Keep doors that connect the garage to the house closed.
Draught proof the connecting door to reduce possibility of fumes leaking into the house.
Never run the engine of your car in the garage other than for entering and exiting.
If there are any pipes running from the garage to the house check that they are tightly sealed to ensure that fumes cannot enter the house in this way.
Car:
Vacuum the fabric seats regularly.
Choose a car with good pollen filtration.
Choose a car with least amount of latex and nickel.
Check for certification of 'allergy friendly' interior.
If buying a car with the 'new car' chemical smell. Leave in the sun with the windows open.
Wipe down the interior of the car with a non perfumed cleaner.