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Total bacteria at 22°C (mean cfu/ml) from weekly testing of filters changed monthly (each n = 18, trial 3)
| Month | cfu/ml | ||||
| week 1 | week 2 | week 3 | week 4 | control before changing | |
| 1st | 12.4 | 13.4 | 23.1 | 14.0 | 540 |
| 2nd | 3.8 | 1.6 | 8.8 | 28.9 | 273 |
| 3rd | 5.6 | 8.8 | 24.7 | 18.1 | 135 |
