We maintained all ventilation systems for the 7-Sprong primary school in Ham. This included a complete air handling unit. The interior was completely cleaned and disinfected manually, and the old air filters were replaced with new F7 and M5 filters. The photo clearly shows the difference in the amount of wear and tear an air filter endures during its life cycle. After cleaning, we always perform a visual inspection to determine any contamination (g/m²).