2.1. Ventilation

Ventilation moves outdoor air into a building or a room, and distributes the air within the building or room. The general purpose of ventilation in buildings is to provide healthy air for breathing by both diluting the pollutants originating in the building and removing the pollutants from it (Etheridge & Sandberg, 1996; Awbi, 2003).

Building ventilation has three basic elements:

• ventilation rate — the amount of outdoor air that is provided into the space, and the quality of the outdoor air (see Annex D);
• airflow direction — the overall airflow direction in a building, which should be from clean zones to dirty zones; and
• air distribution or airflow pattern — the external air should be delivered to each part of the space in an efficient manner and the airborne pollutants generated in each part of the space should also be removed in an efficient manner.

There are three methods that may be used to ventilate a building: natural, mechanical and hybrid (mixed-mode) ventilation.

2.2. Assessing ventilation performance

Ventilation performance in buildings can be evaluated from the following four aspects, corresponding to the three basic elements of ventilation discussed above.

• Does the system provide sufficient ventilation rate as required?
• Is the overall airflow direction in a building from clean to dirty zones (e.g. isolation rooms or areas of containment, such as a laboratory)?
• How efficient is the system in delivering the outdoor air to each location in the room?
• How efficient is the system in removing the airborne pollutants from each location in the room?

Two overall performance indices are often used. The air exchange efficiency indicates how efficiently the fresh air is being distributed in the room, while the ventilation effectiveness indicates how efficiently the airborne pollutant is being removed from the room. Engineers define the local mean age of air as the average time that the air takes to arrive at the point it first enters the room, and the room mean age of air as the average of the age of air at all points in the room (Etheridge & Sandberg, 1996). The age of air can be measured using tracer gas techniques (Etheridge & Sandberg, 1996).

The air exchange efficiency can be calculated from the air change per hour and the room mean age of air (Etheridge & Sandberg, 1996). For piston-type ventilation, the air exchange efficiency is 100%, while for fully mixing ventilation the air exchange efficiency is 50%. The air exchange efficiency for displacement ventilation is somewhere in between, but for short-circuiting the air exchange efficiency is less than 50%.

Ventilation effectiveness can be evaluated by either measurement or simulation (Etheridge & Sandberg, 1996). In simple terms, the ventilation flow rate can be measured by measuring how quickly injected tracer gas is decayed in a room, or by measuring the air velocity through ventilation openings or air ducts, as well as the flow area. The airflow direction may be visualized by smoke. Computational fluid dynamics and particle image velocimetry techniques allow the air distribution performance in a room to be modelled (Nielsen, 1974; Chen, 1996; Etheridge & Sandberg, 1996).

2.3. Comparison of mechanical and natural ventilation

2.3.2. Natural ventilation

If well installed and maintained, there are several advantages of a natural ventilation system, compared with mechanical ventilation systems.

• Natural ventilation can generally provide a high ventilation rate more economically, due to the use of natural forces and large openings.
• Natural ventilation can be more energy efficient, particularly if heating is not required.
• Well-designed natural ventilation could be used to access higher levels of daylight.

From a technology point of view, natural ventilation may be classified into simple natural ventilation systems and high-tech natural ventilation systems. The latter are computer-controlled, and may be assisted by mechanical ventilation systems (i.e. hybrid or mixed-mode systems). High-tech natural ventilation may have the same limitations as mechanical ventilation systems; however, it also has the benefits of both mechanical and natural ventilation systems.

If properly designed, natural ventilation can be reliable, particularly when combined with a mechanical system using the hybrid (mixed-mode) ventilation principle, although some of these modern natural ventilation systems may be more expensive to construct and design than mechanical systems.

In general, the advantage of natural ventilation is its ability to provide a very high air-change rate at low cost, with a very simple system. Although the air-change rate can vary significantly, buildings with modern natural ventilation systems (that are designed and operated properly) can achieve very high air-change rates by natural forces, which can greatly exceed minimum ventilation requirements.

There are a number of drawbacks to a natural ventilation system.

• Natural ventilation is variable and depends on outside climatic conditions relative to the indoor environment. The two driving forces that generate the airflow rate (i.e. wind and temperature difference) vary stochastically. Natural ventilation may be difficult to control, with airflow being uncomfortably high in some locations and stagnant in others. There is a possibility of having a low air-change rate during certain unfavourable climate conditions.
• There can be difficulty in controlling the airflow direction due to the absence of a well-sustained negative pressure; contamination of corridors and adjacent rooms is therefore a risk.
• Natural ventilation precludes the use of particulate filters. Climate, security and cultural criteria may dictate that windows and vents remain closed; in these circumstances, ventilation rates may be much lower.
• Natural ventilation only works when natural forces are available; when a high ventilation rate is required, the requirement for the availability of natural forces is also correspondingly high.
• Natural ventilation systems often do not work as expected, and normal operation may be interrupted for numerous reasons, including windows or doors not open, equipment failure (if it is a high-tech system), utility service interruption (if it is a high-tech system), poor design, poor maintenance or incorrect management.
• Although the maintenance cost of simple natural ventilation systems can be very low, if a natural ventilation system cannot be installed properly or maintained due to a shortage of funds, its performance can be compromised, causing an increase in the risk of the transmission of airborne pathogens.

These difficulties can be overcome, for example, by using a better design or hybrid (mixed-mode) ventilation. Other possible drawbacks, such as noise, air pollution, insect vectors and security, also need to be considered. Because of these problems, natural ventilation systems may result in the spread of infectious diseases through health-care facilities, instead of being an important tool for infection control.

Table 2.1 summarizes the advantages and disadvantages of ventilation systems for hospitals.

Table 2.1

Summary of advantages and disadvantages of different types of ventilation systems for hospitals.

2.4. Mechanical versus natural ventilation for infection control

The decision whether to use mechanical or natural ventilation for infection control should be based on needs, the availability of the resources and the cost of the system to provide the best control to counteract the risks.

For example, in the United Kingdom, the National Health Service policy tends to limit the adoption of mechanical ventilation to the principal medical treatment areas such as airborne infection isolation rooms, operating theatres and associated rooms. Patient wards are usually not required to be mechanically ventilated and natural ventilation through opening windows is usually the most common solution (Mills, 2004). Mills (2004) also states that “One of the major energy users in hospitals is air treatment. The low-energy hospital study identified this as an area for saving by naturally ventilating all ‘nonclinical’ areas, and current NHS guidance has adopted this conclusion.” Conversely, in the American Society of Heating, Refrigerating and Air-Conditioning Engineers design guide (ASHRAE, 2007a, 2007b) all areas are required to be ventilated mechanically.

Mechanical ventilation is expensive to install and maintain in isolation rooms. It often does not deliver the recommended ventilation rate and may fail to maintain negative pressure (and may even be under positive pressure). For example, Pavelchak et al. (2000) evaluated 140 designated airborne infection isolation rooms in 38 facilities during 1992 to 1998 and found that unwanted directional airflow out of the patient room was observed in 38% of the facilities. Primary factors that were associated with the incorrect operation of the airborne infection isolation rooms included:

• ventilation systems not balanced (54% of failed rooms)
• shared anterooms (14%)
• turbulent airflow patterns (11%)
• automated control system inaccuracies (10%).

In addition, a number of problems related to the use of mechanical ventilation can arise from the lack of active collaboration between medical and technical personnel, which can also occur with natural ventilation. For example (ISIAQ, 2003):

• building repair, without adequate control, may adversely affect nearby areas with high cleanliness requirements;
• sophisticated and expensive ventilation systems are often not properly integrated into the building design, and then maintained, or even used; and
• medical staff often have poor knowledge of the intended operational performance of ventilation systems, even with regard to their protective functions; systems that were originally properly designed can be misused to the extent that the intended functionality is reduced, leading to increased risks.

Other problems with mechanical ventilation include the loss of negative pressure differential in isolation rooms due to the opening of the doors; clogged filters; and adjacent, negatively pressurized spaces (Fraser et al., 1993; Dahl et al., 1996; Sutton et al., 1998; Pavelchak et al., 2001; Rice, Streifel & Vesley, 2001).

In response to the 2003 severe acute respiratory syndrome (SARS) outbreak, the government of Hong Kong SAR constructed 558 SARS isolation rooms with more than 1300 beds in 14 hospitals. The negative pressure, airflow path, air-change rate and local ventilation effectiveness were measured in selected isolation rooms in nine major hospitals (Li et al. 2007). Of the 38 rooms tested, 97% met the recommended negative pressure difference of 2.5 Pa between corridor and anteroom; and 89% of the 38 rooms tested met the same requirement between anteroom and cubicle. Although no leakage of air to the corridor was found, 60% of the toilets/bathrooms were operated under positive pressure. More than 90% of the corridor-anteroom or anteroom-cubicle doors had a bi-directional flow when the door was open. Of the 35 cubicles tested, 26% had an air-change rate less than 12 air changes per hour (ACH).

Most of these problems can also occur with natural ventilation.

A comparative analysis of mechanical and natural ventilation systems looked at eight hospitals in Lima, Peru (Escombe et al., 2007). Five of the hospitals had an “old-fashioned” design (built before 1950) and three had a “modern” design (built from 1970 to 1990). Seventy naturally ventilated clinical rooms for infectious patients were studied. These rooms were compared with 12 mechanically ventilated, negative-pressure respiratory isolation rooms built after 2000. The analysis found that:

• opening windows and doors provided a median ventilation of 28 ACH — more than double the recommended 12 ACH in mechanically ventilated, negative-pressure rooms, but relies on correct door and window operation; none of the rooms were normally operated with windows and doors open; and
• facilities built more than 50 years ago, characterized by large windows and high ceilings (larger values of the volume to patient ratio), with windows and doors open, had greater ventilation than modern, naturally ventilated rooms (40 ACH versus 17 ACH).

However, these results should be used with caution. The ventilation rates in the analysis were reported without detailed information on climatic conditions, such as wind velocity and direction. The ventilation rate measurements were also affected by the carbon dioxide measurement device, and the fact that measurements were taken in buildings with multiple, inter-connected spaces, which would have affected the mixing conditions within the measured interior space.

2.5. Summary

The use of outdoor air for natural ventilation, combined with natural cooling techniques and the use of daylight, have been essential elements of architecture since ancient times and up to the first part of the 20th century (ASHRAE, 2007b). Classical architecture with H, L, T or U-shaped floor plans was used, together with open courts, limited plan depth and maximum windows sizes, to exploit natural ventilation and daylight. In recent times, natural ventilation has been largely replaced by mechanical ventilation systems in high- and middle-income countries. At first, full mechanical heating, ventilation and air-conditioning systems appeared to be able to solve all the practical problems of natural ventilation for year-round control of indoor environmental conditions.

However, mechanical ventilation also requires careful design, strict equipment maintenance, adoption of rigorous standards, and design guidelines that take into consideration all aspects of indoor environmental quality and energy efficiency (ASHRAE, 2007b). The same is also true for high-tech natural ventilation. Natural ventilation is not without its problems, particularly for facilities in countries where winters are cold. More work is needed to design low-cost and reliable ventilation systems for rooms that encourage rather than prevent the flow of air and yet allow internal temperature control.

It follows that natural and mechanical ventilation systems can, in practice, be equally effective for infection control. However, natural ventilation only works when natural forces are available, for example, winds or breezes, and when inlet and exhaust apertures are kept open. On the other hand, the difficulties involved in properly installing and maintaining a mechanical ventilation system may lead to a high concentration of infectious droplet nuclei and ultimately result in an increased risk of disease transmission.

In existing health-care facilities with natural ventilation, this system should be maximized where possible, before considering other ventilation systems. However, this depends on climatic conditions being favourable for its use.