Healthcare associated infections are a known burden in hospitals as they are linked to longer stay during hospitalization, long term sequela and higher treatment cost. In Switzerland, 7 to 8% of inpatient are diagnosed with a nosocomial disease per year . Part of it is due to airborne infectious disease.
Airborne infectious disease is defined as disease spreading through the air. Often, they are not dangerous for healthy people, however, they represent a risk for immunocompromised people which are more vulnerable to opportunistic infection . The primary source of airborne diseases are patients and healthcare workers. They produce bioaerosols when they breathe, talk, sneeze, and cough. Medical interventions as intubation are also a source of bioaerosols . Those actions create droplets that can either fall to the floor within 1-2 meter, when they are bigger than 5 µm, otherwise they stay into the air for a long period of time and travel through the hospital due to convection movement . However, droplets can evaporate rapidly to form nuclei (< 5 µm) and stay infectious . Many diseases are known to spread by droplets, this includes tuberculosis, chickenpox, measles, flue , SARS but also, methicillin resistant staphylococcus aureus (MRSA). A dozen of micro-organisms need to be in contact with an open wound or a mucous membrane to generate an infection, or they need to be inhaled. They can deposit on surfaces which may lead to contamination by contact . Ambient air is an unfavorable media for microorganisms. Some bacteria form endospores, allowing them to survive and be infectious for years, but they are difficult to detect as they are not cultivable using the standard method .
Not only humans are a source of bacteria: badly maintained ventilators and humidifiers are reservoirs of biofilm-forming bacteria as Legionella, which is a known waterborne disease but can be aerosolized and suspended into the air. Legionella can also be aerosolized from showers and taps. This leads to a higher risk of infection by inhalation and a higher incidence of lethality . Inadequate ventilation and conditioning air systems are responsible for the spread of airborne microorganisms by resuspending and spreading them into the healthcare facilities .
Airborne disease also includes fungi spores, particles having a low settling velocity, meaning a capacity to spread over long distances . Common species of fungi encounter in hospital are Aspergillus, Cladosporium and Penicillium . They are often encountered during construction and repair activities. Inhalation is the most important mechanism of exposure for fungi infection .
Therefore, the environment act as a reservoir of airborne diseases. The bioaerosols can stay in the air for a long period of time, they can deposit on surfaces, and they can survive in dust. The activities inside the hospital are important. Activities such as bed making allow the resuspension and dispersion of micro-organisms . The composition of micro-organisms is similar throughout the whole facilities; however, the concentration depends on the location in the hospital. The bioaerosol concentration tends to be higher in inpatient facilities than in restricted areas and public areas . The concentration depends on the occupational density, the level of moisture of the ward , temperature, pollutants and UV light .
There is no exposure limit, nor standard based on health for bioaerosols and there is only a recommendation from World Health Organization for the ventilation in hospitals .
To fight against airborne diseases some measures must be taken. In the personal levels, it is important to wash the hands, reduce physical contact and wear face masks, plus when visiting an inpatient, the jewelry must be taken off. Sadly, studies have shown that face masks are often not worn properly and are not used enough, even by infected patients to protect others and contain the infections .
When a patient is diagnosed with an airborne infectious disease, the optimal solution is to isolate the patient into a negative pressure room. Yet, most hospitals do not have enough single room to do it and building this type of room is expensive . Healthcare facilities have a majority of multibed room, which is a source of risk of contracting an airborne disease . Therefore, a key component to control airborne infectious disease is the ventilation. The ventilation rate recommended by WHO is at least 288 cubic meter per hour and per person in order to control the transmission of airborne disease in hospitals  and another guideline, more specific for tuberculosis transmission, but it can be applied to all airborne disease, is to have 6 to 12 air changes per hour (ACH) . The first recommendation can be implemented for wards with a known number of persons inside, however it is not applicable for emergency units with a varying number of patients. Those guidelines are not met in all hospital around the world .
The goal of ventilation is to dilute the concentration of airborne microorganisms indoor by injecting outside air and to control their movement . The ventilation can either be natural (window opening) or mechanical with or without enhanced features (directional flow, filtration). In general, natural ventilation seem to be less efficient than mechanical ventilation .
Natural ventilation is climate and location dependent (high surrounding buildings can decrease the ventilation rate) . Hospitals can have simple architectural modifications in low-income countries to increase the natural ventilation rate as it was done in Lima (Peru), the changes allow to have 16 ACH, which is higher than WHO recommendations .
Mechanical ventilation allows to have a steady airflow and to maintain 12 ACH , but it is controlled by the indoor temperature and odors and not by the concentration of microorganisms . It also permits creating negative pressure in wards, so the air would go from clean to contaminated zone . The negative airflow system is used in many hospitals to isolate infected patients of the rest of the facilities, which is efficient according to research made in Italy during the ongoing Covid-19 pandemic . The negative points are its high price, and an unmaintained mechanical ventilation can become a source of contamination .
As seen, hospital air can be contaminated by many airborne infectious diseases, which can stay suspended for hours. However, a well-maintained ventilation allows to lower the concentration of bioaerosols, meaning lowering the risk of being contaminated by airborne infectious diseases.