A Johns Hopkins University study of Montgomery and Fairfax counties has confirmed what firefighters have been saying for years: Smoke detectors can save lives.

The two-year study, recently released, asserts that as more homeowners equip their residences with smoke detectors, fatal fires decrease.

In studying home-fire deaths from 1972 to 1983, researchers found that over the years, fatalities were cut nearly in half in Montgomery and were down by more than a fourth in Fairfax.

Six persons died in Montgomery home fires last year and four in Fairfax.

But smoke detectors are not a panacea for home-fire deaths, fire officials in the suburban counties said. Not enough detectors are being properly installed and maintained, they said, and families are not discussing or practicing escape plans.

Montgomery requires that all homes have detectors; Fairfax requires only that a home be equipped with a detector by the time a new owner or occupant moves in.

Although the Johns Hopkins survey of 500 households in Montgomery County indicated that 82 percent had at least one working smoke detector, only in 42 percent of those instances were the devices in the right place or installed in sufficient numbers to comply with the county law, adopted in 1978.

"That disturbs me a lot," said Mary Marchone, a fire education specialist with the Montgomery County Fire and Rescue Service who helped coordinate surveys for the study. "A lot of that has to do with the law being difficult to understand. People knew we had a law but didn't know the wording of it."

Marchone also is concerned that only 4 percent of the Montgomery households surveyed had practiced escape plans, while 25 percent had discussed them.

"People think they're completely safe if they have a smoke detector, but a smoke detector is just a noise," Marchone said. "We asked people if they have a plan to escape after the detector goes off, and they say, 'Yes, we have a plan.' And we ask, 'Well, what is it?' And they say 'Get out!'

"They don't have provisions for the children. They don't have a second way out of the house. They don't have a meeting place outside the house. They don't even know who is going to call the fire department."

Sgt. Larry Hanger, of the Fairfax County fire marshal's office, added that in his county, families interviewed said they would "just go out the window" if the door is blocked. "But they never try to open the window before hand," he said. "Some windows may have been painted shut, either on purpose or by accident. Maybe an adult male can open it in a panic situation, but can the kids?"

The Johns Hopkins School of Public Health studied Montgomery because county fire officials were interested in seeing if the county's law had reduced the number of fire deaths. For purposes of comparison, researchers from the Baltimore university selected Fairfax County, a jurisdiction that is similar to Montgomery in population, social and economic statistics, but which has a much less stringent smoke detector law.

The decline in fire deaths in both counties since the early 1970s adds credence to the contention that the more smoke detectors the better, Montgomery officials said. All houses and apartments in the county must have smoke detectors near bedrooms and near stairways, but fire officials are pushing for a revision in the county code that would require detectors on each floor.

Meanwhile, the preliminary results of the study convinced the Fairfax County Board of Supervisors in March that a change was needed. The county now requires that smoke detectors be installed near sleeping areas and in the basement of all residences at the time of a sale, lease or other change of occupancy. Before the revision, Fairfax County required smoke detectors only in new homes, in accordance with Virginia law.

"That still means if you've lived in your home for 20 years, you don't have to have one unless you sell your home or change occupancy. But it's a step in the right direction," said Pam Weiger of the Fairfax County fire marshal's office.

A 1985 Lou Harris poll indicated that 75 percent of households in the United States are equipped with smoke detectors, up from 20 percent in 1978, according to the National Safety Council.

The National Fire Prevention Association recently reported that nationally, deaths from home fires declined by 14 percent last year to 5,240, the largest annual drop recorded.

John Hall, director of NFPA's fire analysis division, said smoke detectors are one reason for the decline, and said that three-fourths of the homes in the United States have at least one of the devices. "Those who do not have them seem to have twice as great a chance of dying in a fire," he said.

Marchone said that, after seven years in which only warnings have been issued, the fire department plans to begin cracking down on homeowners who fail to comply by fining them up to $250.

"People don't understand. You just don't put up a detector and forget about it," Marchone said. "Detectors have to be tested and cleaned every month. The batteries have to be changed about once a year."

She said there will not be a door-to-door campaign to monitor compliance. Detectors will be checked during changes of occupancy and after fires, she said.