FROM THE October Fire Log:

A fire in a Gaithersburg townhouse killed a young man and sent another to the hospital.

In the Gunston area of Fairfax County a malfunctioning furnace burned a house to the ground. The occupants managed to escape.

A postman delivering mail on North Columbus Street heard an alarm and saved the life of an elderly man whose house was burning.

The uncommon denominator here is smoke detectors. Some had them and some did not. Those who did, lived. Those who did not, died.

Area fire officials say smoke detectors, more than anything else, are saving the lives and property of people who have installed them. In Montgomery County, says Earle B. Poole of the fire marshal's office, the little device may be responsible for a 50 percent drop in the number of deaths caused by home fires. The county averaged 12 deaths each year for the past 17 years. This year, that figure stands at five.

And at the other end, officials bemoan the lives lost needlessly. "We've had three deaths (since the beginning of the year) where a smoke detector would have made a difference," said Randolph Kirby, the fire marshal in Alexandria.

Yet despite the tales of woe (some 6,000 persons died in home fires nationally last year) and despite robust sales figures (Americans gobble up some 12 million smoke detectors annually), not everyone has bothered to go out and get one. "We haven't even scratched the surface," says Ronald Peck, deputy chief fire marshal in Fairfax County.

With winter coming on, windows batted down and furnaces fired up, the dangers of fire and smoke inhalation will soon be at their peak. (It is usually the toxic gases produced by fire, not the fire itself, that kill. The majority of deaths occur at night, when people are asleep.)

There are two types of detectors on the market (though that may be changing). One is called "ionization," the other "photoelectric." The ionization detector uses a radioactive element that creates a field sensitive to smoke particles. The photoelectric model is activated when smoke causes light to be reflected on a sensor.

More than a hundred such models are available just about anywhere household goods are sold - from Dart Drugs to Sears to Hechinger's. And, as with any consumer item, there has been no end to the discussion of which type is best and what consumers should look out for.

Whatever the pros and cons, however, playing with the risks of fire is a fool's game. Every home should have at least one smoke detector. In some places every home must have one.

It is now law in Montgomery and Prince George's counties that all dwellings have detectors. The District of Columbia has given owners three years to put them everywhere people live. In Virginia all new residences must have them.

Those still without can count on an investment ranging from as little as $10-$20 for the simplest battery units to $40-$50 for models that run on household current to $100-$150 or more for centralized systems installed by experts.

Which type of detector to install has been debated. In the past, consumer groups, such as Consumers Union (which publishes Consumer Reports) advocated installing one of each type for maximum safety - this because the ionization model is more sensitive to flash fires that give off little smoke, while the photoelectric variety is quicker to pick up the particles emitted by smoldering fires.

(In its most recent story on smoke detectors, May 1977, Consumer Reports rated the Guardian FBI, the Norelco HB0933 and the Sunbeam 4521 as best among ionization models; Captain Kelly and Nutone for photoelectric. There have been a number of new models since then.)

But authorities such as Richard Bright, a research engineer at the National Bureau of Standards, and the Consumer Product Safety Commission (CPSC), regard both the photoelectric and ionization types as equally effective. Bright says the "technology is moving so fast" that it may be "wise to replace (your) detectors every two or three years with the new models." (The CPSC plans a major study, over several years, to determine how effective smoke detectors are and whether either type is, in fact, better than the other.)

There are major differences between battery operated models and AC units that hook up directly with the house current. Batteries have to be changed. "Think permanent," says Bright. (Some have argued that wired units can be knocked out in an electrical fire. Other experts, however, see that possibility as slim. The Insurance Information Institute (III) urges a battery unit in homes that are frequently without power during storms, or models that carry a battery as back-up.)

Older models use 12-volt batteries that can cost up to $7. The newer detectors take a 9-volt alkaline battery that goes for a little over a dollar. Batteries last anywhere from one year, in a home without air conditioning, to two years or more in a home that maintains constant temperature and himidity levels.

If a battery-operated detector is a must (as it may well be for some apartment dwellers), it should give an audiblie signal when the battery is wearing down.

One difference between the ionization and photoelectric detectors is the life of the sensing device. The life of the radioactive element in the ionization detector is said to be around 450 years - just about permanent. The photoelectric models have been carrying an incandescent bulb that can burn out within three to five years, says Peter Bocknak of the National Fire Protection Association (NFPA).Look for newer models that use light-emitting diodes which, says Bocknak, "shouldn't need replacing." Those with bulds should signal the owner when the light is dying.

At least two major manufacturers of smoke detectors - Wells Fargo and Pittway - are coming out with a unit that has both features.

The new Wells Fargo model is presently listed at around $40, the Pittway around $55. But Bright expects those figures to come down, as sometimes happens in the electronics business. Wells Fargo and Pittway both have their new model at the UL (Underwriter's Laboratories) for testing. As of mid-October reports had not been issued, but company spokesman say they expect their detectors to be in major retailers available to the public sometime after the beginning of the year. ("All the people taking shots at us can't take shots at us any more," said one such spokesman, who asked not to be identified.)

Most detectors are easy to install, though Bright recommends calling in an electrician to mount cordless AC models. The NFPA, which issues guidelines on smoke detector use, urges one device on each floor. In Montgomery County, the law demands a smoke detector at each stairwell leading to an occupied area.

One detector should be located in the hallway outside the bedroom area; one in the living room, if the house has more than one level; and one at the bottom of the stairs in the basement.

Smoke detectors should not be placed near an air register in homes that have forced air heating or too near an air conditioner. A smoke detector is a lame duck if it sits near a fan that blows the smoke away. Bochnak says they should be at least four inches away from corners. Though detectors can be mounted on the wall, he says the ceiling is preferable.

And if someone you know is already fed up with his detector because it screams at the first sniff of broiling lamb chops, that person probably has it too close to the cook stove. Smoke detectors should be kept out of the kitchen and away from fireplaces.

As important as buying a detector is keeping it in shape. There is no lack of stories about fires in homes that had a detector that was just so much plastic when a fire broke out because the owner neglected to clean it or change the batteries. Accumulated dirt and dust in the detector is another reason for false alarms.

The one you buy should have maintenance instructions which, says Peck, "should be followed religiously." Regular cleaning, as with a vacuum cleaner, is a must. All need regular testing.

Battery-operated models require weekly testing (preferably by blowing smoke into the detector) and whenever you return from an extended vacation, says, Bright.Detectors that operate off house current should be checked every month.

Buying a detector could also mean a break on your insurance costs. Bruce Butterfield of the III says a spot check of area fire insurers revealed a "widespread" practice of offering discounts to home owners who install smoke detectors. Butterfield said that credit starts at about 2 percent on homes using the simplest system, but can range up to 10 percent on larger homes with more sophisticated, centrally installed and professionally maintained systems.

And if you are concerned about the dangers of leaking gas, look for new gas detectors (Hechinger's is one store that carries them).

Whichever smoke detector you buy, is should carry either the UL or the FM (Factory Mutual) label. If you have a difficult floor plan, or just can't figure out the directions, call your local fire house or: 691-2331 in Fairfax County; 558-2481 in Arlington; 750-6650 in Alexandria; 779-3850 in Prince George's; 468-4153 in Montgomery; and 745-2250 in the District.