The survey was quick and informal. A Web-based survey link was sent to 175 health-care HVAC design engineers. Names came from our address books and Web searches for certified professionals. Recipients were encouraged to forward the survey link to others. Participants were anonymous and not screened. Seventy-two responses were received.
Use of the ACH convention. In two questions, we asked about the convention of specifying ACH (Figure 2). Respondents fell primarily into two camps. A substantial group (42 percent, 55 percent) said air-change rates are required for infection control. Most of the remaining group (44 percent, 34 percent) said ACH rates are not beneficial in many spaces or could be replaced by more mainstream metrics.
A minority of respondents (14 percent, 11 percent) said ACH specifications enable comfort and ensure good indoor-air quality (IAQ). In other human-occupied spaces, such as offices, schools, and churches, however, comfort and good IAQ are achieved without minimum ACH rates. In those spaces, cubic feet per minute per person, cubic feet per minute per square foot, or both are used.
Some engineers believe dilution ventilation requires ACH rates (in lieu of more mainstream metrics). We interviewed several engineers by phone before and after the survey. Several initially stated dilution ventilation, by nature, requires the use of ACH. This is somewhat concerning, as it potentially reveals a lack of understanding of dilution ventilation. (Confused? See the sidebar at the end of this article, “What Does Volume Have to Do With It?”)
For a large fraction of respondents, the ACH rates are sacred, based on infection control. Undoubtedly, patient safety is uncompromisable and must be the primary concern of hospital designers, but the connection of specific ACH rates to infection control is unclear.5,6 Many of the ventilation rates in the table, authored by consensus committees across decades, are not tied to evidence or objective research. For example, the requirement of six total ACH in X-ray (diagnostic and treatment) rooms can be found as far back as the American Society of Heating and Air-Conditioning Engineers’ (ASHAE’s) 1959 Heating and Ventilating Guide.7 There are no references in today’s standard or ASHRAE Handbook chapter indicating where it came from or on what it was based. One of the most well-known ACH rates, six ACH for patient rooms, was, in fact, based on achieving comfort,8 not infection control.
Infection control. Respondents showed sound understanding of infection-control basics (Figure 3). Through three questions, we tested knowledge of airborne-infection-control basics. Sixty-nine percent correctly answered that airflow has little or no effect on large-droplet transmission.9 Sixty-one percent answered that source control is more important than any ventilation measure in controlling environmental risks.10 Sixty-two percent correctly answered that isolation is the key to airborne disease infection control.9,11 (Within the last group, 72 percent indicated air-change rates are critical in isolation spaces; 28 percent answered they are not.)