Perhaps you've found mold or mildew in the home or apartment. Cleaning does wonders, but a good treatment with ozone will kill the spores so that there is no reoccurrence.
Have you ever had a tenant who smoked? How did you try to explain it away to the next tenant? Maybe by reducing the rent!
Not anymore. Use the A7K Air Ozone Generator. This portable machine gets rid of all odors, including cigarette smoke.
At 7,000 mg / hour, the concentration is strong enough for the toughest odors. Here is a handy guide: