Safety information on the use of unvented, portable kerosene heaters, which were recently permitted for use in one- and two-family homes in Maryland, has been issued by Montgomery County fire prevention officials.

Such heaters are not allowed in facilities such as stores, schools, day-care centers, boarding homes, places of public assembly or nursing homes.

Fire officials urge consumers to read and carefully follow manufacturer's instructions and precautions concerning operation, placement, fueling and maintenance.

Consumers should use only pure or "water-clear" kerosene, which has the transparency of water. Use of "yellow" or otherwise contaminated kerosene can cause the wick to gum up, resulting in incomplete combustion, smoke odors and other harmful byproducts, fire officials say.

Consumers are advised to leave a window or door to the outside open at least three square feet to prevent possible asphyxiation, especially in tightly weatherized homes. They also are warned to never use gasoline in the units, since an explosion could result.

The heater also should be kept at least three feet away from any furniture, drapes, decorations or walls, fire officials say. Flammable liquids, papers and clothes should be removed from the area.

Storage containers for kerosene, which should be stored outside the home, must be of metal or heavy-duty plastic. All containers one gallon or less in size must have a child restraint cap. The use of food-type plastic or glass containers is forbidden.

In addition, kerosene containers must carry the following warning label, prescribed by the State Department of Health and Mental Hygiene: "Danger: Kerosene; petroleum distillate. Harmful or fatal if swallowed. Keep away from heat, sparks, or fire. If swallowed, do not induce vomiting; call a physician immediately. Use with adequate ventilation; avoid prolonged contact with skin. Keep out of reach of children."

Heaters should be refilled only out of ors and only when they are cool to the touch. Fuel vapors produced from the hot surfaces are explosive, fire officials warn. When refilling, do not top the tank or overfill. Allow for expansion of the liquid.

The kerosene should be strained through a filter each time it is transferred to ensure no dirt or other particles foul the heater unit.

Kerosene or any liquid fuel should never be transferred in the presence of any open flame, lit smoking materials, welding, metal cutting or heat-producing operation.

When transporting kerosene in a vehicle, fire officials say, never transport more than five gallons at a time and keep it in a vented space. If it is in the passenger compartment, keep the windows open. If it is in the trunk, prop the trunk lid partially open.

Kerosene heaters should not be operated unattended or while occupants are sleeping, fire officials say. They also advise owners of the units to practice a home fire escape plan and install smoke detectors.