In hot summer day you sit in your car and turn on conditioner. At the same time pushing your nose you begin to open window, because unpleasant smell instead of nice coolness.
Unpleasant conditioner smell is seldom phenomenon though it depends on car owner. The reason of this is formation of bacteria on internal evaporator. As a rule it is the reason of untidy car keeping (there are cigarette ends, dust in the salon) and also problems with draining off of water from evaporator.
Remember that under good system of draining off of water it flows from evaporator while conditioner works. Amount of water depends on air humidity.
If smell appears you should take measures for its elimination immediately. You must buy aerosol killing bacteria which are sold in many auto shops. In any case if you consult to any service center you will be recommended how to get rid of unpleasant smell. The problem is that in neglected cases these measures will not help and there is only one way out: to take evaporator into pieces and its disinfection with the help of chlorinated liquid. But if even chlorine didn’t cope with a problem you will have to change evaporator which is not cheap.
So it will be better and cheaper to keep a car in clearness not smoking and not eating in the salon.