The stale and fusty odour you get when turning on your air conditioning comes from bacteria and fungus multiplying in the vents and ducting. Lack of regular use allows condensation to form in the ducting, which stagnates and encourages bacteria and mould.
It can sometimes be the case that the cabin filter (sometimes called a pollen filter) contributes to the foul odours if the moisture build-up within the system reached a point where the filter also became damp.
To resolve the problem, we run a disinfecting vapour through the system for around 20 minutes. Unlike many others, the product we use is designed and manufactured in Germany and independently certified to have cleansing properties.
At the same time, we recommend replacing the cabin filter since these have carbon layers within them that capture and neutralise odours, in the same way charcoal filters water.
Once the system is clean and smell-free, remember that air conditioning does not only cool but also dries the air coming into the car. Therefore, using your air conditioning daily means the dry air blowing through the ducts will stop the condensation from forming again.