Car air conditioners have the dehumidifier built in as part of the system.
From about.com: A system that cools and
dehumidifies air entering the passenger compartment. The system uses a refrigerant to cool the air and carry heat away from the passenger compartment. Major system components include a compressor, condenser, evaporator,
accumulator or receiver/dryer, and orifice tube or expansion valve.
from:
How to Fix Your Car's Air ConditionerBecome familiar with the major components to auto air conditioning:
-the compressor, which compresses and circulates the refrigerant in the system
-the refrigerant, (on modern cars, usually a substance called R-134a older cars have r-12 freon which is becoming increasingly more expensive and hard to find, and also requires a license to handle) which carries the heat
-the condenser, which changes the phase of the refrigerant and expels heat removed from the car
-the expansion valve (or orifice tube in some vehicles), which is somewhat of a nozzle and functions to similtaneously drop the pressure of the refrigerant liquid, meter its flow, and atomize it
-the evaporator, which transfers heat to the refrigerant from the air blown across it, cooling your car
-the
receiver/dryer, which functions as a filter for the refrigerant/oil,
removing moisture and other contaminants
Air conditioning units may or may not have a dehumidifying function built into the system. You certainly wouldn't need to dehumidify the air when living in a desert, and would only want the air cooled. But certainly cold or dry air alone isn't enough to cause headaches. Otherwise people living in dry desert (hot) or tundra (cold) areas with very low humidity wouldn't be able to function, eh? And we'd all be getting headaches all winter long as temperatures and humidity tends to drop naturally.