After setting off a panic among state officials nationwide last week, the Environmental Protection Agency has scaled down substantially the number of counties it plans to punish for failing to clean up their air by New Year's Day.

Fewer than 150 counties, not the 471 originally estimated, are likely to be targeted for bans on new construction or the loss of federal aid under the Clean Air Act, several EPA officials said yesterday.

The government plans to publish the slimmed-down list of violators next month, but the confusion over the federal government's tactics in enforcing the complex air pollution law is expected to live on.

The havoc reached a peak last week when the EPA named the 471 counties deemed too polluted to meet the Clean Air Act's Dec. 31 deadlines. The list was publicized in news reports across the country, alarming officials in dozens of counties that had cleaned up their air as long as four years ago.

EPA officials, flustered by the furor, explained that the list was preliminary, based on data gathered in 1978, and that only 144 counties are likely to face cutoffs once more data is studied. But they reasserted that the Clean Air Act "mandates" the sanctions, and yesterday specified how the localities would be punished.

Meanwhile, members of Congress and beleaguered local officials continue to insist that the administration is overstating the threat of penalties in hopes of scaring Congress into weakening the law next year.

"I'm not going to panic because I think they're nuts," said George P. Ferreri, director of Maryland's clean air program. "We can't figure out what they're doing. First they rattle everyone's cage saying all this god-awful stuff is going to befall you because you haven't met the standards. But up until a few weeks ago, they were telling us to hold off because they might change one of the standards."

Certain areas of Baltimore and Baltimore County are on the list for failing to reduce dust and dirt particles to healthy levels, Ferreri said. But the EPA is expected to propose changes in those requirements, and Ferreri said he was instructed several months ago to "hold off" on the problem until the issue is resolved.

In Virginia, 13 localities were on the list for ozone pollution, although state officials contend that all but three -- Chesterfield and Henrico counties and the city of Richmond--have met the requirements or have obtained waivers until 1987. The District of Columbia also has a waiver until 1987 for reducing its ozone and carbon monoxide levels.

In outlining their penalties, EPA officials promised to apply sanctions in ways that would "minimize economic disruption." Early next year, the agency will publish in the Federal Register a list of the areas that have not met standards for nitrogen dioxide, sulfur dioxide, particulates, ozone and carbon monoxide.

The public will be given several weeks to file challenges to the list and to the EPA's proposed sanctions for each area. If the EPA proceeds to cut off federal aid, several governors have indicated they will file lawsuits challenging the move.

Sen. Robert T. Stafford (R-Vt.), chairman of the committee that handles the Clean Air Act, has argued that the EPA is not forced to impose sanctions on areas with polluted air, as long as the state government is making a good faith cleanup effort.

However, EPA Assistant Administrator Kathleen Bennett said yesterday the law is too rigid on sanctions and other matters. "The Clean Air Act remains in immediate need of updating . . . ," she said. "We cannot interpret the law in a way that would clearly rewrite years of legislative and legal history, and ignore the act's mandatory sanctions."