By District of Columbia law, owners of public and private residential buildings in the District have until October 1981 to install smoke detectors, but the city Fire Department strongly recommends that residents don't wait that long.

"I suggest that people buy smoke detectors as soon as possible - you know, like yesterday," said firefighter Michael Smith of the department's community relations office.

Conservative estimates are that "at least 40 percent of the lives lost in fires" could have been saved with the use of smoke detectors, Smith said. "I personally think the percentage is much higher."

Under a law passed last year by the City Council, homes, apartments, hospitals, hotels, motels, nursing homes and other residential and custodial care facilities are required to install smoke detectors by October 1981. Buildings in these categories constructed or issued building permits since October 1978, must already have detectors. Facilities, including jails and prisons owned by the District government, must have smoke detectors by October 1980.

The law specifies that hotels and motels must install detectors in each guest room or guest suite. Hospitals, nursing homes, prisons, jails and custodial care facilities can install them in hallways adjacent to rooms used for sleeping or in the rooms themselves.

Detectors in new buildings must be wired directly to the power supply of the building. Old buildings may have the detectors plugged into electric outlets or wired into the power supply.

Owners of single-family homes and condominiums may use either electric or battery-operated smoke detectors. The law states that detectors must be placed in the vicinity of any rooms used for sleeping.

The D.C. Fire Department, however, recommends that detectors be installed on every level of a house.

The landlord is required to install detectors in rental apartments, and the management is required to install them in cooperative apartments.

Those not complying with the law by Oct. 1, 1981, will face 10 days in jail or a $300 fine.

Consumers will find two basic types of detectors on the market. One uses ionization to detect smoke and trigger the alarm. The air is ionized by a radioactive material, but Smith said that tests have shown the level of radiation is so minute that it does not pose a hazard.

The U.S. Consumer Product Safety Commission says that tests performed by the Nuclear Regulatory Commission shwoed that "if you held an ionization smoke detector close to you for eight hours a day through a whole year, you would receive only a tenth as much radiation as you get on one round trip airline flight across the USA."

However, Ralph Nader's Public Citizens Health Research Group flatly recommends against ionization smoke detectors. Sidney Wolfe, of the research group says that although the risks of ionization detectors may be "minimal in the home, the research group is concerned about the radiation level workers may be exposed to in the manufacturing process.

Disposal is another problem. "If 5 or 10 million (used ionization) detectors are thrown in the trash, a lot of radiation could leak out," Wolfe says. He adds that "it is just dumb to purchase the ionization smoke detectors when the photoelectric type work just as well."

Photoelectric detectors reflect light from smoke particles onto a photocell, which in turn sets off an electric current and triggers the alarm.

Ionization smoke detectors are very sensitive to particles produced by blazing fires, while the photoelectric types detect smoke more easily. But Smith siad that the difference between the two types in reacting to flames or smoke is very slight, and that both are effective.

Detectors range in price from $8 to $200. Some of the more elaborate systems can be hooked into a neighbor's house while you are away. If you can't decide between a battery-powered or an electric smoke detector, you can now purchase one that runs on electricity, but is automatically transferred to a battery if your power supply goes out.

Any smoke detector should bear a label of approval by either Underwriter Laboratories or Factory Mutual Laboratory.

Smith offers these suggestions to home and apartment dwellers for installing and maintaining smoke detectors:

Place them in a central location in the sleeping area, either on the ceiling or the wall.

Detectors placed on the ceiling should be at least six inches away from the wall.

Wall detectors should be placed between six and 12 inches from the ceiling.

Do not install them in the path of an air register, fan or air conditioner.

Do not use an extension cord, but plug the smoke detector directly into an outlet.

Do not install a smoke detector within eight inches of a light source.

Check the smoke detector once a month by blowing on it or by striking a match near it. At the same time, dust or vacuum it.

Replace the batteries at least once a year even if they still seem to be operating.

Smith said that escape plans are as important as smoke detectors. He offered this advice:

If you don't have a porch roof you can crawl out on, place portable ladders or heavy knotted ropes near a window (and not in closets where you can't find them). Make sure escape windows open freely.

If you are in your bedroom when a fire starts, don't open the door if the door is hot. If it's cool to the touch open it slightly, but if hot air and smoke hits you, quickly shut the door. Either telephone the fire department, or yell out your bedroom window for help, or go down the rope or ladder.

If you are able to get out of your house or apartment safely, don't stop to pick up mementos, but head for a telephone in the closest house or apartment to call for help.

"You must practice," Smith says. "If you wake up groggy and there's a fire, you're going to be confused or panicked if you haven't practiced ahead of time."

D.C. residents seeking more information about smoke detectors can contact the D.C. Consumer Protection Office at 727-1158 or can call the Consumer Product Safety Commission at 800/638-8326 to get its booklet on "What You Should Know About Smoke Detectors."

The Community Relations Office at the D.C. Fire Department, 745-2347, can provide information on how and where to install smoke detectors. You can make an appointment through the office to have a firefighter visit your home to make a safety survey.

Meanwhile, Smith says, "The bottom line is to get that smoke detector."