Quaternary ammonium compounds, commonly named QAC or quats, have been used for their antimicrobial property since the beginning of the 20th century (Jacobs et al., 1916a, Jacobs et al., 1916b; Jacobs, 1916). QAC have the general structure of a positively charged nitrogen atom, bound to four carbon atoms, and a negatively charged anion, which is biologically inactive. In 1916, Jacob and colleagues discovered the germicidal activity of quaternary salts of hexamethylenetetramine and derivates by investigating the relation between the chemical structure and the bactericidal activity (Jacobs et al., 1916a, Jacobs et al., 1916b; Jacobs, 1916). A few years later, the German Nobel prized Gerhard Domagk realized the potentiality of QAC with the discovery of the well-known and currently widespread benzalkonium, or BAC, in 1935 (Domagk, 1935). He found that if at least one of the groups bound to the QA was a long alkyl chain, the molecule could kill micro-organisms. Indeed, the positive head of the molecule adheres to the negatively charged surface of micro-organisms (bacteria, virus), and the long alkyl chain inserts into the lipid membrane rendering it porous and causing cellular content to leak out. BAC was the first QAC approved by the United States Environmental Protection Agency in 1947 (US EPA, 2006). Since then, different structures of QAC have been described, among which the trimethylalkylammonium based-QAC, from which cetyltrimethylammonium bromide or CTAB is the most famous, or the dialkyldimethylammonium based-QAC, whose main component is the dimethyldidecylammonium chloride or DDAC (Table 1).

QAC are the first used biocides before nitrogen-containing scaffolds, phenolic compounds and aldehydes (Vereshchagin et al., 2021). They represent a broad class of molecules whose market was estimated at $963.7 million in 2019 (XiaoZhi, 2020). They are used in various areas such as the medical field (disinfection of surgical and medical instruments, cleaning the surfaces…), industry, agriculture, or household. Considering their intrinsic properties (antimicrobial, antistatic, film-forming, or preservative), they can be found in numerous products such as cleaning products, herbicides, disinfecting products (wipes, soaps, sprays…), cosmetics (shampoo, shower gel, toothpaste…), eye drops, pharmaceutical products (throat lozenges, nasal spray…), which make them widespread use products.

QAC have been reported in immediate hypersensitivity reactions (asthma, contact urticaria) (Bernstein et al., 1994; Burge and Richardson, 1994; Purohit et al., 2000; Houtappel et al., 2008; Oropeza et al., 2011) but also delayed reactions (CD, eczema) (Klein et al., 1991; Cusano and Luciano, 1993; Dejobert et al., 1997; Mauleón et al., 2006; Ulicki et al., 2022) in the exposed populations. They also have been suggested to be the sensitizing molecules at the origin of NMBA anaphylaxis (Florvaag et al., 2005; de Pater et al., 2017). The underlying mechanisms are still controverted, as QAC have been described as irritants, adjuvants, or sensitizers, depending on the studies. This review firstly aims to describe the different QAC currently used and the relationships between their structure and use. Then, we will focus on the different reports of their implication in human health, with a deep study on the immunological mechanisms involved.

Several authors have incriminated QAC in the onset of some immediate hypersensitivity reactions. Those reactions occur within a few minutes after the exposure to an allergen. They are classically mediated by an IgE-dependent immune response. Symptoms, like asthma, urticaria, dermatitis, conjunctivitis, or anaphylaxis, result from histamine release by basophils and mast cells. Symptoms of asthma have been reported following BAC occupational exposition in factory workers, pharmacists or nurses (Bernstein et al., 1994; Burge and Richardson, 1994; Purohit et al., 2000). In those case reports, patients had no QA-specific IgE but experienced pulmonary symptoms during the re-challenge, confirming the incrimination of BAC (Bernstein et al., 1994; Burge and Richardson, 1994; Purohit et al., 2000). BAC used as a preservative in asthma medicine Atrovent^® has also been reported to induce paradoxical and severe bronchoconstriction (Beasley et al., 1987). DDAC was also responsible for contact urticaria in a laboratory technician (Oropeza et al., 2011). Another QAC was reported in immediate reaction (urticarial eruption and respiratory discomfort) in an anaesthetic nurse a few minutes after using a disinfectant product for an endoscope containing N, N-didecyl-N-methyl-poly(oxyethyl) ammonium propionate. The diagnosis was confirmed by performing prick-test (Raison-Peyron et al., 2009). In those case reports, QAC are clearly incriminated, but sometimes, QAC responsibility is only suggested, and definite proof is lacking (Rosenman et al., 2003). To better characterize the effect of cleaning products on pulmonary functions, Vandenplas et al. (2013) realized a retrospective study of workers with suspected occupational asthma. Workers were explored by specific inhalation challenge (SIC), bronchial hyperresponsiveness to histamine, and sputum cell count. Their results showed that 10/17 were exposed to QAC (mainly BAC and DDAC) among the positive SIC in the patients (Vandenplas et al., 2013). Noticeably, among the participants who developed a positive bronchial response to QAC, a post-challenge increase in sputum eosinophils and/or in the level of airway hyperresponsiveness was documented in 9 of 10 instances which was according the authors, in favor of a specific reaction (Vandenplas et al., 2013). Importantly, anaphylaxis was also reported after BAC exposure contained in nose drops (Mezger et al., 2012) and in eye drops (Anderson et al., 2009). Another anaphylactic reaction happened during a skin prick test with BAC, in the exploration of a suspected allergy to the nebulizer solution used to treat asthma (Kim and Ahn, 2004).

Apart from those case reports and SIC investigations, epidemiological studies have been performed to study the link between hypersensitivity reactions and the exposure to QAC. For example, Vogelzang et al. (1997) realized a cross-sectional study of pig farmers and showed that bronchial hyperresponsiveness to histamine, a hallmark of asthma, was detected in farmers using QAC as disinfectants for animal cleaning, especially if QAC were the principal active components of the disinfectants. However, those results were not found in a 3-year follow-up study by the same author (Vogelzang et al., 2000).

Occupational asthma is a significant concern for hospital workers and cleaners widely exposed to QAC. As a result, epidemiological studies are linking exposure in these workers to the development of asthma. For example, a large cohort of American nurses was studied for asthma control regarding their occupational exposure. In this study, the authors found no association between QAC exposure and asthma control (Dumas et al., 2017). In contrast, another French study of healthcare workers showed a strong association between the exposure to QAC and asthma or nasal symptoms (Gonzalez et al., 2014). Surveillance networks of occupational health can also provide information about the role of QAC in the onset or the exacerbation of asthma. In this context, Paris et al. showed an increase in occupational asthma related to QAC exposure in France in 2001–2009, although overall work-related asthma tends to decrease over time (Paris et al., 2012).

However, studies that explore occupational asthma in cleaners do not always precise the composition of the products responsible for respiratory symptoms so that quaternary ammonium are not specifically incriminated (Medina-Ramón et al., 2006, 2005; Bernstein et al., 2009), even if QAC are known to be the main component of cleaning agents. Lakind and Goodman recently published an excellent review about occupational asthma and QAC (LaKind and Goodman, 2019).

As already described earlier, immediate hypersensitivity symptoms are the results of histamine release. BAC has been shown to induce the release of histamine by mastocytes in vitro, and to inhibit the 48/80 compound-, NMBA-, and substance P-induced histamine release (Read and Kiefer, 1979). The latter compounds are three agonists of the recently discovered Mas-related G-protein-coupled receptor member X2 (MRGPRX2) (McNeil et al., 2015), but further studies will be necessary to understand the underlying mechanisms. However, this direct histamine release was suspected in asthmatic patients as histamine antagonists were partly able to inhibit BAC induced-bronchoconstriction (Miszkiel et al., 1988b).

Delayed hypersensitivity reactions to QAC have also been described. Those reactions generally occur within 24–72 h after allergen exposure, and the classical mechanism involves macrophages, specific T lymphocytes, and interferon γ (IFN-γ) release. Symptoms are caused by inflammatory cytokines released by IFN-γ activated macrophages and cytotoxic T cells that can mediate direct cytotoxicity (Pichler, 2003). Symptoms of delayed hypersensitivity range from mild reactions such as eczema and CD, to severe systemic reactions (i.e., DRESS syndrome or Steven-Johnson syndrome).

CD to QAC has been well described in healthcare workers (HCW) and cleaners. For example, BAC has already been reported in CD in a nurse which used BAC in sterilizing solution (Cusano and Luciano, 1993) and in surface disinfectants (Klein et al., 1991). Airborne CD was also described in a cleaning woman due to a spreading on the floor of QUAT-10^® containing BAC (Mauleón et al., 2006). In a hospital cleaner, BAC and DDAC were also recently incriminated in severe dorsal and palmar hand dermatitis (Ulicki et al., 2022). DDAC had already been described in other case reports of CD in hospital workers (Dejobert et al., 1997; Dibo and Brasch, 2001; Geier et al., 2013). CD to PQ7 was also described in a nurse with a history of work-related eczema of the hands spreading to the arms, face, and neck (Gallo et al., 2001). Another QAC, N, N-didecyl-N-methyl-poly(oxyethyl) ammonium propionate, was reported in CD in a dental assistant (De Quintana Sancho et al., 2014).

Cohorts of HCW and non-HCW with CD were compared to provide information on the origins of sensitization. Three studies found a significantly higher incidence of CD to BAC in HCW than in non-HCW (Shaffer and Belsito, 2000; Nettis et al., 2002; Suneja and Belsito, 2008). The prevalence of BAC sensitization in HCW with CD ranges from 7.1% to 9.7% (Shaffer and Belsito, 2000; Nettis et al., 2002; Suneja and Belsito, 2008). Concerning quaternium-15 Suneja and Belsito found a significantly higher rate of sensitization in HCW than in non-HCW (34% vs. 13.7%, p< 0.001), but it was not found in an older study of the same researchers (Shaffer and Belsito, 2000).

Contact allergies were also described with QAC contained in topical medicinal products. For example, contact allergy was found to be common in patients using BAC-containing topical medicine for chronic leg ulcers or stasis dermatitis, with a sensitization prevalence ranging from 7% to 18.6% (Saap et al., 2004; Barbaud et al., 2009; Valois et al., 2015; Raudonis et al., 2019; Silverberg et al., 2021). Another condition, flexural eczema (eczema affecting the skin where the joint bends with low pruritic lesions) and irritant dermatitis were also reported after the use of BAC in antiseptic bath oil at high concentrations (Ling and Highet, 2000; Loo et al., 2003; Storer et al., 2004; Hann et al., 2007). The fact that these patients present alteration of the skin barrier could constitute a risk factor for sensitization by facilitating the cutaneous penetration of QAC.

BAC and benzethonium (BE) are also used as preservatives in medicines. Using patch testing, sensitization to BAC and/or BE were found in patients with contact allergies to ear drops (Fräki et al., 1985), nose drops (Lechien et al., 2018), eye drops (Fisher and Stillman, 1972; Afzelius and Thulin, 1979; Aoki, 1997), and inhaled or intranasal corticosteroids (Bennett et al., 2001). The diagnosis in those cases might be difficult as symptoms of mucosal contact hypersensitivity can be mild and may appear only as a failure to the treatment or a worsening of the underlying condition (asthma, conjunctivitis, allergy…).

CD has also been described in patients using BAC and quaternium-15 contained in cosmetics (Amin and Belsito, 2006; Dastychová et al., 2008). Moreover, a significant association was found between eyelid dermatitis and sensitization to BAC (Amin and Belsito, 2006). A series of six patients with eyelid dermatitis to shellac (insect-derived texture agent) contained in mascaras revealed the co-sensitization to quaternium-22 in one patient. Quaternium-22 is used as a film-former, antistatic, and hair-conditioning agent, mainly in mascaras, hair conditioners, soaps, shampoos, and detergents (Le Coz et al., 2002). This co-sensitization has already been described with mascara (Scheman, 1998). More anecdotally, DDAC was incriminated in foot dermatitis by using a shoe refresher spray (Mowitz and Pontén, 2015).

BAC has been the most reported QAC in the literature for contact allergies. This is probably because it is the most widespread QAC as preservative and antiseptic in medicines, cosmetics, and cleaning agents, but it might also be due to its intrinsic properties. A study of patients with contact allergy and CD found a trend to increase in BAC sensitization in the recent years, and the main sources of exposure were ophthalmic drops, topical antiseptics, and cosmetics (Dear et al., 2021). Moreover, BAC was the sixth allergen giving the highest reaction within a large retrospective cohort of patch testing (Veverka et al., 2018).

QAC are therefore ubiquitous products, and their use is very frequent. Numerous proofs of their irritant, sensitizing and adjuvant properties have been reported, but all the underlying mechanisms are still unknown.

It appears that all the QAC do not exhibit the same effect on the immune system. Regarding the irritancy and the adjuvant properties, BAC seems the most potent among all (Larsen et al., 2004a; Larsen et al., 2012), and it is also the most common QAC found in case reports of asthma or CD symptoms. Thus, the reported effects of BAC could be due to its intrinsic properties although the overrepresentation in case reports is probably also related to its wide use. The fact that all QAC do not have the same adjuvant effects highlight an important role for their hydrophobic associated chain in this property. A classification has thus been proposed about the adjuvant effect of QAC, and the potencies were BAC ≈ CPC > DDA in a mice model of OVA sensitization (Larsen et al., 2004a; Nielsen et al., 2007). On the other hand, the sensitizing properties seem to be related to the quaternary ammonium group. Considering the possible sensitization of NMBA by QAC, the pattern responsible for the cross-sensitization is the quaternary ammonium epitope (Baldo and Fisher, 1983a; Baldo and Fisher, 1983b). Moreover, the IgE sensitization to QAC can be demonstrated by measuring the presence of QA-specific IgE. However, how QAC can lead to the production of specific IgE still raises debate. To induce the production of specific IgE, it is likely that QAC should bind to proteins and that those proteins might differ regarding the route of exposure. However, BAC could not bind to HSA (Aleksic et al., 2007) and DDAC could not bind to any peptides in a DPRA assay (Lee et al., 2019). However, HSA can bind drugs non-covalently through its two pockets [binding site I (in subdomain IIA) and II (in subdomain IIIA)] (Ghuman et al., 2005), and also bind fatty acids (Petitpas et al., 2001b). These bindings involve both hydrophobic and polar interactions between the ligand and the pocket and have already been well studied for warfarin (Petitpas et al., 2001a). Given the amphiphilic structure of QAC, this type of binding could also be considered. Alternatively, another mechanism could be involved, as is the case for nickel, which is capable of forming reversible complexes with proteins (Thierse et al., 2005).

Another hypothesis to overcome the inability of QAC to covalently bind to proteins is a presentation by CD1 molecules. Indeed, CD1 molecules are known to present amphiphilic membrane lipids from self or foreign sources. Their antigen binding clefts are well suited to these molecules by binding the hydrophobic tail inside the pocket and leaving the hydrophobic head on the outside (Salio and Cerundolo, 2015). Contact allergens such as benzyl benzoate and benzyl cinnamate (constituents of Balsam of Peru) have been shown to interact with CD1a molecule and induce a T cell response (Nicolai et al., 2020). Given their structure, such a mechanism could be hypothesized for QAC.

To present antigens, antigen-presenting cells (APC) need to get activated with a danger signal. For example, the ability to upregulate co-stimulation receptors on monocytes, such as CD86, has been shown on THP-1 for DDAC (Lee et al., 2019). This means that some QAC could induce the maturation of APC through their innate properties and then be presented by the APC. The sensitization phase could also be potentiated through the effect of an irritant. Indeed, irritants can alter the epithelial skin or lung barrier, which can increase the permeability and facilitate the entry of the sensitizer. This is particularly important because QAC themselves have irritant properties and because QAC are often associated with other antimicrobials in products that have irritant effects. On this last point, it is necessary to study the effect on the immune system of the association of QAC with other chemicals as they are found together in commercial products. For example, the combination of DDAC and ethylene glycol has been studied by intratracheal instillation of mice. The results showed that the association of the two compounds induced amplifying and dose-dependent cytotoxicity and inflammation in BALF of the mice treated, compared to the chemicals alone (Kwon et al., 2016). It is also essential to consider the global exposure sequence and not assess the effect of a group of chemicals without considering previous exposure to other substances. Indeed, the immune system could be skewed by a chemical, and the response to another chemical applied after that could be different. For example, the effect of DDAC has been studied in a mice model with prior exposure to ortho-phthalaldehyde, another disinfectant known for its Th-2 sensitizing abilities (Shane et al., 2020). In this model, prior exposure to ortho-phthalaldehyde dramatically increased the IgE response to DDAC and the Th-2 skewing, although DDAC alone is known to induce a Th-1 response (Shane et al., 2020). This also implies that the results of epidemiological studies might have been biased with respect to the overall exposure of the individuals studied.

The majority of the immunotoxicological studies on QAC concern aliphatic and aromatic QAC. Despite the huge number of polyquaterniums used in cosmetics, only few studies have been conducted so far to assess their involvement in hypersensitivities. Among them, studies on quaternium-15, -22 and -39 have been published (Becker et al., 2010; Scientific Committee on Consumer Safety, 2011; Johnson et al., 2016), but data are missing for most of polyquaterniums. Currently, 109 polyquaterniums are registered at CosIng, the European Commission for information on cosmetic substances and ingredients, but many without any CAS of EC number (CosIng—Cosmetics—GROWTH—European Commission, 2022).

Recently, QAC levels were assessed in blood samples of healthy volunteers. In this report, the authors found a detectable level of QAC in 80% of the samples. More interestingly, QAC levels were positively correlated with inflammatory cytokines (IL-6, IL-10 and TNF-α), decreased mitochondrial function, and disruption of cholesterol homeostasis in a dose-dependent manner (Hrubec et al., 2020). However, Vincent et al. (2007) could not detect DDAC in air samplings collected indoors in hospital settings. Indeed, QAC are non- or low-volatile substances and do not likely to contaminate the atmosphere. This indicates that the mode of exposure of QAC could rather be due to an aerosolization procedure, considering the pulmonary route.

Pending further studies, and to reduce the possible harmful effects of QAC, they could be either replaced by other cleaning products, such as green products and methods (steam cleaning, ultraviolet light…), which are less associated with dermal and respiratory health hazards (Garza et al., 2015), or changed in their formulation (avoid spray formulation for example to reduce the aerosolization) (Tarlo et al., 2017). Considering the dermal route, wearing gloves could also be a simple action to reduce the effect of QAC, especially for workers handling QAC.

Interestingly, during the COVID-19 pandemic, QAC has been extensively used. It would be interesting to evaluate the sensitization to QAC before and after the pandemic, and to compare the incidence of dermal and bronchial symptoms, but also to assess whether NMBA anaphylaxis increased during this period, knowing that NMBA have been also much more used than usual due to the need of intubation of patients with severe COVID.

To conclude, accumulated proofs exist concerning the implication of QAC in hypersensitivities. To date, the mechanisms underlying their effects on the immune system suggest that they can either be considered as irritant, sensitizer, or adjuvant according to the studies and the molecules. However, to clearly understand the sensitizing potential, further studies are still expected, in particular to unravel the processes leading to antigenic presentation and the possibility to act as haptens.