Air conditioners have been linked to an increase in respiratory illnesses, such as colds. According to one 2004 study, women who worked in an office with air conditioning had more absences due to sickness and more frequent visits to ear, nose and throat doctors. Another study found that most employees who worked in air-conditioned offices suffered more illnesses than those who worked in offices with natural ventilation systems.
The overwhelming conclusion of the studies that have been conducted is that the answer lies in germs. If you think about it, this makes perfect sense. Air conditioners circulate air continuously and with that air comes the germs it contains. Air conditioners are essentially circulating germs, mold and bacteria all day long. It is no wonder that a continuous exposure to airborne microbes would lead to an increase in respiratory illnesses.