Keep windows and doors closed. Avoid the urge to air out the house by opening the windows. Use an air conditioner to keep the temperature cool and filter out pollens. You can buy a special high-efficiency particulate accumulator filter called a HEPA filter that will remove most of the pollen from the air.
When driving, use the air conditioner on the recirculate setting. Have the car's air conditioner serviced regularly to remove mold and dust from the system.