Pat Boone is crooning a different tune these days.

He is singing the praises of Living Air, a high-tech air purifier that promises to scrub indoor air clean of everything from fish odors to cigarette smoke.

"You can survive weeks without food, days without water but only minutes without air," Boone says in radio ads. "It just makes sense for your family to breathe fresh air."

Helping consumers breathe easier has become a big business. In the past five years, U.S. sales of air-cleaning systems has shot up 34 percent, to $395 million in 2003, according to a study by market research firm Freedonia Group Inc. That figure is expected to grow 5.4 percent annually and exceed half a billion dollars by 2008, as more companies barrage consumers with messages touting the benefits of pristine air, retail consultants said.

"Fifteen years ago, people thought it was crazy to buy bottled water," said Cynthia R. Cohen, president of Strategic Mindshare, a retail consulting firm in Florida. "There is now consciousness-raising about poor air quality. We've built too many buildings where we cannot open windows."

Much of the competition is focused on high-tech air purifiers, the kind represented by EcoQuest International's Living Air Classic and Sharper Image Corp.'s Ionic Breeze. They sell for about $300 or more, although other manufacturers' models that rely on older filter technology can be had at discount stores such as Wal-Mart for as little as $30.

Few experts expect air purifiers to become the next microwave oven, an appliance that once struck consumers as bizarre but is now found in 90 percent of U.S. households.

Yet for the growing number of families where someone suffers from asthma or allergies, air purifiers may soon be considered a necessity, Cohen said.

Other consumers concerned about healthful living, including many aging baby boomers, already have added an air purifier to their anti-aging arsenal of vitamins, organic vegetables and filtered water.

Some are even wearing miniaturized air purifiers around their necks on airplane trips. Sharper Image sells air cleaners for use in cars and office cubicles.

The case for air purification was bolstered by a study conducted by the Environmental Protection Agency in the mid-1990s. The government agency best known for cleaning up toxic waste sites came to a surprising conclusion: "The air within homes and other buildings can be more seriously polluted than the outdoor air in even the largest and most industrialized cities."

Because people spend about 90 percent of their time indoors, "the risks to health may be greater due to exposure to air pollution indoors than outdoors," the EPA said. The pollution levels are greater in many newer, energy-efficient homes that have less air leakage.

"The EPA is advertising for us better than anyone else," said Shane Cohen, marketing product manager for Oreck Corp. in New Orleans.

Even so, the competition to sell clean air is becoming downright dirty.

The Federal Trade Commission fined several air purifier companies for making claims about their products that are not supported by science.

After Consumer Reports gave Sharper Image's air purifier poor grades in 2002 and 2003, Sharper Image sued the magazine, alleging its findings were false and misleading. It also sued some competitors, alleging they infringed its patents by copying the Ionic Breeze's design or technology too closely.

Oreck says EcoQuest's Living Air purifiers can be risky for people with respiratory problems because their machines use ozone to clean the air. Ozone, an extremely energetic form of oxygen found in the upper atmosphere, can be hazardous when it makes its way into the air we breathe.

EcoQuest says its first-generation purifiers did use ozone but the level was carefully monitored to make sure it stayed within safe parameters. Its second-generation machines use much less ozone.

Michael Jackson, president and founder of EcoQuest, said results speak for themselves. Privately held EcoQuest, which sells its purifiers through a nationwide network of home-based businesses, has sold more than $1 billion of air purifiers, the bulk of them in the past five or six years.

The battle is likely to get uglier.

Sales of Sharper Image's Ionic Breeze purifiers appear to be softening as more competitors enter the lucrative game. That is a blow to the San Francisco high-tech gadget retailer, which relies on purifiers for 40 percent to 50 percent of sales, according to BusinessWeek magazine. Sharper Image says that percentage is too high but declines to be more specific.

In August, Sharper Image reported record second-quarter revenue but warned it expected to post a loss in the third quarter because of slowing sales. It also lowered its earnings outlook for the rest of the year.

A competitive market could lead to a price war, retail experts say, which would hurt manufacturers but benefit consumers. In the meantime, makers are beefing up their products in the hopes of maintaining some pricing power.

In September, Sharper Image launched a new generation of the Ionic Breeze featuring an easier-to-clean collection tube and patented technology that increases air flow around the machine.

In August, EcoQuest acquired a division of RGF Environmental Group, which has a patent pending on an air purification process called radiant catalytic ionization that uses metal ions and low levels of ozone to clean the air.

In the long run, though, the free-standing air purifier may become a thing of the past, retail experts predict.

As houses are built, an increasing number feature air-purification systems tied into the home's heating and ventilation systems.

That is fine with EcoQuest's Jackson. As part of the RGF acquisition, his company has a residential unit, which makes purifiers that tie into ductwork.

"There's no question in my mind a day will come when everyone has an air-purification system in their home," Jackson said. "I wouldn't be in a home without one."

Jaime Garland browses Ionic Breeze purifiers at a Sharper Image in Chicago. In the past five years, U.S. sales of air-cleaning systems has gone up 34 percent.Sales of Sharper Image's Ionic Breeze purifiers appear to be softening as more competitors enter the high-tech air-cleansing market.