A PASS device also known as an ADSU (Automatic Distress Signal Unit), is a personal safety device used primarily by firefighter

Firefighter

Firefighters are rescuers extensively trained primarily to put out hazardous fires that threaten civilian populations and property, to rescue people from car incidents, collapsed and burning buildings and other such situations...

s entering a hazardous (IDLH) environment such as a burning building, which sounds a loud audible

Audible

Audible may refer to:* Sound that is capable of being heard* Audible, a tactic used by quarterbacks in American football to change a play at the line of scrimmage* Audible.com, an online audiobook store-See also:*Audio...

 alert to notify others in the area that the firefighter is in distress

Distress

Distress may refer to:*Distress , occurring when an individual cannot adapt to stress*Suffering*Distress signal, an internationally recognized means for obtaining help...

.

The PASS device will automatically activate if the device does not detect motion for a certain short period of time, typically 15-30 seconds, so that the alert will sound if the firefighter is seriously injured or otherwise incapacitated. Some devices use a ball bearing on a track to sense firefighter movement and others utilize an infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 beam against a mirror mounted on a spring. When activating for a lack of motion, the PASS device will typically emit a few seconds of a muted warning that activation is about to occur, so that a firefighter who has simply been motionless for a time but is otherwise safe will be able to move slightly and thus reset the activation timer before a false activation occurs. The PASS device can also be activated manually in an emergency, such as when a firefighter is lost or trapped.

While older models of PASS devices required manual arming by firefighters prior to entering a dangerous environment, the current application integrates the PASS device into the self-contained breathing apparatus (SCBA) worn by firefighters so that it automatically arms when the SCBA air supply is engaged or when the SCBA is removed from its mounting bracket. These style devices are powered by battery

Battery (electricity)

An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

, are easily activated while wearing gloves, and are intrinsically safe to operate in flammable or explosive atmospheres.

When activated, according to OSHA

Occupational Safety and Health Administration

The United States Occupational Safety and Health Administration is an agency of the United States Department of Labor. It was created by Congress of the United States under the Occupational Safety and Health Act, signed by President Richard M. Nixon, on December 29, 1970...

 standards which apply in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, the PASS device emits a high-pitched audible alert of at least 95 decibel

Decibel

The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...

s. On a fireground, the sound of an activated PASS device indicates a true emergency

Emergency

An emergency is a situation that poses an immediate risk to health, life, property or environment. Most emergencies require urgent intervention to prevent a worsening of the situation, although in some situations, mitigation may not be possible and agencies may only be able to offer palliative...

 and results in an immediate response to rescue

Rescue

Rescue refers to responsive operations that usually involve the saving of life, or prevention of injury during an incident or dangerous situation....

the firefighter(s) in distress.