The following was written by Hirofumi Miyoshi, et al., Osaka Medical College, Department of Internal Medicine, Osaka, Japan.
Introduction A great number of bacteria and fungus (hereafter referred to as "germs" collectively), which are floating in the air do not infect healthy people under normal conditions. In a hospital, however, patients are subject to opportunistic infection due to various reasons of the patients who have less resistance to germs, and what we call "nosocominal infection" could occur through the air. Therefore, keeping the air in a hospital as clean as possible is one of the important countermeasures to prevent infection. Germ-free rooms are available in most hospitals, but are available to a limited number of patients due to the expensive running cost.

We took the opportunity to install an inexpensive, noise-free, electron-type of air purifier, the SilentAir Air Purifier, into our hospital patients' rooms and tested its effectiveness in terms of the number of germs which fell onto the floor before and after installation of the SilentAir Air Purifier.
Method One SilentAir Air Purifier was installed in the middle area and near the wall of each of the patients' rooms at the height of approximately 1.7 meters. Nine small containers containing culture medium of vegetable gelatin were placed on fixed positions of the floor of each room three days before and after the SilentAir Air Purifier installment, and the fallen germs were counted. The lid of each container was left open for 15 minutes at fixed times. The total number of fallen germs for the three days were divided by the number of containers, and for the statistical evaluation, the Paired-t Test or Chi-Squared Test were applied. Results less than p<0.05 were judged significantly useful.

Test periods were two periods: wintertime (February – March) and summertime (June – July), when the windows and doors in the rooms would remain almost closed because of the heating or air conditioning times of the year. Two rooms were chosen for each period (four rooms in total). Patients were requested to avoid opening windows and doors during these test periods as much as possible.

Furthermore, cut parts (1 cm x 2 cm) of the used Dielectric Collection Sheet were collected to be soaked well in a germ-free physiological salt solution, and were put into a normal culture medium for counting of germs.

Chart 1 depicts the number of germs which had fallen into the containers before and after installation of the SilentAir Air Purifier.

Chart 1
Room Season Heating or A/C in Use Before Installation After Installation
A Winter Heating 23/9 (2.55) 4/9 (0.44)


B Winter Heating 26/9 (2.89) 10/9 (1.67)


C Summer Air Conditioning 39/9 (4.33) 9/9 (1.00)


D Summer Air Conditioning 26/9 (2.89) 15/9 (1.67)


Totals     114/36 (3.16) 38/36 (1.20)

As a result, the total number of germs before installation was 114/36 (3.16) and after installation was 38/36 (1.20), which shows statistical significance (p<0.01 Paired-1 Test). Furthermore, there was not a large difference between each of the rooms with respect to the number of fallen germs before installation.


Chart 2 shows the number of germs that were collected on cut parts of the Dielectric Collection Sheets used.

Chart 2
Room Clean Dielectric Collection Sheet Used Dielectric Collection Sheet
C 0 109


D 0 78


Totals 0 187

We used the SilentAir Air Purifier in our hospital patients' rooms and evaluated its effectiveness in terms of the number of germs in the rooms. The results are shown in Chart 1. Each room (A through D) had similar conditions and no big difference between wintertime and summertime. Indoor airborne germs before installation decreased to about 1/3 (p<0.01) after installation, which suggests SilentAir's significant usefulness.

Also, a large number of germs were found on the Dielectric Collection Sheets after usage, while no germs were found before usage, which leads us to the conclusion that the germs in the patients' rooms, which decreased after installation of SilentAir, were attracted to and collected by the Dielectric Collection Sheets.

The germs collected on the Dielectric Collection Sheets were those with leukemia and terminal cancer, are easily infected by various types of germs transmitted in the air. Keeping rooms as clean as possible is one of the important measures to prevent infectious diseases. This could be achieved by completely germ-free rooms, however, it is nearly impossible to receive all patients in such germ-free rooms due to financial reasons.

Running costs of the SilentAir Air Purifier is very low, in addition to the cost of the unit. Because it has almost no noise, it is beneficial to patients. Furthermore, it can be said that SilentAir has a wide range of clinical applications and has many advantages. For instance, since the machine's dimensions are not big, it can be installed in various places. In our tests, the influences upon patients using an air purifier (decrease of fallen germs in the indoor air) were not studied but this kind of study should be done in the future.


An air purifier, SilentAir was installed in patients' rooms.

After the installation, the number of fallen germs in the air significantly (p<0.01) decreased.

Numerous germs were found on the used Dielectric Collection Sheets, on which no germs were confirmed before use.

The above determinations led to the conclusion that the decrease of fallen germs found in the patients' rooms was made by means of the SilentAir Air Purifier.

The difference between filtering air and purifying it.