According to research, at a relative humidity of 50%, bacteria die after floating for 10min; at a higher or lower relative humidity, most bacteria are still alive even after 24h. At normal temperatures, mold can develop when $varphi=60%$ or above, and when $varphi=80%$ or above, mold will develop regardless of the temperature, as shown in Figures 4.7 and 4.8. It is understood that the relevant Japanese hospital standards require maintaining 50%, while the German standard stipulates that the relative humidity within the entire operating department shall not exceed 65%. The 1999 edition of the U.S. ASHRAE Handbook requires a relative humidity between 45%～55%, whereas the 1991 edition specified between 50%～60%, which is the same as the requirement of the "Guidelines for Construction and Equipment of Hospital and Medical Facilities" published by the American Institute of Architects. However, it relaxes the requirement to 45%～60% for obstetric and surgical operating rooms. Obviously, 50% is the most ideal. However, considering domestic technical conditions, the "Code" sets the relative humidity of Class Ⅰ and Ⅱ operating rooms at 40%～60%, and relaxes it to 35%～60% for Class Ⅲ and Ⅳ operating rooms.

Figure 4.7 Relationship between mold development and temperature and humidity

Figure 4.8 Relationship between relative humidity and bacterial survival rate

For auxiliary rooms, sometimes only the upper limit is specified, and sometimes the lower limit is relaxed. The above-mentioned "Guidelines" also require 30%～60% for recovery rooms, while imposing no requirement on anesthetic gas storage rooms and treatment rooms. Therefore, this clause further relaxes the requirement to 30%～60% for occupied rooms, while for unoccupied rooms only the upper limit is specified. [Reference] The latest versions of foreign standards have all raised the requirements for indoor humidity control. A large body of facts shows that although purification air conditioning can effectively filter out bacteria in the supply air, humidity control throughout the entire clean surgical department must still be emphasized, because as long as there is appropriate moisture, bacteria have a nutrient source and can multiply anywhere and at any time in the system, ultimately causing the failure of the entire control scheme; therefore, the harm of humidity has received great attention. When an independent fresh-air handling unit is installed, emphasis is placed on the final state point of the treatment. Since indoor units in China cannot yet operate under dry-coil conditions, it is hoped that, where conditions permit, the treated fresh air can undertake part of the indoor moisture load.