Discussion on using of recirculating air in negative pressure isolation ward: Part 2 of the series of research practice of national standard Technical code on clean nursing and isolation unit

 It is considered that the full outdoor air system is unnecessary in negative pressure isolation ward, while some foreign standards also support that the recirculating air is utilizable when there are high efficiency particulate air (HEPA) filters. Taking class C HEPA filter for example (bacteria filtration efficiency reaches 99.999 997%), if a sneeze produces up to 300 000 bacterial particles per minute and a patient sneezes five times per hour in the ward with air exchange rate of 12 h-1, there are 0.34×10-4 particles/L being transferred to the room when using recirculating air. However, compared with the concentration of bacteria in the patients environment, it can be neglected. In addition, according to several foreign standards, the indoor recirculating air is also allowed after it passes through high efficiency filters.