The findings of our systematic review identified significant morbidity and mortality associated with chemical irritant agents CS and OC. Chemical irritants, like many other crowd-control weapons, are typically justified as a safe tool to disperse potentially dangerous groups or incapacitate threatening individuals as part of the effort to ensure public safety. The prevailing presumption about these chemical agents is that they cause minimal and transient irritation to the skin and eyes, but are generally safe for use on diverse populations. However, we found that, by design or by inappropriate use, chemical irritants can cause significant injuries as well as permanent disabilities. While deaths were rare, we identified one death directly caused by the blunt trauma from the projectile and another from high dose exposure to the chemical agent in a closed environment. These health consequences may be related to the chemical agents themselves, the total exposure dose, the deployment technique, or the way these weapons are used in different settings.

Our findings indicate that agent CS and agent OC were used both in protests and during arrest scenarios and training exercises; contact also occurred through accidental exposure. Chemical irritants caused injuries to many different body systems in addition to the expected pain to the skin and eyes. We also documented a range of injury severity for neurological, oropharyngeal, cardiac, pulmonary, and musculoskeletal systems. The psychological impact of the use of crowd-control weapons has not been well studied or documented in the medical literature, but cases described in this review indicate that exposure to CCWs may result in significant psychiatric symptoms and long-term disability.

In addition to documenting injuries, we identified factors that may impact injury severity. Intrinsic characteristics of the chemical agents themselves play a role. Chemical irritants, especially those deployed in aerosolized forms, are inherently indiscriminate and can affect not only the intended targets but also peaceful demonstrators, bystanders, nearby communities and residences, and law enforcement officers themselves. The majority of people injured are young adults, consistent with typical protest demographics [63, 64]. We found a relatively equal gender distribution of injuries. But many studies also found injuries among children and elderly people that appear to validate concerns about the indiscriminate nature of chemical irritants and their potential impact on bystanders and nonviolent demonstrators [65]. Children are more vulnerable to severe injuries from chemical toxicity [66, 67]. The elderly and those with chronic diseases are also prone to worse outcomes from chemical irritants [68, 69]. Because of the indiscriminate nature of chemical irritants, limiting the exposure to individuals or small groups is difficult. Most often a large, diverse, and differentially susceptible group will be exposed, posing the risk of unnecessarily injuring nonviolent, potentially vulnerable people.

Perhaps even more concerning are the effects of these chemical agents in settings where people are chronically exposed to these chemicals, either by repeated use near their homes or businesses, or because of occupational use in which safety has never been studied and cannot reasonably be assumed [34, 55, 58]. Repeated exposure may be particularly concerning for law enforcement officers, people who attend protests frequently, and health workers who may experience multiple occupational exposures.

The decision to use chemical agents in specific environmental conditions and social contexts may also play a role in injury severity. Clinical effects are likely dose-dependent and excessive exposure may exacerbate severity. Studies included in this review show that the use of chemical irritants in enclosed spaces without safe avenues of egress increases exposure to the agent and exacerbates ensuing injuries [33, 35, 37, 39, 40, 61]. Although our study excluded secondary injuries, we note reports that there were several cases of chemical irritants sparking mass panic and stampedes that contributed to significant morbidity and mortality. These include at least 20 deaths in a sports stadium in Egypt in 2015, [70] 15 deaths in the Democratic Republic of the Congo in 2014, [71] 11 deaths in a stampede in Zimbabwe in 2014, [72] and 43 in South Africa in 2001, [73] all during protests or in other crowded contexts. Deliberately aiming the munition as a projectile weapon into dense crowds or at individuals can cause severe traumatic injury [42, 51, 54, 57, 62].

In conducting the broader research, we also identified significant public concern over lack of transparency by law enforcement and manufacturers about the agent(s) used during specific events. Manufacturers often do not provide adequate information on concentrations of chemicals or the solvents and non-active ingredients that may contribute to toxicity. In addition to the difficulties this may pose to health workers trying to appropriately manage patient injuries, lack of transparency can break down trust and negatively impact relationships between communities and law enforcement. Though this may be an intended outcome in some repressive regimes, we noted this concern in all uses of toxic chemical agents against primarily unarmed civilian populations.

Policy implications and recommendations

The legal protections of the rights to freedom of expression and peaceful assembly, along with general principles on the proportionate use of force by law enforcement, provide some general guidance that the use of chemical irritants, along with other crowd-control weapons, should be limited. Specifically, CCWs should only be used in situations where particular individuals pose an imminent violent threat, or where a protest requires dispersal because of widespread violent acts that pose an imminent threat to public safety [74, 75]. In most situations where we found these weapons being used, neither of these conditions was documented. The use of chemical irritants as crowd-control weapons must be considered in the broader context of human rights, public safety, use of force, and law enforcement practices necessary to maintain order in the context of demonstrations. Open communication with demonstrators and the community, arrests of violent individuals, and safeguards for legal demonstration may obviate much of the demand for these chemicals. Given the frequency of serious injury, disability, and death, the use of chemical irritants should be strictly limited to situations of imminent harm that cannot be policed effectively with safer methods.

The Chemical Weapons Convention (effective since 1997 with 192 state signatories excluding only Egypt, Israel, North Korea and South Sudan) prohibits the use of these riot control agents’ during warfare. Significant questions exist on the legality of military use of these weapons in civilian protest. We recommend that law enforcement and the military be obligated to maintain transparent and accurate data on use-of-force incidents, particularly those that employ chemical irritants and other crowd-control weapons. Active surveillance of injuries caused by chemical irritants is vital for manufacturers, law enforcement, and the community in order to understand the risks and dangers of these weapons. This data should be available and accessible to the public for independent analysis.

We also note that combinations of OC and CS are becoming more common, both in spray and aerosol forms as well as within projectiles such as the “pepper ball” [16, 76, 77]. Several newer agents are also in development, including agents CS1 and CS2 (which may extend the half-life of CS or facilitate higher dermal penetration) and agent CX, which is reported to be more potent than agent CS [16]. Each of these potential enhancements to weapons may compound the already large number of injuries. It is important that we address the human costs of current chemical irritants before developing new, more potent ones.

While making law enforcement protocols publicly available may not be possible due to security risks, we recommend that police and military departments make every effort to communicate with health workers and the community in order to minimize potential injuries from chemical irritants and to maintain trust. Training of police officers must include education on human rights principles and the obligation of the police to protect peaceful protestors. Police should also be trained in the dangers of chemical irritants, guidelines on the safe utilization of chemical irritants, the risks of repeated exposures, environmental factors, and the risks of direct trauma from poorly-aimed projectile munitions, as well as other risk factors.

Limitations

Our systematic review had several limitations. In the absence of systematic reporting requirements on deaths and injuries in crowd-control settings, it is likely that we have largely underestimated the prevalence of deaths and injuries. The limited follow-up in many of the articles also highlighted the lack of data on the chronic health impacts of these weapons, which are likely underreported. We note that in our attempt to ensure valid and reliable injury data, we have not accounted for a large number of injuries and deaths from chemical irritants that have been reported by the news media, by social justice organization reports, and by social media, many of which include photographic or videographic evidence of injuries. We also excluded reports that lacked injury specifics or clear causation from chemical irritants, but were likely linked to chemical irritant utilization. We also excluded the significant number of case reports in the published literature to avoid biasing our results towards the publication of the most severe injuries. In addition, there is wide variability in how weapons are used and in the specific concentrations of chemical agents; there is also a lack of data on the number of people exposed. Given these concerns, we were not able to calculate population estimates of the impact of chemical irritants or compare any specific agents or numerical study results. However, severe injuries from chemical irritants are not rare or isolated incidents. They have occurred in many nations and under different types of regimes and law enforcement protocols.

This review was also limited by the quality and methodology of the available literature on chemical irritant injuries. There are several potential biases, including the potential over-publication of the most dramatic incidents and independent limitations on individuals’ decision to seek medical care. On the other hand, difficulty in gathering and publishing data in repressive regimes may limit the availability of injury data from many instances of chemical irritant utilization. There was also significant heterogeneity in the participants and medical treatments in different regions and clinical settings. There was considerable methodological variability in the study designs and settings. Each setting had variable standards on the use of chemical irritants. However, the overall quality of the studies was comparable to observational and case series-type studies addressing chemical irritants. Given the multiple confounding factors, we could not compare the chemical agents or deployment mechanisms. Included studies did not provide enough data to reliably estimate the risk of injury from any given chemical irritant in an exposed population.