The use of chemical-weapons represents one of the most serious violations of international humanitarian law and poses enduring risks to public health, regional stability, and global biosecurity. From antiquity to the present, there have been at least 121 instances of chemical-weapon use globally up to October 2023 (1). Despite the near universal prohibition of chemical weapons under the Chemical Weapons Convention (CWC), their use has persisted in modern conflicts, exposing critical weaknesses in international non-proliferation, verification, and enforcement mechanisms. Among contemporary cases, Syria stands out as the most extensive and systematically documented instance of state-level chemical-weapon use since the late twentieth century (2-10), with profound humanitarian and regulatory implications.

Syria’s chemical weapons programme emerged under Hafez al-Assad during the Cold War in the 1970’s, reportedly through the acquisition of sulphur mustard and sarin from Egypt prior to the Yom Kippur War, and subsequently expanded through procurement of precursor chemicals, production equipment and munitions from multiple international suppliers (Germany, Czechoslovakia, the Soviet Union, China, and North Korea) (6,11,12). Human-rights violations, arbitrary detentions, extrajudicial killings, and suppression of opposition movements defined the Assad family’s rule (13). Although some early allegations, such as the detection of chemical capabilities during the 1973 conflict or cyanide use during the suppression of the Muslim Brotherhood in Hama in 1982, remain unverified, international concern regarding Syria’s chemical capacity intensified by the late 1980’s, including United States export restrictions on chemical precursors in 1986 and further in 1987 (11,14). By the early 1990’s, Syria was reportedly producing VX and developing delivery capabilities, with multiple suspected production and storage sites identified (11). Throughout this period, the programme was publicly framed as a strategic deterrent against Israel (6,12).

Following Hafez al-Assad’s death in 2000, Bashar al-Assad consolidated power after a constitutional amendment lowered the minimum age for presidential eligibility (15,16). Despite early portrayals as a reformist leader, governance remained characterised by repression and political control (17,18). Prior to the 2003 United States invasion of Iraq, some intelligence and journalistic sources alleged that elements of Saddam Hussein’s chemical weapons stockpile may have been transferred to Syria; however, these claims remain contested and unverified (11). Despite periodic elections, Assad’s victories were widely regarded as fraudulent (19). Syria’s weapons posture persisted amid regional insecurity, while social and environmental stressors, including a prolonged drought between 2006 and 2010, contributed to rising instability, exacerbated social unrest, driving rural migration, unemployment, and discontent (20,21).

In March 2011, mass protests over mounting grievances over corruption, repression, and lack of political freedom escalated into a multi-actor civil war involving domestic factions and substantial international involvement (19-22). Inspired by the Arab Spring uprisings in Tunisia, Yemen, and Egypt, demonstrators in Deraa demanded reform after the arrest and torture of Syrian schoolboys who had written pro-democracy graffiti (22). Security forces responded with lethal force, triggering nationwide protests and armed resistance (21). The conflict rapidly escalated into a full-scale civil war involving multiple rebel factions and foreign actors. Regional and international powers, among them Iran, Turkey, Russia, Israel, and the United States, became directly or indirectly engaged (21). By January 2012, an estimated 5,000 civilians and protestors had been killed, and in August 2012, the United States declared chemical-weapon use in Syria a “red line” (20,23).

From 2012 onwards, chemical weapons were repeatedly documented in attacks affecting civilian populations, occurring across urban neighbourhoods, besieged enclaves, and contested frontlines (20,24). Syria acceded to the CWC in 2013 and, under the supervision of the Organization for the Prohibition of Chemical Weapons (OPCW), declared and removed its chemical arsenal; however, subsequent investigations and reporting indicated continued chemical-weapon incidents after the declared disarmament, raising concerns regarding undeclared stockpiles, residual precursors, and verification limits (25-28).

The collapse of the Assad regime in December 2024 transformed, but did not eliminate, chemical-weapons-related biosecurity risks, shifting them from state-controlled arsenals to post-conflict oversight and diversion challenges (27-30). Against this backdrop, this study examines documented chemical-weapon incidents in Syria between 2012 and 2020 using open-source investigative data from the Global Public Policy Institute (GPPI) (24), supplemented by international monitoring reports, to inform understanding of patterns of use, associated health impacts, and implications for chemical weapons governance and post-conflict risk reduction.

This study aimed to describe the scale, temporal and geographic distribution, and health impacts of documented chemical-weapon incidents in Syria during the civil war, and to situate these findings within the context of international chemical disarmament and post-conflict biosecurity risk. Specifically, it examined patterns of chemical agent use and attribution, characterised casualty profiles, and contextualised chemical-weapon events in relation to major military offensives and siege operations to inform chemical weapons governance and non-proliferation efforts.

This analysis demonstrates that chemical-weapon use in Syria was characterised by marked heterogeneity in agent type, delivery mechanism, and health impact. Although chlorine accounted for the majority of documented incidents, sarin-related events were responsible for a disproportionate share of fatalities. This disparity is consistent with differences in agent toxicity and exposure dynamics, but also reflects contextual factors such as attack location, population density, and access to timely medical care. Variation in CFRs across governorates may further reflect differences in health system capacity, emergency response, and reporting completeness during periods of intense conflict.

Importantly, these findings should be interpreted as descriptive patterns rather than definitive estimates of burden. Casualty figures derived from open-source investigations are subject to under-ascertainment, survival bias, and inconsistent documentation, particularly in besieged or inaccessible areas. Nevertheless, the concentration of high-fatality events associated with nerve agents underscores the continued public health relevance of chemical-weapon exposure, even when such agents are used relatively infrequently.

Beyond immediate mortality, the health consequences of chemical-weapon use were compounded by prolonged conflict, mass displacement, and health system degradation. By 2024, more than half of Syria’s pre-war population of 22 million had been displaced, with millions seeking refuge in neighbouring countries (20). In this context, chemical exposures occurred in settings with limited medical infrastructure, likely amplifying both acute and longer-term health impacts. Of the 157,634 individuals reportedly detained between 2011 and 2024, at least 6,786 were documented to have died in government custody (13).

The temporal concurrence between documented chemical-weapon incidents and major military offensives suggests that chemical weapons were used within broader patterns of siege warfare and territorial contestation. However, these associations cannot be interpreted as evidence of uniform strategic intent. Fluctuations in external military support, including the evolving roles of the Russian Federation, Iran, Türkiye, and the United States, contributed to a fragmented conflict landscape characterised by shifting control and variable operational pressures (20).

By 2019-2020, Kurdish forces had consolidated autonomy in the north, Turkish military operations intensified following United States troop withdrawals, and the Islamic State sought to reassert influence over strategic resources, including oilfields (20). These dynamics complicate attribution and interpretation, particularly where access for independent verification was constrained. As such, the observed patterns likely reflect a combination of military, logistical, and political factors rather than a singular or consistent deployment strategy.

The persistence of documented chemical-weapon incidents following Syria’s accession to the CWC and the declared destruction of its stockpiles highlights enduring limitations in chemical weapons verification and enforcement mechanisms. While the OPCW certified the destruction of stockpiles in 2014, subsequent investigations indicated the probable existence of undeclared agents, precursors, or production capacity.

These findings do not necessarily imply systematic non-compliance across all phases of disarmament but instead illustrate the inherent challenges in verifying chemical weapons destruction in active conflict settings. Restricted access, contested territory, and reliance on state declarations constrain the verification and reporting regimes. Syria, therefore, represents a critical case study for understanding how chemical weapons governance frameworks operate under conditions of protracted war and limited international leverage.

The collapse of the Assad regime in December 2024 marked a significant political rupture but did not eliminate chemical-weapons-related risks. A 7.8-magnitude earthquake in February 2023 devastated parts of Turkey and Syria, killing at least 6,000 people in Syria and over 50,000 in Turkey (20). The disaster deepened Syria’s humanitarian crisis but also exposed the regime’s fragility. Rebel groups such as Hay’at Tahrir al-Sham (HTS) began preparing a coordinated campaign to oust Assad, with preparations commencing in 2023 (38). The regime’s dependence on the Iranian Revolutionary Guard Corps (IRGC), Hezbollah, and other Shia militias weakened as these groups shifted focus to conflicts elsewhere, including Hezbollah’s prolonged confrontation with Israel (38). Meanwhile, Russia had already redirected resources toward its invasion of Ukraine in 2022 (38).

In late November 2024, HTS led a coalition offensive capturing Aleppo, Syria’s second-largest city and economic hub (39). Assad, caught off guard, abandoned the city while awaiting reinforcements (36). Hamah and Homs fell soon after, with limited Russian air support failing to stem rebel advances (33). Iran, preoccupied with the Lebanon-Israel conflict, provided no aid (40,41). Facing total collapse, Assad fled Damascus on 8 December 2024. His prime minister surrendered to HTS forces, marking the end of 54 years of Assad family rule (30).

Following Assad’s departure, fears grew that residual chemical stockpiles could fall into rebel or extremist hands. On 9 December 2024, Israel conducted airstrikes on suspected chemical-weapons and missile facilities, including the Syrian Scientific Studies and Research Center (SSRC) (26). The OPCW later confirmed that unaccounted quantities of sarin precursors and chlorine likely remained in the country (27). International inspectors estimated that roughly 360 tonnes of mustard gas and five tonnes of sarin precursors were still missing (28). In response, HTS issued a statement pledging cooperation with the OPCW and renouncing the use of chemical weapons (42). Nevertheless, concerns persist that hidden caches may resurface amid Syria’s continuing political instability and the presence of designated terrorist organisations controlling parts of the country.

This analysis relies on open-source investigative data and is subject to several limitations. Reporting bias, under-ascertainment, and differential access to affected areas may have influenced the number and characteristics of documented incidents. Attribution of responsibility and identification of chemical agents remain uncertain in some cases, particularly where independent verification was restricted. Casualty figures may be incomplete or inconsistent across sources. As such, findings should be interpreted as conservative estimates and descriptive patterns rather than definitive counts or causal inferences.

The Syrian case demonstrates both the catastrophic public health consequences of chemical-weapon use and the structural vulnerabilities of existing chemical weapons governance in protracted conflict settings. Despite formal accession to the CWC and the declared destruction of stockpiles, chemical-weapon incidents persisted, underscoring the limitations of verification mechanisms reliant on state declarations and access during constrained active hostilities. The post-collapse environment further highlights the enduring biosecurity risks posed by residual agents, precursors, and fragmented governance, particularly where non-state actors control territory. Together, these findings emphasise the need for strengthened international oversight, adaptive verification strategies, and sustained post-conflict monitoring to prevent diversion and re-emergence of chemical weapons. Lessons from Syria should inform future non-proliferation efforts by reinforcing the integration of public health surveillance, open-source intelligence, and international inspection regimes to enhance early detection, accountability, and long-term chemical weapons risk reduction.

No ethics application was required for this study.