An all-clear is a term used by civil defense

Civil defense

Civil defense, civil defence or civil protection is an effort to protect the citizens of a state from military attack. It uses the principles of emergency operations: prevention, mitigation, preparation, response, or emergency evacuation, and recovery...

 or military authorities to inform others that an imminent physical danger has passed. This typically will end an alert

Alert

To be alert is to be in a state of alertness.Alert or ALERT may also refer to:-Places:* Alert, Indiana, a community in the United States* Alert, Nunavut, Canada, the northernmost permanently inhabited place in the world...

 or warning

Warning

Warning may refer to:- Signal :* Precautionary statement* Warning sign* Warning system* Warning , issued by a police officer in lieu of a citation following a traffic stop- Music :* Warning...

 status that was previously issued. The all-clear can be given vocally, through media — such as television

Television

Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

 or radio

Radio

Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

, or via means of civil defense broadcast systems such as air raid

Civil defense siren

A civil defense siren is a mechanical or electronic device for generating sound to...

 and tornado sirens.

The term gained notoriety during World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

 when deadly chemical agents were deployed on the battlefield

Battle

Generally, a battle is a conceptual component in the hierarchy of combat in warfare between two or more armed forces, or combatants. In a battle, each combatant will seek to defeat the others, with defeat determined by the conditions of a military campaign...

 — the all-clear would be given once the area was free of contamination

Contamination

Contamination is the presence of a minor and unwanted constituent in material, physical body, natural environment, at a workplace, etc.-Specifics:"Contamination" also has more specific meanings in science:...

. Since this time, the all-clear has been used to notify that conventional

Conventional warfare

Conventional warfare is a form of warfare conducted byusing conventional military weapons and battlefield tactics between two or more states in open confrontation. The forces on each side are well-defined, and fight using weapons that primarily target the opposing army...

, nuclear

Nuclear warfare

Nuclear warfare, or atomic warfare, is a military conflict or political strategy in which nuclear weaponry is detonated on an opponent. Compared to conventional warfare, nuclear warfare can be vastly more destructive in range and extent of damage...

, biological

Biological warfare

Biological warfare is the use of biological toxins or infectious agents such as bacteria, viruses, and fungi with intent to kill or incapacitate humans, animals or plants as an act of war...

, chemical

Chemical warfare

Chemical warfare involves using the toxic properties of chemical substances as weapons. This type of warfare is distinct from Nuclear warfare and Biological warfare, which together make up NBC, the military acronym for Nuclear, Biological, and Chemical...

, or environmental threats are no longer of immediate concern

Worry

Worry is thoughts, images and emotions of a negative nature in whichmental attempts are made to avoid anticipated potential threats. As an emotion it is experienced as anxiety or concern about a real or imagined issue, usually personal issues such as health or finances or broader ones such as...

.