3.1. The association between ventilation and infection

There is little evidence that ventilation directly reduces the risk of disease transmission, but many studies suggest that insufficient ventilation increases disease transmission. A number of studies have looked at the possible transmission routes of diseases, but few have looked at the direct impact of ventilation on disease transmission.

Historically, the concept of airborne spread was first described by Wells (1934, 1955) and then by Riley & O'Grady (1961). The Wells–Riley equation (Riley, Murphy & Riley, 1978) was used to evaluate the effect of ventilation, filtration and other physical processes on transmission through droplet nuclei (Nardell et al., 1991; Fennelly & Nardell, 1998).

Detection of pathogens in room air and buildings may suggest a possible, indirect association between ventilation and disease transmission (Artenstein et al., 1967; Sawyer et al., 1994; Aintablian, Walpita & Sawyer, 1998; Mastorides et al., 1999; Suzuki et al., 2002, 2003; Booth et al., 2005; Chen & Li, 2008; Huynh et al., 2008). However, other aspects (e.g. necessary infecting dose, susceptibility of the host, infectivity of the pathogen, other environmental factors) are important for determining the ability of a pathogen to be transmitted. Therefore, data on presence of pathogens in the air does not provide the full evidence for disease transmission, and should be used in conjunction with other data (e.g. epidemiological data).

To develop this guideline, a systematic review of scientific literature up to June 2008 was used (see Annex A) to answer two questions.

1. Does ventilation rate (measured by air changes per hour — ACH — or flow rate in m³/s) have an effect on decreasing (i) rates of infections or (ii) outbreaks of infectious diseases by agents that are transmitted by each of the modes of transmission listed in Table 3.1, in (a) patients, (b) health-care workers and/or (c) other caregivers such as household members? If yes, what ventilation rate has been associated with each infectious agent?
2. Does airflow or direction have an effect on decreasing (i) rates of infections or (ii) outbreaks of infectious diseases by agents that are transmitted by each of the modes of transmission in (a) patients, (b) health-care workers and/or (c) other caregivers such as household members? If yes, what conditions of airflow or direction have been associated with this?

Table 3.1

The scope and definitions of three transmission models for the systematic review.

The final selected studies (n = 65) (see Annex A for a list of these studies) were included based on an association of ventilation rate or airflow direction with spread of certain infectious diseases. The diseases that showed a possible association between transmission among humans and ventilation were chickenpox (Gustafson et al., 1982), measles (Bloch et al., 1985), smallpox (Wehrle et al., 1970) and pulmonary tuberculosis (TB) (Hutton et al., 1990; Calder et al., 1991; Menzies et al., 2000). In this guideline, these four diseases are referred to as airborne diseases.

There were five main findings of the systematic review.

• Lack of ventilation or low ventilation rates are associated with increased infection rates or outbreaks of airborne diseases.
• High ventilation rates could decrease the risk of infection. For non-isolation rooms, ventilation rates lower than 2 ACH (e.g. equivalent to 13 l/s for a 4 × 2 × 3 m³ room) are associated with higher tuberculin skin test conversion rates among staff. A higher ventilation rate is able to provide a higher dilution capability and consequently reduce the risk of airborne infections. For this reason, better ventilated areas have a lower risk of transmission of TB and other airborne infections. Annex D contains a more detailed explanation of how ventilation rates reduce the transmission of airborne infections.
• No information exists on the impact of ventilation rate on transmission of droplet-transmitted diseases. This agrees with the physics of droplet transmission, which shows that general ventilation should not affect large droplet transmission.
• The airflow from a contaminated source can lead to infection further away from the source. The rate of infection (attack) reduces as the physical distance from the source increases. One of the essential conditions for airflow-induced infection is that the airborne pathogen concentration in the source location must be sufficiently high (either due to high source strength or a low ventilation rate).
• Although there are not enough data to support this, it appears that the airflow from a contaminated source with sufficiently high dilution may not lead to further infection. No information is available on the exact amount of minimum dilution needed.

Despite more than 100 years of ventilation and infection study, the information is still sparse and incomplete. There are insufficient data to estimate minimum ventilation requirements in isolation rooms or in non-isolation areas in hospitals to prevent the spread of airborne infection. There are also insufficient data to estimate the minimum ventilation requirements in schools, offices and other non-hospital buildings to prevent the spread of airborne infection.

3.2. Ventilation requirements relating to airborne infection control

Central to the difficulties in developing ventilation guidelines for infection control is that there are not enough data to recommend a minimum ventilation flow rate for infection control against droplet nuclei. Ventilation can reduce the concentration of airborne pathogens through removing or diluting airborne droplet nuclei. A higher ventilation rate can provide a higher dilution capability and consequently potentially reduce the risk of airborne infections. In line with this assumption, Menzies et al. (2000) found that the tuberculin conversion among clinical personnel was significantly more rapid and frequent among those working in average ventilation lower than 2 ACH. A higher ventilation rate can dilute the contaminated air inside a space more rapidly than a lower ventilation rate, and can also decrease the risk of transmission of infectious droplet nuclei to individuals in the space. However, the maximum ventilation rate (above which there is no further reduction of infection risk) is not known. The choice of the minimum ventilation flow rate may be influenced by the need to reduce energy consumption (because higher ventilation rates have a higher energy cost for mechanical ventilation).

In this guideline, the rationale for determining the minimum ventilation rate requirements is based on two main aspects (see Annex E):

• the effect of air-change rate on decay of droplet nuclei concentration; and
• mathematical modelling of risk using the Wells–Riley equation to estimate the effect of ventilation rate on infection risk for known airborne diseases.

These underlying principles indicate that the higher the ventilation rate, the more rapid the decay of particles (e.g. droplet nuclei) in the room air. Also, according to the Wells–Riley equation, the probability of infection through infectious droplet nuclei is inversely correlated to the ventilation rate. The parameters used in the Wells–Riley equation include ventilation rate, generation of droplet nuclei from the source (quanta/minute) and duration of exposure

where:

P = probability of infection for susceptibles

D = number of disease cases

S = number of susceptibles

I = number of infectors

p = breathing rate per person (m³/s)

q = quantum generation rate by an infected person (quanta/s)

t = total exposure time (s)

Q = outdoor air supply rate (m³/s).

Based on this model, in situations of high quanta production (e.g. high-risk, aerosol-generating procedures), the estimated probability of infection with 15 minutes of exposure in a room with 12 ACH would be below 5% (see Annex E for more details).

When ACH is used to measure ventilation performance, the volume of the enclosed room is clearly an important parameter. For a given ACH, a ward with a larger volume can provide a larger airflow rate (m³/h or l/s) than a room with a smaller volume.

In some existing guidelines of mechanical isolation rooms (CDC, 2003), a minimum negative pressure needs to be maintained while the minimum ventilation rate is ≥12 ACH. As discussed, a major disadvantage of natural ventilation is the difficulty in achieving a consistent airflow direction, and major fluctuations may occur. Although negative pressure is difficult to achieve with natural ventilation, if dilution is sufficient, air being emitted to the open air presents a minimal risk.

Still, the choice of airborne precaution areas and the placement of patients within the areas need to be planned and designed carefully, so as to further reduce the risk of infection for people in the surrounding areas.

Based on the discussions above, the World Health Organization makes the recommendations contained in section 3.3, below.

3.3. World Health Organization recommendations relating to natural ventilation requirements

Please see the explanations for the overall ranking (i.e. strong versus conditional recommendation) of the recommendations in the respective appraisal tables in Annex B.

1. To help prevent airborne infections, adequate ventilation in health-care facilities in all patient-care areas is necessary (Gustafson et al., 1982; Bloch et al., 1985; Hutton et al. 1990; Calder et al. 1991).

Strong recommendation

Remarks: There is moderate evidence available to suggest that insufficient ventilation is associated with an increase of infection risk and favours the use of ventilation for airborne infection control.

2.

For natural ventilation, the following minimum hourly averaged ventilation rates should be provided:

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160 l/s/patient (hourly average ventilation rate) for airborne precaution rooms (with a minimum of 80 l/s/patient) (note that this only applies to new health-care facilities and major renovations);

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60 l/s/patient for general wards and outpatient departments; and

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2.5 l/s/m³ for corridors and other transient spaces without a fixed number of patients; however, when patient care is undertaken in corridors during emergency or other situations, the same ventilation rate requirements for airborne precaution rooms or general wards will apply.

The design must take into account fluctuations in ventilation rate.

When natural ventilation alone cannot satisfy the recommended ventilation requirements, alternative ventilation systems, such as a hybrid (mixed-mode) natural ventilation system, should be considered, and then if that is not enough, mechanical ventilation should be used.

Conditional recommendation

Remarks: The application of natural ventilation depends on climatic conditions being favourable.

3.

When designing naturally ventilated health-care facilities, overall airflow should bring the air from the agent sources to areas where there is sufficient dilution, and preferably to the outdoors (Gustafson et al., 1982; Bloch et al., 1985; Hutton et al. 1990; Calder et al. 1991).

Conditional recommendation

Remarks: Despite some evidence suggesting a possible association of airflow direction with spread of airborne infections, such spread was observed at a very low (lower than 4 ACH) ventilation rate (Bloch et al, 1985). It is hypothesized that if the ventilation rate in adjacent spaces is sufficiently high, the risk would be very low to minimal (e.g. as in an open space). However, the precise ventilation rate required in closed spaces adjacent to airborne precaution rooms to reduce the risk of spread is not known. The application of natural ventilation depends on climatic conditions being favourable.

4.

For spaces where aerosol-generating procedures associated with pathogen transmission are conducted, the natural ventilation requirement should, as a minimum, follow Recommendation 2. Should the agent be airborne, Recommendations 2 and 3 should be followed.

Conditional recommendation

Remarks: There is indirect evidence to show that some aerosol-generating procedures are associated with an increased risk of infection. Ventilation may play a role, but the minimum ventilation requirements for aerosol-generating procedures deserve further investigation.

3.4. Summary

The design of proper, general ventilation systems can play an important role in preventing the spread of infections. Patients with infectious diseases that spread easily through air (e.g. chickenpox, measles, tuberculosis) should be placed in airborne precaution rooms. However, there is often a delay between admission of these patients to the health-care facility, and the diagnosis of their infectious disease. Disease transmission to other patients or staff can occur while these patients are waiting in common areas (e.g. waiting room, emergency departments). Paying more attention to ventilation requirements in these common, non-isolation spaces could lead to significant infection-control benefits.

However, the strategies for disease control and prevention involve the assessment of threats and resources, and then applying appropriate administrative controls, environmental and other engineering controls, and PPE, in conjunction with using a suitable ventilation system.