When Congress passed the 1990 amendment to the Clean Air Act, many people believed the problem of acid rain had been solved. The amendment called for 50 percent reductions in emissions of sulfur dioxide and nitrogen oxides, the primary causes of acid precipitation, by 2010. But a recent report by the Hubbard Brook Research Foundation indicates that these measures aren’t enough: many forest and aquatic habitats in the northeastern U.S. are still suffering the effects of acid rain and show few signs of recovery.

Acid rain forms when sulfur dioxide and nitrogen oxides react with water, oxygen, and other chemicals in clouds to form weak sulfuric acid and nitric acid solutions. Although these acid-forming chemicals occur in nature ? for example volcanic eruptions and forest fires release sulfur dioxide ? natural processes account for less than 10 percent of their current levels. The rest is primarily the result of power plants burning fossil fuels and vehicle exhaust.

Rainwater is naturally slightly acidic, with a pH of about 5.6. But the pH of rain in some areas of the eastern U.S. has measured as low as 4.3. (Remember that in the pH scale, a compound with a pH of 4 is ten times as acidic as one with a pH of 5.)

Scientists first discovered the damaging effects of acid rain in the early 1970s, at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. Hubbard Brook was also one of the sites studied for the 2001 report. Fish have died out in lakes and streams because they could not survive in the acidic water. In the thin mountain soil, acid rain has also depleted nutrients, wiping out an estimated 25 percent of the red spruce trees in the region.

The Clean Air Act and its 1990 amendment have yielded some positive results; sulfur deposits are down 38 percent from their 1990 levels. However, nitrogen levels have not declined significantly since the new emissions standards took effect. Researchers have also found that the environmental impact of acid rain is more complex than originally thought.

Minerals such as calcium and magnesium normally help neutralize acids in soils, much the way antacids relieve stomach upsets. But over time these minerals have been depleted, allowing acids to accumulate. In addition, the loss of these nutrients leaves trees more vulnerable to insect damage and cold weather. The red spruce and sugar maple ? source of maple syrup ? are especially threatened by soil depletion.

Meanwhile, some 41 percent of lakes in New York’s Adirondack Mountains, and 15 percent of lakes in New England, exhibit either chronic or occasional acidification. Although the disappearance of fish from these lakes receives the most attention, the acidic waters can affect entire food webs.

Utility company officials say it’s too early to conclude that the current emissions standards are inadequate. But the Hubbard Brook scientists say the most sensitive habitats need more immediate attention. They have called for an additional 80 percent decrease in sulfur emissions in order to achieve ecosystem recovery in the next 20 to 25 years.

Said Gene E. Likens, one of the report’s authors and director of the Institute of Ecosystem Studies in New York, “In 1990, the businesses, politicians, and public took a collective sigh of relief and said ‘that problem is over,’ and it’s not.”

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