When the fastidious folks at NASA studied home air quality, they found that it was five times worse than the air outside. This is why the air in your home is your final allergy-proofing frontier. Start by purchasing anair purifier with a HEPA filter. For a cheaper alternative, fill your home with air-filtering plants. Plants such as ficus, snake plants, and gerbera daisies clean your air by removing carbon monoxide, formaldehyde, and other indoor air pollutants. Ask your local plant store for other suggestions. Lastly, change the filters in your heating and air-conditioning systems and units four times per year.