San Francisco is a neighborhood where homes and businesses live side by side. Mid-rise apartment and office buildings, medical offices, clinics and restaurants sit a few blocks from one another, and every property has its own air conditioning system. It is one of the closest areas: about 3 km by road from the Climatiza office, in an approximate calculation.
Almost all of those systems are switched on daily for many hours, so each duct network works without a rest and struggles to dry out inside. In a mid-sized building, moreover, the air from each branch always reaches the same people, and any buildup shows up among a small group of occupants.
Data point: In an air system that looked clean to the naked eye, 10⁵ to 10⁶ viable bacteria per cm² were counted on the coil fins (Hugenholtz and Fuerst, 1992). It is a case measured in a single system, not a general figure, which is why the camera looks inside before deciding.