This study explored factors associated with the transmission of airborne infectious disease in HCS using a mathematical modelling approach and carbon dioxide as a surrogate for exhaled air. Although it is not the most effective intervention for airborne infectious disease control, natural or mechanical ventilation plays a key role in disease control by reducing (diluting) the concentration of airborne infectious particles expelled by infectors. Mechanical air disinfections, such as negative pressure ionization and ultraviolet germicidal irradiation in healthcare wards, can act as disease control interventions and may replace natural ventilation if installed properly [4, 6]. The WHO recommend a ventilation rate of 12 ACH for HCS, including isolation wards, though in exceptionally hazardous areas, such as autopsy rooms, airflow rates should be increased to 15 - 25 ACH [4].

The transmission of airborne infectious diseases, such as TB, has been noted in different healthcare sections, including autopsy rooms and hospital wards [4, 12]. Several studies have demonstrated the spread of TB infection and disease among HCW, including nurses and medical students. For example, a study conducted in 1924 in Oslo, Norway, by Heimbeck, noted that from 220 tuberculin-negative tests from 420 nursing students on entry into the degree, 210 (95%) became tuberculin-positive by graduation [5, 12]. Furthermore, in his 1946 work, Heimbeck demonstrated that 105 (37%) of 284 nurses who were initially tuberculin-negative became infected and developed TB disease [12]. Based on these findings, he concluded that nurses are at excessively high risk of acquiring TB infection and disease. Apart from Heimbeck’s work, numerous other studies have reported higher rates of TB transmission risk among nurses than the general population, and one of the studies concluded that the risk of developing TB among nurses was 500 times higher than the general population. Another study in 1953 noted that the risk of developing TB for hospital workers in contact with patients, including nurses and technical workers such as doctors, is 8 to 10 times higher than for other workers in the same institution without such contact [12]. Numerous studies since then, including empirical evidence demonstrated by Myers et al [5] also show that HCW, such as nurses and doctors, are at a high risk of acquiring TB infection and developing clinical disease in their working environments. The main reservoir and spread of TB strains among HCW is not clearly understood; however, there are several factors and conditions attributable to TB transmission in HCS. Poor infrastructure is a key factor fostering airborne infectious disease transmission in HCS, particularly in low- and middle-income countries, where healthcare facilities are of poor quality and can accelerate the transmission of infectious diseases.

It is important to note that HCW can be infected from three main sources: the general population to HCW, patients to HCW, and from HCW to HCW. It is assumed that the highest transmission might be HCW to HCW in the tea rooms, which do not have high ventilation and require the removal of PPE, such as masks. Apart from HCW, susceptible individuals, such as HIV infected individuals, children, and the elderly, are at high risk of acquiring airborne infectious disease in the HCS because of sharing environments and thus expired air with patients. For example, due to either overpopulation or limited ward rooms in some HCS, particularly in developing countries, HIV-infected and TB patients are admitted to the same ward and share exhaled air, some of which is derived from infective sources. This is one of the factors fostering the spread of co-infection in HCS and the community at large. Furthermore, since they don’t receive treatment, unidentified and asymptomatic patients are an additional threat in HCS and in the general population [9]. Since they are not yet identified as cases for treatment and interact with people in different locations, this group may be among the unidentified super-spreaders in the community¹¹.

Adequate ventilation, a short duration of exposure, identification, and successful treatment of cases are some of the most effective interventions for disease control in HCS and the community. If identified cases can be isolated and treated with effective chemotherapy, it will have a significant impact on disease control strategies. Separating infectious individuals from vulnerable and susceptible people may help reduce the rising number of new cases. Individuals infected with COVID-19, for example, should be separated from other patients, such as TB cases, because some of these people may have underlined conditions, increasing their risk of infection and developing active disease. The force of infection that accelerates airborne infectious disease transmission in HCS is driven by high disease prevalence and the virulence of infecting pathogen strains, and if cases can be successfully treated, this will potentially reduce the likelihood of new cases.

Competing interests: The authors declare no competing interests.

Acknowledgements: This (publication) was made possible (in part) by a grant from Carnegie Corporation of New York. The statements made and views expressed are solely the responsibility of the author.