Detonating the Air – The Legality of the Use of Thermobaric Weapons under International Humanitarian Law


| Jun 12, 2023


This post is drawn from the author’s article-length work, “Detonating the Air: The Legality of the Use of Thermobaric Weapons under International Humanitarian Law,” appearing in the International Review of the Red Cross.

Weapons law, principally by way of treaty law but also through customary international humanitarian law (IHL), prohibits particular weapons and related technologies or restricts the conditions in which weapons may lawfully be employed. The basic principle of weapons law prohibits harm that is not essential to achieve a legitimate object of armed conflict.

The development of more formidable and efficient weapons has produced, among many other examples, thermobaric weapons that use oxygen from the air to create wide area dynamic negative overpressure, thermal heat and sustained mechanical and thermal impulse blast waves. Thermobaric explosions produce a higher total energy output over an extended period than conventional explosives. These weapons also cause tertiary and quaternary damage due to the pressure effects generated by the structures around the explosion and from suffocation due to the consumption and depletion of the ambient oxygen, as well as from toxic gases and smoke. The combined attributes produced by thermobaric weapons make them extremely effective against buildings, bunkers, trenches and hard or deeply buried subterranean structures. However, the use of thermobaric weapons also creates significant challenges for belligerents as their use in populated areas potentially exposes civilians to harm.

Possible Prohibitions of Thermobaric Weapons

No international instrument specifically addresses the legality of the possession or use of thermobaric weapons. Even so, the debate on the legality of these weapons may indirectly implicate specific provisions of some existing instruments. The fuses of some thermobaric weapons are constructed of non-metallic parts that produce fragments that may not be detectable by X-rays. The incorporation of these parts in thermobaric weapons will not in itself result in their prohibition under Protocol I of the Convention on Certain Conventional Weapons (CCW) because the primary effect of thermobaric weapons is not intended “to injure by fragments which in the human body escape detection by X-rays.”

Thermobaric explosions have the potential to cause choking, suffocation and poisoning from processes that cause burns, chemical reactions on or in the human body and infections due to contamination. The removal of oxygen from the surrounding area will be enhanced where thermobaric weapons are employed in confined spaces, such as caves. However, thermobaric weapons are not primarily designed to asphyxiate or poison. When present, these effects are regarded as secondary or additional effects, and thus the use of thermobaric weapons is not prohibited by the 1899 Hague Declaration, the 1907 Hague Regulations or the 1925 Geneva Gas Protocol.

The composition of the fuel mixtures in thermobaric weapons contains toxic chemical substances and chemical agents, which are selected based on exothermicity (the release of heat during a chemical reaction). The thermobaric explosions may also create toxicity in specific circumstances or may, due to the presence of these substances and the gases released, incidentally poison personnel. However, the fact that a thermobaric weapon contains chemicals does not render it a prohibited chemical weapon in the terms of the Chemical Weapons Convention, as thermobaric weapons are not primarily designed to produce harm by poisoning.

On initial scrutiny, thermobaric weapons may violate Protocol III to the CCW. Even so, Protocol III’s definition of incendiary weapons is excessively narrow and does not adequately deal with multi-purpose incendiary weapons. Instead, the focus in the definition is on the purpose for which a weapon is designed, as opposed to the weapon’s impact. Thermobaric weapons are, as a result, excluded from the definition of incendiary weapons as they are not primarily designed to cause fire or burns, even though these weapons frequently produce “incendiary effects” that are substantial but “incidental” or secondary.

Additional Protocol I to the Geneva Conventions (AP I) provides that “[i]t is prohibited to employ methods or means of warfare which are intended, or may be expected, to cause widespread, long-term and severe damage to the natural environment.” AP I only apply to weapons that have extreme effects on the environment, and the thresholds of “widespread,” “long-term” and “severe” are cumulative as all three requirements must exist for the rule to be breached. The use of thermobaric weapons, in accordance with their primary design purpose, will thus not achieve the cumulative threshold requirements set in AP I.

Customary International Humanitarian Law

The Hague Regulations prohibit the employment of means and methods of warfare that are “calculated to cause” superfluous injury or unnecessary suffering beyond that which is required to accomplish the destruction of material or rendering combatants hors de combat. The phrase “calculated to cause” implies an element of deliberate design and, therefore, weapons that incidentally, unintentionally or accidentally inflict superfluous injury would probably not violate this principle. AP I states that it is “prohibited to employ weapons, projectiles and materials and methods of warfare of a nature to cause superfluous injury or unnecessary suffering.” When used as intended, thermobaric weapons may cause severe injuries and extreme suffering. However, the increased lethal effects will not render these weapons unlawful. The purpose for which thermobaric weapons were developed was, and still is, to achieve a unique military advantage, especially when these weapons are employed to defeat hard targets and subterranean objectives. A ban on the use of thermobaric weapons would thus contradict prevailing military judgement on the military advantages these weapons offer. Accordingly, it is inappropriate to assume that using thermobaric weapons would inevitably breach the prohibition against superfluous injury and unnecessary suffering.

The more proximate question is whether a weapon with a wide blast radius and effects is inherently indiscriminate. It is important to acknowledge the basic distinction between an indiscriminate weapon and an indiscriminate attack, even though the attack and the weapon used to prosecute the attack cannot be practically separated. The potential classification of a thermobaric weapon as indiscriminate must consider its ability to engage a specific military objective and the likelihood of limiting that weapon’s effects to the intended military objective. The International Committee of the Red Cross database of customary IHL reveals no evidence that any State has expressly declared thermobaric weapons inherently indiscriminate. As a result, none of the forms of thermobaric weapons are automatically and inherently indiscriminate. Belligerents must consider the reasonably foreseeable, direct or reverberating civilian harm that may be expected to result from an attack with a thermobaric weapon. Special care must be taken when there are any civilians, critical civilian infrastructure, means of livelihood and cultural sites near the intended detonation point of the weapon. The use of thermobaric weapons may create doubt about the reliability of the mitigation measures taken to reduce the weapon’s area effects and possible harm to civilians. Thermobaric weapons, especially those with inaccurate delivery systems, should therefore not be used in urban or populated areas.

Public Conscience

The Martens Clause, as codified in AP I, states that “[i]n cases not covered by this Protocol or other international agreements, civilians and combatants remain under the protection and authority of the principles of international law derived from established custom, from the principles of humanity and from the dictates of public conscience.” Nonetheless, the principles of humanity and the dictates of public conscience will not, except in remarkable instances and where general agreement exists, cause a particular weapon to be unlawful. An intense dislike of a particular weapon by the public, in general, may become a significant consideration in the development of weapons law. However, public opinion on the use of thermobaric weapons, unlike the previous campaigns to prohibit landmines and cluster munitions, cannot be regarded as an extreme or uncontested instance where the dictates of public conscience alone have created enough impetus to delegitimise the use of thermobaric weapons.

Concluding Thoughts

The application of the IHL rules governing the conduct of hostilities has progressively increased the protection of the civilian population, notwithstanding the military utility of particular weapons. There is, accordingly, an obligation on belligerents, where thermobaric weapons are used, to minimise or avoid injury to, or incidental loss of, civilian life and damage to civilian objects. As with any other heavy explosive weapon, the use of thermobaric weapons should, wherever possible, be avoided in urban or populated areas, as the multiple mechanisms to inflict harm and their dispersed wide area effects make it extremely difficult to mitigate or appreciably reduce their harmful effects on civilians.


Arthur van Coller is an Associate Professor in the Faculty of Law, University of Fort Hare.


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