Last week, I testified before Congress about a rarely recognized public policy success: the stunning improvement of America's air quality.

The federal agency responsible for air quality, the Environmental Protection Agency, typically issues gloomy pronouncements on the topic. Former EPA Administrator Lisa Jackson told the public, "Don't breathe the air. It may kill you." Yet buried on its own website (under "Our Nation's Air: Status and Trends"), EPA's data document a record of dramatic improvement. Since 1970, aggregate emissions of the six major pollutants covered by National Ambient Air Quality Standards have decreased more than 60 percent and are still falling. These air quality achievements occurred while the U.S. gross domestic product increased 200 percent.

Virtually the entire country now attains the national standards. Some urban areas still struggle with smog (ozone) and soot (particulate matter), but most are within reach of the standards and making steady progress. In 1997, EPA classified 113 metropolitan areas as ozone nonattainment areas. That number has now fallen below 30.

Over the past several decades, tailpipe emissions have been reduced 90 percent even as vehicle miles traveled have increased 165 percent. Improvement will continue with the turnover of vehicles. EPA's Toxic Release Inventory, which tracks "releases" of more than 600 chemical compounds from more than 20,000 businesses, reports a decline of 65 percent since 1988.

With a reduction of 97 percent, lead has been practically eliminated from the air we breathe. In the 1970s, 88 percent of children between ages 1 and 5 had lead in their blood above the Center for Disease Control's threshold of risk. In 2006, it was down to only 1.2 percent.

Airborne emissions of mercury in the U.S. have declined by approximately 60 to 70 percent. The Centers for Disease Control and Prevention's most recent survey finds mercury levels among young women are well below EPA's already conservative risk level for exposure.

Benzene, a well-known carcinogen and the most widespread hazardous air pollutant, has declined by more than 64 percent. And then there are the much-feared dioxins. According to EPA's measure of "toxic equivalents," dioxin emissions have declined 92 percent over the past 20 years.

Life expectancy has increased by at least 40 percent over the past century. Medical science and disease prevention have reduced disabling and fatal diseases, with the greatest gains in preventing strokes and fatal heart disease. And although the incidence of asthma has increased, this occurred over the period of the most dramatic reduction of air pollutants in EPA's 40-year history.

You might think EPA would want to trumpet these successes. Think again. After great environmental gains and ever-stricter air quality standards now approaching naturally occurring (and thus unpreventable) background levels, EPA now implausibly claims that there is no threshold of pollution below which risk of premature mortality ceases.

Over the past four years, during which EPA has promulgated regulations unprecedented in number, stringency and cost, its administrator told the public that aggressive new regulation was necessary to prevent the deaths of thousands of people.

But facts are facts. The air has become much healthier. And while EPA's regulation under the Clean Air Act played a key role, the engine driving this transformation was technological advancement in emissions control and efficiency -- innovation spurred and made possible by economic growth within the dynamics of the free market. The U.S. now produces much more with less input, less waste and less pollution.

In most developing countries, environmental quality remains an elusive goal. Some cities in China have pollutant levels 50 times higher than the most polluted cities in our country. The prosperity made possible by economic growth in the U.S. over the past 30 years enabled business and consumers to absorb the steep cost of elaborate environmental controls. And as 19th century historian Thomas Macaulay asked: "When we see nothing but improvement behind us, why would we expect nothing but deterioration before us?"

Imagine environmental optimism.

Kathleen Hartnett-White is a distinguished senior fellow at the Texas Public Policy Foundation and director of its Armstrong Center for Energy and the Environment.