Northern Virginia motorists will do their bit to clean up the nation's polluted air starting Jan. 1, when mandatory vehicle emissions tests begin in the area.

The tailpipe exhaust tests, required annually beginning in 1982 by federal and state law, will cost $3.50 each and take about five minutes.

But Virginia State Police officials predict that one of every four vehicles -- or 125,000 -- could flunk the test the first time around, which could mean more time and more money for the owners.

As the program is organized in Virginia, motorists will not be able to renew their vehicle registrations until their cars pass the emissions inspection or they're given a waiver from a service station indicating they have spent at least $75 to fix pollution-causing problems in their vehicles.

"This program is not designed to create any undue hardships," said State Police Sgt. John A. Bowden, program administrator. "Any motorist who has reasonably maintained his vehicle will not have any problems. We're out to clean up the air with minimal discomfort to the public."

The emissions tests, which Bowden said will be available at approximately 100 Northern Virginia safety inspection stations by Jan. 1, are mandated under the federal Clean Air Act. The law requires the inspections in 36 metropolitan areas around the country where there is excessive carbon monoxide or ozone (the main component of smog) in the atmosphere.

Northern Virginia will be the first area in the Washington region to start the tests, but the District of Columbia and Maryland suburbs will follow a year later. The other jurisdictions were given more time, Bowden said, because they plan to construct special facilities for the tests, instead of using existing inspection stations.

Nearly 500,000 cars and light trucks manufactured in the last eight model years and registered in Fairfax, Arlington and Prince William counties, Alexandria, Fairfax City, Falls Church, Manassas and Manassas Park must undergo the test in 1982, according to Paula Kripaitis, spokeswoman for the Division of Motor Vehicles. Loudoun County is excluded because it is less densely populated, officials said, and other sections of Virginia do not need to offer the tests because their ozone levels satisfy U.S. Environmental Protection Agency standards.

To conduct the test, the inspector inserts an infrared probe into the tailpipe while the vehicle engine is idling. The probe measures the amount of carbon monoxide and ozone-forming hydrocarbons in the exhaust. Heavy concentrations of those pollutants can be harmful to human health and can damage crops and buildings, officials said.

The Virginia state legislature passed a law in 1980 setting up the inspection program, but not without resistance from lawmakers who felt the federal government had no business ordering such tests. The legislation finally passed after the state was threatened with the loss of more than $100 million in federal highway and sewer funds -- a penalty provided for in the Clean Air Act.

Even as Virginia starts the inspections, however, Congress is tinkering with the Clean Air Act. If Congress relaxes some of the air quality rules, the nationwide emissions inspection program could be scrapped or made optional.

Kenneth Van Auken, director of planning for the State Air Pollution Control Board, said the Environmental Protection Agency has never shown state officials evidence of a direct correlation between vehicle inspections and a reduction in ozone levels. EPA officials maintain, however, that if all cars and light trucks nationwide were inspected, hydrocarbon and carbon monoxide emissions would drop 25 to 35 percent by 1987.

When the Virginia legislature debated the issue two years ago, representatives of state automobile clubs were among the most vocal critics of the program, and they remain so today.

"We oppose the emissions inspection and maintenance program because we think it is too costly with too little return," said Stokes Grymes, community affairs director of the Automobile Club of Virginia. "We feel the automobile owner is forced to do a whole lot of things that we're not sure are going to have much effect on the quality of the air."

While State Police Lt. Jerry Conner, manager of the inspections program, said that as many as 25 percent of the vehicles tested could fail the inspection, based on rejection rates in other states, he said most cars will need only minor carburetor adjustments or simple tuneups to lower emissions. In most states now offering emissions tests, average repair costs range from $25 to $40, Sgt. Bowden said.

As a protection against exorbitant costs, the Virginia law sets a $75 ceiling on the amount of repair work required after a vehicle fails the inspection, Bowden said. If a car owner has spent $75 and the problem still is not fixed, he can receive a waiver from an inspection station -- and will not have to have the vehicle inspected for emissions again until it's sold or transferred.

Bowden said an exception would be made for owners who have tampered with pollution-control equipment on their cars, however, such as removing catalytic converters, the exhaust-cleansing devices in most new cars. In such cases, motorists may be required to spend hundreds of dollars before they can renew their vehicle registrations and license plates, he said.

Another consumer protection built into the program is the requirement that each inspection station be licensed by state police and employ at least one mechanic who has passed a special certification course, separate from the safety inspection course, Bowden said. He said the state police will operate three "referee vans" to check the test results of motorists who disagree with the findings of an inspection station.

By Jan. 1, or possibly sooner, the emissions tests will be available at about 100 of the 450 Northern Virginia service stations and new car dealers that now offer semi-annual safety inspections, Bowden said. State police hope that another 300 stations will offer the test by the end of next year, he said.

Lt. Conner said he thinks some station managers are hesitant about participating in the program because they believe Congress might eliminate the requirement for inspections and they don't want to invest in the equipment needed to do the tests. Station managers said the infrared analyzer needed to measure hydrocarbon and carbon monoxide levels in vehicle exhaust could cost as much as $4,000.

While some station owners said they signed up for the program as a convenience for customers who ordinarily come in for safety checks, others said they fear the emissions tests will only create animosity.

"We decided not to do it," said Henry Seay of Jim's Gulf Service in Arlington. "If you turn 'em down on this, they won't come back. You make a lot of your customers mad."

Kripaitis, the DVM spokeswoman, said the emissions tests will be staggered throughout the year and motorists will be encouraged to take the test around the time of their vehicle registration renewal. Motorists can ask for the test for an extra $3.50 when they receive their safety inspections, however, and the results would apply when they renew their registrations, she said.

A special windshield decal, separate from the safety decal, will be issued to identify vehicles which pass the emissions test.

The first group of motorists required to take the test is those whose vehicle registrations expire Jan. 31. Kripaitis said notices of the tests will be mailed in the next few days to the affected 32,800 Northern Virginians, along with their January renewal notices.