Mechanisms for airborne transmission of pathogens
Surprisingly little progress has been made in understanding how pathogens pass from one host to the next. 28 Since the 1930s, four mechanisms of transmission have been described. These are: contact; dust; ‘respiratory droplets’ and ‘droplet nuclei’. There is confusion in the literature regarding the definition of these particles and their mechanisms of spread. For example, ‘contact’ may be used to indicate inhalation of large droplets from contagious individuals when they cough or sneeze, i.e. droplet transmission, but ‘contact’ may also refer to infectious particles transmitted directly from contaminated surfaces. Aside from particle size, the potential for transmission depends upon dynamic factors, such as number of particles produced; velocity at which they are produced; number of micro-organisms contained within the spectrum of droplet sizes; infectious longevity of those microbes; and proximity of a susceptible target…
Natural versus mechanical ventilation
There is some evidence that natural ventilation can be more effective than mechanical systems for preventing transmission. During the 1918 influenza pandemic, sick patients who were accommodated in the open air survived in greater numbers. Eighty years later, during Operation Desert Shield, respiratory tract infections were more Frequent in military personnel in air-conditioned barracks than among those housed in tents.
Background:
Infections caught in buildings are a major global cause of sickness and mortality. Understanding how infections spread is pivotal to public health yet current knowledge of indoor transmission remains poor.
Aim:
To review the roles of natural ventilation and sunlight for controlling infection within healthcare environments.
Methods:
Comprehensive literature search was performed, using electronic and library databases to retrieve English language papers combining infection; risk; pathogen; and mention of ventilation; fresh air; and sunlight. Foreign language articles with English translation were included, with no limit imposed on publication date. Findings: In the past, hospitals were designed with south-facing glazing, cross-ventilation and high ceilings because fresh air and sunlight were thought to reduce infection risk. Historical and recent studies suggest that natural ventilation offers protection from transmission of airborne pathogens. Particle size, dispersal characteristics and trans- mission risk require more work to justify infection control practices concerning airborne pathogens. Sunlight boosts resistance to infection, with older studies suggesting potential roles for surface decontamination.
Conclusions:
Current knowledge of indoor transmission of pathogens is inadequate, partly due to lack of agreed definitions for particle types and mechanisms of spread. There is recent evidence to support historical data on the effects of natural ventilation but virtually none for sunlight. Modern practice of designing healthcare buildings for comfort favours pathogen persistence. As the number of effective antimicrobial agents declines, further work is required to clarify absolute risks from airborne pathogens along with any potential benefits from additional fresh air and sunlight.
