When I researched this myself, all I found were terrifying articles about something called the “air conditioning lung”. This is a very rare and serious immune reaction to small particles suspended in the air, but according to Dr. Raj Dasgupta, a pulmonologist at the Keck School of Medicine at USC, it's not the only problem caused by a dirty air conditioner. When you don't maintain your air conditioner, it accumulates high concentrations of dust, pollen and mold.
At best, this could aggravate allergies and asthma. In the worst case, it can lead to infections. All parts of the air conditioning system work together to circulate air and heat or cool a room. When air passes through dirty components, the machine consumes more energy and dirty coils don't move cold, clean air efficiently.
This will result in an inefficient HVAC system, costing you hundreds of dollars over time...