Ronald Reagan attacked the Environmental Protection Agency and the Clean Air Act yesterday as obstacles to industrial growth, promising to reorient both of them to help the coal and steel industries.

And earlier, he repeated his belief that nature is the chief air polluter, saying that Mount St. Helens has produced more sulfur dioxide than 10 years of American automobiles "or things of that nature."

EPA officials jumped on the remark, pointing out that automobiles do not produce much sulfure dioxide and are not regulated for it. Public utilities, which do face stringent EPA rules for sulfur dioxide emissions, have put out 200 million tons of it over the last decade, compared with perhaps 400,000 tons from Mount St. Helens so far, officials said.

The Republican presidential nominee, speaking to coal and steel workers in Youngstown, Ohio, said air pollution has now been "substantially controlled" and that President Carter's regulatory policies are responsible for closed mills in the area.

"EPA appointees apparently don't know and don't care about coal," Reagan said. He promised that, as president, he would "turn to your industries for help in coming up with reasonable rules," and that he would "see to it that EPA has leaders who know and care about the coal industry."

The Clean Air Act was written based on evidence available in 1970, Reagan said, "But these 1970 rules have helped force factories to shut down and cost workers their jobs . . . and they will certainly slow the use of coal." He said that as president he would "thoroughly review these regulations to bring them in line with all new scientific knowledge."

Reagan's remarks about Mount St. Helens came during one of his increasingly rare rambling, off-the-cuff speeches late Tuesday in Steubenville, Ohio. He also said "some doctors" are investigating the possibility that oxides of nitrogen "might be beneficial to tubercular patients." He defended his previous statement that trees are responsible for 93 percent of the nation's nitrogen oxides.

Reagan then added that winds blowing over natural oil seepage in the Santa Barbara, Calif., ship channel were advertised at the turn of the century as having "purified the air and prevented the spread of infectious diseases."

EPA Administrator Douglas Costle said Reagan apparently had mixed up nitrogen dioxide, a regulated pollutant, with nitrous oxide, a natural product of plant respiration. "The fact is that trees simply do not emit nitrogen dioxide, which is a public health concern. They emit nitrous oxides, which are not regulated," Costle said.

Reagan, he said, "has once again fumbled the facts" on both cars and trees.

Automobiles are regulated for "an entirely different kind of air pollution" than Reagan mentioned, Costle said: particles, nitrogen oxides and hydrocarbons.

William Lewis, executive director of the National Commission on Air Quality, said Reagan's statement was "one of the more preposterous things I've ever heard." It constituted, he said, "either an intention to mislead or a misstatement."

He added that the commission's report on its examination of the Clean Air Act, expected in March, will include recommendations for changes. "But I don't anticipate there will be any proposals for major restructuring of the act," he said. "The basic principles of the program are generally desirable."

Reagan told the Youngstown audience, in remarks prepared in advance, that he would seek to replace federal regulations that require certain procedures with performance standards that are more flexible. He would require a cost-benefit analysis of each proposed new regulation, add automatic expiration dates to them all, give Congress and the president more authority to veto agency regulations and review them in light of new technical knowledge, he said.

Edward Tuerk, EPA's acting assistant adminstrator for air quality, said most of those proposals already were being implemented.

While there has been "material progress" in cleaning up the air, he said, "much still remains to be done" in many areas such as Los Angeles and most of the East Coast where the air is still not clean.