Our results suggest that in this setting a single air velocity measurement at all openings in a room using a vaneometer is sufficient to assess ventilation in that room through the calculation of ACH. Ventilation assessed with the vaneometer was classified as adequate in most of the rounds. These findings do not compare well with the “20% rule” because both methods agreed in only 56%.
The weather condition had a rather small effect on the measured air velocity and may be due to a difference in the temperature gradient between in- and outside temperature. Because we did not measure outside temperature we cannot verify this. However, the effect was rather small on the estimated air velocity and will probably not affect the ACH. Though different weather conditions may affect the opening of windows and doors compared to the routine working situation, which would affect ACH. Therefore, we recommend assessment of ACH under various weather conditions to verify the ventilation in this different condition.
The finding of (potentially) adequate ventilation in more than 94% (177/189) of the rounds was surprising. We did expect poorer ventilation based on other studies from Africa reporting less than 50% of rooms adequately ventilated, though with a different assessment method [10, 14]. The “20% rule” ventilation assessment of 50% adequately ventilated rooms agreed with another study from Uganda [10]. Deciding on the most appropriate assessment of ventilation systems would require a validation study using for example tracer gases.
We used 12 ACH as cut-off for adequate ventilation. This cut-off recommendation applies to mechanically ventilated airborne precaution rooms [2]. The recommended cut-off for laboratories is 6-12 ACH [4]. No clear recommendations on ACH exist for the other rooms such as TB clinics, OPD consultation and waiting rooms, or wards. In a systematic review, Li et al. did not find evidence for a recommended quantification of ventilation requirements [15]. A study in Canada found an association between general or non-isolation rooms having less than 2 ACH and the conversion of the tuberculin skin test in health care workers [16]. The study did not find an association between skin test conversion and inadequately ventilated isolation rooms for which at the time of the study the cut-off was 6 ACH. If a lower cut-off of more than 6 ACH instead of more than 12 ACH would be acceptable to define adequate ventilation, only one room in one round in Uganda would have inadequate ventilation.
Natural ventilation has been shown to achieve higher ACH than mechanical ventilation [5, 7]. The disadvantage of natural ventilation is its variability in both velocity and direction [17]. However, given the costs of mechanical ventilation systems and the need to maintain these systems, and the weak evidence available for specific recommendations regarding the quantification of ventilations requirements, natural ventilation seems the way forward for resource limited settings. Our study shows that in Uganda natural ventilation provides adequate ventilation in at least 50% (“20% rule”) or 71% (vaneometer) of the facilities and rooms assessed.
Our method is easy and simple to use and provides a rough estimate of the ACH. It will give health care workers an idea whether their place of work is probably safe with regard to ventilation as prevention for air-borne transmission. However, if the assessment needs to be precise because of working with high risk patients such as patients with MDR-TB, then a rough estimate is insufficient.
Health facilities would need practical guidelines to assess ventilation using the vaneometer in their rooms. Based on our findings, not validated by a reference method, we suggest that such practical guidelines could include at least the following items:
A single measurement of air velocity at each opening using a vaneometer and measurements of openings and rooms provides adequate input for the ACH calculation;
If ACH is above 12 the ventilation is deemed adequate;
If the ACH is between 6 and 12, several measurements of air velocity provide insight into the variability of ventilation; if persistently between 6 and 12, opening more openings will probably increase ventilation;
Because of a potential effect of the weather, assessment of the ACH under different weather conditions is necessary;
If opening of more openings is not possible, or the ACH is below 6, then health facility management should consider improving health care worker safety through additional measures for infection prevention and control; and.
Training and support for ventilation assessments: infection control officers could conduct the assessments after a practical training on how to measure air velocity and how to calculate ACH.
Additional measures to reduce the TB transmission risk in rooms with inadequate ventilation assume that all administrative controls are in place [2]. Additional measures include positioning of health care workers such that they would not inhale potentially infected air, and fans to direct airflow out of the room. Construction adaptation such as addition windows to allow cross-ventilation or latticed walls, seem most effective, though not easily implemented [18]. Each situation with inadequate ventilation would need individual assessment on how to improve ventilation in the particular circumstances of that situation. Should all these measures be insufficient to contain the transmission risk health care workers may need to wear particulate respirators. To do that effectively, they need clear instructions on how and when to use these and how to handle the respirators in-between use should the respirators be used more than once [19].
Limitations
This method of ACH calculation assumes perfect mixing of air in the entire room. This may not happen in rooms that have obstacles such as partition walls or patient screens. Imperfect mixing means that some areas in the room are better ventilated than other areas.
A further limitation to this study is that, in common with many resource-constrained settings, we lacked the resources to validate the vaneometer against a reference test for ACH assessment using trace gases [20] or carbon dioxide dilution [5, 7]. Such validation is urgently needed. Until such research is done, our findings should be interpreted cautiously.
We did not measure outside wind speed, which has been shown to influence ACH [7]. Therefore future research should also measure ambient, outside wind speed and test the extent to which this influences vaneometer assessment of natural ventilation ACH. For example, on still days, with little wind, airflow through room openings may be too low to measure with the vaneometer, possibly causing ACH to be under-estimated.
Although the manufacturer instructions for the vaneometer states accuracy to ±10% of the full scale, the reading of vaneometer is not straightforward because of the constant movement of the vane. However, the data collectors were trained and experienced in taking the readings as such minimizing reading variability. This inter-reader variability potentially results in different assessments of the ventilation in a room, and becomes especially important when the resulting ventilation is below 6 ACH. We therefore recommend taking more than one air velocity measurement if the resulting ACH is between 6 and 12.
In addition, the lower detection limit of the vaneometer device may assess ACH insufficiently in situations with low air velocity.
We assessed the area of the open openings only on the first day, which limits the comparison between the ACH assessment with the “20%-rule”. Data collection took place on three consecutive working days, which may have resulted in the same openings being open or closed during all measurements, it would have been better to assess this at reach round of measurements.
Our study does not capture the complexity of ventilation that is influenced by many factors such as in- and outside temperature and surrounding structures. This was on purpose because we wanted to assess ventilation with simple to use tools and methodology which can be used in the many health facilities in settings with limited resources where more complicated ventilation assessment methods are not widely available. Also, the technical expertise to do such assessment is not or limited available. Our proposed method is easy to implement after a short training and provides a reasonable assessment of the ventilation status. Though we consider a single measurement sufficient for assessing ventilation, we do acknowledge that this method needs further validation. This method is probably of less value in situations where good infection control is highly important such as places where patients with MDR-TB receive treatment. However, it can provide an initial assessment that informs policy makers for further requirements.