What is better for a young infant: purified air or "fresh" air with all its pollutants?

Infant's immune system responds to viruses and bacteria by developing an immunity to them.Is it the same thing for pollens, fungus, chemical pollution? Would having an industrial grade air purifier(such as the Oreck) prevent the infant from developing it's immune system? At what age would the air purifier be an advantage over the stimulation of pollutants? Also: is an air purifier a healthy option for adults with sinus, asthma or allergy problems? What purifiers would you recommend? Please list any professional qualifications you have.