Air vent replacement?

Huntington Beach CA

Description

Hello, I manage a small apartment community of 10 units of very old little houses (like 60 years old)and one tenant believes that his house was infested with mites (like dust mites) through the vents. His doctor could not find any evidence of mites. However, his 2 vents (bathroom and kitchen) are very old and have yellow residue. I agree with having them cleaned with new fans, or replaced with new fans. The duct length is only a few meters because it is a tiny studio apartment with a low ceiling. Is this possible? Does anyone know someone that can help? Here is a picture of the duct as it exits the studio roof at about 7 feet high. There are 2. Thank you