Penobscot McCrum will test the ammonia warning siren at its Pierce Street potato processing factory today at 1 p.m.

The siren will emit a long steady tone and a pulsing tone. Each tone may last up to three minutes. Tests are conducted the third Wednesday of every month at 1 p.m.

The siren is meant to warn the public in the event of a leak in the factory's refrigeration system, which uses the poisonous gas anhydrous ammonia.

Anhydrous ammonia is common in commercial refrigeration systems. In Belfast, it is used at two facilities, Penobscot McCrum and Midcoast Frozen Foods on Northport Avenue (Route 1).

The gas can cause chemical burns to skin, eyes and lungs. It is attracted to water and migrates to moist areas, including eyes, mouth, nose, throat and moist skin.

If released, anhydrous ammonia rapidly absorbs moisture from the air and forms a dense, visible white cloud. The gas tends to travel along the ground on a cool day.

The gas does not always form a cloud but can be detected by its pungent odor.

In low concentrations, between 5 and 50 parts per million, it can cause headaches, loss of sense of smell, nausea and vomiting. Higher concentrations can cause irritation to the nose, mouth and throat, as well as coughing and wheezing. Concentrations of 300 to 500 ppm are life-threatening.

Ammonia is also flammable in high concentrations, such as a release inside a building.

What to do in the event of a real ammonia release

Residents who hear the siren at times other than the scheduled monthly tests should take the following actions:

• Call 911.

• Shelter in place in your home or apartment. This means move or stay indoors, close all windows and doors, turn off heating and air conditioning appliances. Go to an interior room with the fewest doors and windows. Take your telephone. If possible, place a wet towel at the bottom of the door. Stay there and tune in to TV and radio stations for further information. Keep pets with you, along with their food and water.

• If there is a need to evacuate the area, emergency officials will notify you and direct you to go to a certain place. Otherwise, stay where you are and wait for an all-clear announcement.