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Q: I live in a second-floor condo unit. The woman who lived below me for around 10 years was a heavy smoker. I had a cigarette smell in my unit constantly, and although she moved away nearly 20 years ago, I can still smell it. It's worse in my closets, and it still seeps into clothes stored in drawers. If it is spring or fall when the heat pump fan is not running often, I smell the lingering cigarette odor when I walk in. How can I eradicate this?

Alexandria

A: Cigarette stink lingers long after the smoke has cleared because it penetrates into fabrics, water-based paint and other materials, but isn’t locked in there. For many years, the smelly ingredients gradually seep back into the air or into other objects.

For years, the best advice was to clean or toss out everything absorbent and then to seal the walls, ceiling and floors. To get rid of any remaining odor, people could set out containers of charcoal or other odor-absorbing material. Then ozone treatment came along, and today it’s the go-to solution offered by numerous companies that specialize in cleaning up after fires.

But there are some serious caveats about resorting to ozone, said Brian W. Christman, a professor of medicine at Vanderbilt University who volunteers as a national spokesman for the American Lung Association. “Ozone makes it hard for people with asthma or heart trouble, and at high enough levels, it can bother anyone,” he said, noting that ozone is a key factor in air pollution.

And ozone is not an all-in-one solution, said Mike Calamus, owner of Calamus Enterprises (301-275-5640; cigarette.odorremoval.us ), a company in Rockville that does ozone treatments throughout the Washington area. You have to clean up first. That means removing carpet and carpet pads and scrubbing everything, even the windows. And after the ozone treatment, you have to seal the walls and ceilings. Calamus recommends oil paint for that because it is a one-step solution, but you could instead prime with an oil- or shellac-based primer, and then recoat that with whatever paint you want. Using only a water-based paint won’t work though.

The ozone treatment gets rid of the smelly bits you can’t scrub away. With three oxygen atoms in each molecule, rather than the two oxygen atoms in the air we breathe, ozone is unstable, so its extra oxygen atom can detach and grab onto many other kinds of molecules. If the molecules are stinky ones, they can change to be non-smelly. But if the molecules are inside the lungs of a person or pet, the result can be lung damage, chest pain, coughing and shortness of breath.

Unfortunately, to effectively eliminate odor, the ozone concentration needs to be higher than what is considered safe to breathe, Christman said. The Environmental Protection Agency, which recommends against buying home air-filtering equipment that generates ozone, reviewed numerous scientific studies and concluded that ozone concentrations that do not exceed public health standards have little likelihood of removing odors. But the EPA warned that concentrations high enough to work can cause damage to plants, rubber, electrical wire coatings, fabrics, and the dyes and pigments in some artwork, in addition to causing health problems.

Calamus agrees with the warnings. “This is a serious thing,” he said. “People can’t be around. It would be like breathing bleach.” He warns his customers to remove especially vulnerable items before the treatment. To prevent damage to the plastic coating on electrical wires, trained technicians run an ozone generator for only two to four hours at a setting that is safe for wiring. Then they leave the house closed for two full days, allowing the ozone molecules to do their work. The technicians air out the space before people and pets move back in.

The cost for ozone treatment starts at $600, which generally covers about 1,000 square feet, Calamus said. Numerous other factors can also affect the cost.

Could someone just skip the ozone treatment? “No,” Calamus said. “It’s what takes away that smell. The other things are just helping. If you don’t do ozone, you still have the smell underneath the paint.” But with ozone and the proper prep and follow-up steps, an odor problem can be cured “99 percent of the time,” he said. The exceptions he’s seen involved basements with mildew and other issues.

If you don’t think you need the full odor-removal process, there are a few other things you can try. Consider getting a portable air purifier that does not generate ozone, or upgrade the filter in your heating and air-conditioning system so it works as an air purifier. The level of particulate filtering isn’t relevant for your issue, but do search for equipment that includes a thick filter of activated charcoal or another absorbent material. The EPA’s “Guide to Air Cleaners in the Home, 2nd Edition,” updated over the summer, includes shopping advice. (You can find this document online by searching for the title.)

You can also try to spot-treat places where the odors are most noticeable. For example, paint the closet interiors and the insides of your bureau drawers with a shellac-based primer or clear shellac. (It’s wise to avoid oil finishes in confined spaces because they can add unpleasant odors as they age.) Clean everything before putting it back into these treated spaces.