Besides disinfectants, they are used in other products such as wood preservatives, herbicides, eye drops, mouthwashes, nasal sprays, detergents and shampoos, dryer sheets and fabric softeners.

While QACs demonstrate effectiveness in laboratory settings, their real-world impact on reducing infectious disease transmission, especially in healthcare, is limited.

Despite widespread use, many QACs lack thorough regulatory assessment for potential health and environmental effects. Adverse ecological effects include acute and chronic toxicity to susceptible aquatic organisms, with concentrations of some QACs approaching levels of concern.

QACs show great persistence in indoor environments, which is concerning as it increases the potential for exposure, especially for susceptible populations such as children, and because of the role that QACs may play in antimicrobial resistance. Exposure to QACs can occur through dermal contact, hand-to-mouth transfer, inhalation of contaminated dust or aerosols, and ingestion of food and water. Acute high-level exposure is typically accidental. Exposure to QAC concentrates can cause dermal corrosion and burns, respiratory effects, developmental and reproductive toxicity, disruption of metabolic function such as lipid homeostasis, and impairment of mitochondrial function. QACs have been detected in breast milk, suggesting breastfeeding as a potential exposure route.

Workers in cleaning-related occupations, icluding housekeeping, health care, hospitality, food production, processing and services, face higher exposure. If not wiped off after disinfection, QACs can stay on surfaces, which would lead to postapplication exposure. In the latter case, exposure routes include touching disinfected hard surfaces, unintended hand-to-mouth contact (e.g., ingestion of both surface residues and dust-bound QACs), and dermal absorption of chemicals present on hands after surface-to-hand contact.

Since these products are widel used in hospitals, schools and other facilitites, those regularly visiting, working or living in these settings, e.g. nurses, teachers, students may also have elevated exposures. According to a study, children (age 3 to 14) have 14–55 times higher daily uptake of QACs compared to teenagers and adults due to their more frequent hand-to-mouth contact.

After usage QACs enter the wastewater treatments plants where they continue persisiting. High levels have been measured in sludge and biosolids, but they were also detected is soils irrigated with wastewater. There is limited konwledge about the leveles and persistence of QAcs in soils. From wastewaters QACs can migrate to aquatic ecosystems.

QACs, like benzalkonium chloride (BAC), dissolve membranes at lethal concentrations but, at sublethal levels, favor resistant microorganisms, with spores exhibiting high intrinsic resistance. Pseudomonas, particularly P. aeruginosa, poses a significant challenge due to efflux pumps and QAC tolerance, linked to lipid production and biofilm formation. Salmonella species, relevant in food production, show resistance to unspecified QACs in swine slaughterhouses. Other organisms in the environment enhance survival against disinfectants.

Studies on the impact of QACs on antimicrobial resistance are rare, but exposure of soil microbes to BAC increased resistance genes. Microorganisms may use antimicrobials as a nutrient source. Antimicrobial resistance, exacerbated by QACs, poses a critical problem, especially in drug-resistant pathogens like P. aeruginosa. The post-COVID-19 increase in antibiotic resistance may be linked to the heightened use of QAC-based disinfectants, resulting in resistance to both QACs and antibiotics. Problems associated with tolerance include disinfection failure and cross-resistance to medically relevant antibiotics.

The authors also identified the most pressing knowledge gaps and recommending research and policy actions to address these chemicals of emerging concern. Some examples from the recommendations:

• widen the spectrum of studied QAC compounds and develop new analytical methods
• monitor the presence of QACS in the environment, icluding wastewater and surface waters
• conduct quantitative exposure surveys on QACs, characterize human exposure and carry out human epidemiological and animal toxicity studies
• include QACs on lists of contaminants of emerging concern used for reporting, monitoring, assessment
• assess or re-evaluate the safety and effectiveness of QACs.