emissions of harmful substances into the atmosphere, with its Environmental Protection Agency recently sending out back-to-back issuances against known pollutants.

The agency issued two rules that aimed at cutting down the availability and use of hydrochlorofluorocarbons as refrigerants.

The first prohibits the use of specific hydrochlorofluorocarbons to manufacture new air-conditioning and refrigeration equipment starting 2010. However, limited use of hydrochlorofluorocarbons in servicing existing equipment will be allowed.

At the same time, the sale, distribution and import of air-conditioning and refrigeration appliances and their components that also contain certain hydrochlorofluorocarbons manufactured or imported after January 1, 2010, will be prohibited.

Hydrochlorofluorocarbons contribute to the harmful depletion of the ozone layer. A diminished ozone layer allows more radiation to reach the earth’s surface, leading to serious illnesses such as skin cancer, cataracts, and weakened immune systems.

The United States is obliged under the Montreal Protocol to cut emissions of substances that deplete the ozone layer.

Separately, the agency released new data that show power plants across the United States have decreased emissions of acid rain-causing sulfur dioxide to 7.6 million tons last year, falling by 52 percent compared with 1990 levels.

The environmental agency said 2008 levels are already below the statutory yearly emission cap of 8.9 million tons for 2010. The 2010 cap is about one-half of the emissions from the power sector in 1980.

The power plants are covered by the Acid Rain Program under the 1990 Clean Air Act Amendments. It requires major emission reductions of sulfur dioxide and nitrogen oxides from the electric power industry.

Sulfur dioxide, along with compounds of nitrogen and carbon, causes acid rain.

All 3,572 electric generating units subject to the program’s sulfur dioxide requirements reportedly held enough allowances to cover their sulfur dioxide emissions, thus resulting in 100 percent compliance in 2008.

Emission reductions under the program have also led to improvements in air quality and the recovery from acidification of sensitive water bodies, the agency said.