Less-lethal weapons, less-than-lethal weapons, non-lethal weapons, non-deadly weapons, pain-inducing weapons or, more recently, compliance weapons are weapons intended to be less likely to kill or to cause great bodily injury to a living target than a conventional weapon. Less-lethal weapons are used in riot control, prisoner control, crowd control, such as refugee control, non-lethal battle, and self-defense.
In the past, police or soldiers faced with a riot were able to use truncheons or similar club-like weapons, or bayonet or saber charges, or fire live ammunition at crowds. Less-lethal riot control weapons were developed to provide more effective riot control than truncheons with less risk of loss of life or serious injury than with bladed weapons or firearms . Before the development of less-lethal weapons, police officers had few if any non-lethal options for riot control. Common tactics intended to be less lethal included a slowly-advancing wall of men with batons, or a charge into a riot using the flats of sabers. Other resonably successful approaches included shotguns with lower-powered cartridges, "salt shells", and richocheting the shot off of the ground. In the mid 1900s, with the integration of fire-control systems into major cities, police found that high-pressure fire hoses could be effective in dispersing a crowd (the use of water cannons and fire trucks has remained an effective nonlethal tactic to disperse riots). Trained police dogs were also commonly used to scare and disperse rioters and apprehend individuals.
In the 1980s the development of the high-tensile plastics Kevlar and Lexan revolutionized personal armor and shields, and led to new tactics for riot squads and other special-purpose teams. Officers could now stand up against violent rioters throwing dangerous projectiles without having to resort to lethal methods to quickly disperse the danger. Coupled with the introduction of effective non-lethal chemical agents such as tear gas and offensive odor canisters, and non-lethal impact rounds such as rubber bullets and "bean bag" flexible baton rounds, riot tactics were modified to rely less on violent response to attacking rioters than on a return to the slowly-advancing wall, with supporting officers firing non-lethal ordnance into the crowd to discourage advance.
Police officers on patrol were traditionally armed with a baton or pistol or both, and non-lethal methods of subduing an attacker centered on hand-fighting techniques such as Jujutsu and baton use. In the 1980s and 1990s officers began deploying non-lethal personal sidearms such as pepper sprays, and eventually electroshock weapons such as Tasers, which were developed for use by police and also found a market in self-defense by private citizens. However, these weapons were developed for non-lethal resolution of a one-on-one conflict.
During the 1990s and early 2000s interest in various other forms of less-than-lethal weapons for military and police use rose. Amongst other factors the use of less-than-lethal weapons may be legal under international law and treaty in situations where weapons such as gases defined as chemical are not. Less-than-lethal weapons are also useful in keeping the peace in the aftermath of violent conflict.
In the late 1990s and early 2000s police began to adopt a new pepper spray delivery system based on the equipment used in paintball. A specialized paintball, called a "pepperball", is filled with liquid or powdered capsaicin, the active ingredient in pepper spray, and is propelled by compressed gas using a paintball marker similar to those used for the sport but operating at higher pressure. The impact of the capsule is immediately painful (a pepperball's shell is thicker than a standard paintball and is fired at higher velocity), and it breaks open on impact, dispersing the capsaicin with similar effect to aerosol-delivered pepper spray.
In 2001 the United States Marine Corps revealed its development of a less-than-lethal energy weapon called the Active Denial System, a focused microwave device said to be capable of heating all living matter in the target area rapidly and continuously for the duration of the beam, causing intolerable pain. The skin temperature of a person subjected to this weapon can jump to approximately 130 °F (54 °C) in 2 seconds depending on the skin's starting temperature. The system is nonlethal if the subject can escape from the beam immediately, but the device has caused 2nd degree burns after several seconds of exposure.
In 2004 author Jon Ronson revealed a military report titled "Non-Lethal Weapons: Terms and References" 21 acoustic weapons were listed, in various stages of development, including the Infrasound ("Very low-frequency sound which can travel long distances and easily penetrate most buildings and vehicles...biophysical effects: nausea, loss of bowels, disorientation, vomiting, potential internal organ damage or death may occur. Superior to ultrasound...)", however no such effects had been achieved as of 2002.
Less-lethal weapons are intended to minimize injury or death. While people are occasionally seriously injured or killed by these weapons, fatalities are relatively infrequent. Causes of death from non-lethal weapons are varied and occasionally uncertain. Misplaced or ricocheting shots, pre-existing medical conditions, inadequate user training, repetitive applications and intentional misuse have been implicated in different cases.
As different parts of the body differ in vulnerability, and because people vary in weight and fitness, any weapon powerful enough to incapacitate may be capable of killing under certain circumstances. Thus "non-lethal force" does have some risk of causing death: in this context "non-lethal" means only "not intended to kill". For this reason, two new terms, "less than lethal" and particularly "less-lethal", were coined and are now being used in place of "non-lethal" by many weapons manufacturers and law enforcement agencies, and by those who oppose their common use in riot control. This term is intended to emphasize that they tend to kill or injure far fewer people than traditional weapons.
Several groups maintain there is great room for improvement in non-lethal weapons and procedures for their use. Claims for the relative safety of such weapons are usually contingent on their being used "properly." For example, the rubber bullets developed during the 1960s were supposed to be fired at the ground and hit the target only after ricochet, and other non-lethal bullets are designed to be fired at the lower body; they can be lethal if fired directly at the head, as commonly happens.
Non-lethal or less-lethal rounds are firearm rounds which are designed to incapacitate, but not kill, a target. The rounds rely on the transfer of kinetic energy to accomplish this incapacitation. Rubber bullets, wax bullets, plastic bullets, beanbag rounds, and rubber bullets with electroshock effect (e.g. Taser XREP rounds) are less lethal than conventional metal bullets, and are also propelled at lower speed. (Gunpowder is not used, only a percussion cap.) "Bean bag" type bullets are sometimes referred to as flexible baton rounds. More recently, high-velocity paintball guns are also used to launch less-lethal rounds, including the FN 303 launcher and PepperBall  commercial products. There is also the Variable Velocity Weapon Concept, for which a propulsion energy source may not yet have been clearly established and/or finalized. In any case, all of these technologies apply the same basic mechanism, which is to launch a mass at the target that interacts kinetically.
Because the impact energy varies based on engagement range, it is accepted that less lethal ammunition in some scenarios may cause injury. The risk of adverse effects: contusions, abrasions, broken ribs, concussions, loss of eyes, superficial organ damage, serious skin lacerations, massive skull fractures, rupture of the heart or kidney, fragmentation of the liver, hemorrhages, and death are considerable. Policies for determining if medical assistance should be contacted immediately after an actual deployment of a less-lethal munition (even if no physical injuries appear on subject or subjects) are usually determined by the organization using such devices. There is currently no universal consensus policy paper to suggest an alternative.
Water cannons are commonly used in crowd and riot control, for dispersal or to prevent movement on a particular position.
Unlike less-than-lethal rounds that rely mostly on the kinetic energy of the projectile to incapacitate the opponent, darts use sedative agents to incapacitate the target. Darts have been used for centuries in wildlife parks and zoos to incapacitate large and/or dangerous animals (e.g. leopards, tigers, elephants) in order to transport them. Darts can be fired with repeating crossbows, compressed air rifles or blowpipes, or with electromagnets as used by the Gauss pistol.
Malodorants produce smells so horrible they cause people to leave the affected area. In 2008 The Israeli Defence Forces have begun using Skunk for crowd control. It is a form of mist sprayed from a water cannon, which leaves a terrible odour of rot or sewage on whatever it touches, and does not wash off easily.
An estimate by the International Association of Chiefs of Police suggested at least 113 pepper spray related fatalities had occurred in the United States, mostly from positional asphyxia, which is caused by airway-restrictive immobilizing holds. Such holds can be exacerbated by the use of pepper spray and the resulting airway inflammation.
The use of chemical weapons such as tear gas (CS) and pepper spray (OC) has come under increasing scrutiny and criticism due to studies showing serious long term side effects. Many police forces are no longer exposing their members to the chemicals during training. Additionally, tear gas and pepper spray are banned in warfare by the international Chemical Weapons Convention.
Electroshock weapons are incapacitant weapons used for subduing a person by administering electric shock aimed at disrupting superficial muscle functions. One type is a conductive energy device (CED), an electroshock gun popularly known by the brand name "Taser", which fires projectiles that administer the shock through a thin, flexible wire. Other electroshock weapons such as stun guns, stun batons, and electroshock belts administer an electric shock by direct contact.
Directed energy weapons are weapons that emit energy in an aimed direction without the means of a projectile. They are non-lethal and can immobilize people as well as machines (e.g. vehicles). Directed energy weapons include electromagnetic weapons, (including laser weapons), particle beam weapons, and sonic weapons.
In the United States of America, the University of Texas-Austin Institute for Advanced Technology (IAT) conducts basic research to advance electrodynamics and hypervelocity physics related to electromagnetic weapons. Although generally considered 'non-lethal weapons', electromagnetic weapons do pose health threats to humans. In fact, "non-lethal weapons can sometimes be deadly."
United States Department of Defense policy explicitly states that non-lethal weapons "shall not be required to have a zero probability of producing fatalities or permanent injuries." Although a Human Effects Advisory Panel was established in 1998 to provide independent assessment on human effects, data, and models for the use of 'non-lethal weapons' on the general population, the TECOM Technology Symposium in 1997 concluded on non-lethal weapons, "Determining the target effects on personnel is the greatest challenge to the testing community," primarily because "the potential of injury and death severely limits human tests." However, "directed energy weapons that target the central nervous system and cause neurophysiological disorders" may violate the Convention on Certain Conventional Weapons of 1980. And weapons that go beyond non-lethal intentions and cause "superfluous injury or unnecessary suffering" could violate the Protocol I to the Geneva Conventions of 1977." Safety and evaluation of the physical and psychological effects of the long-term or repetitive uses of the pain-inducing non-lethal weapons on humans have not been well understood or studied in any great details. Any such studies require explicit consent of all participants so as not to violate the UN Convention against torture and other cruelties.
Both pepper spray and electroshock weapons have been misused in so-called "pain compliance" techniques against people attempting to practice nonviolent civil disobedience. For instance, pepper spray has been swabbed directly into the eyes of protesters who were being held immobile with their eyelids forcibly pulled back. Amnesty International in 1997 released a report titled USA: Police use of pepper spray is tantamount to torture. The repetitive use of pain-inducing non-lethal weapons on a human may be considered cruel, if not torture by itself. Such use is likely to be considered abusive or in violation of the 1984 United Nations Convention against torture and other cruelties.
Loren Thompson, chief operating officer of the Lexington Institute in Virginia states that: "The relevant (electromagnetic weapon) technology is well within the grasp of some countries and transnational terrorist groups", and further states that U.S. hardware is susceptible to microwave and other directed-energy weapons.