A thick layer of salt crystals lines the walls, and the coarse-ground mineral carpets the floor like sand. A machine known as a halogenerator gives off a soothing hum as it blows microscopic particles of high-grade sodium chloride into the air.

The room’s setting isn’t so much beach as cave. In fact, it is designed to replicate one of Eastern Europe’s salt caves. Over the centuries, the quiet, salt-infused environments of these caves have become famous among Europeans for their purported powers to ease symptoms of a variety of respiratory and skin conditions.

Costa has long suffered from sinus allergies and asthma. Since starting her 45-minute sessions at the salt spa in July, she’s enjoyed so much relief she has been able to scale back her use of decongestants and a corticosteroid inhaler.

“For my first tour of the spa, I walked in, and right away I could taste it in my mouth. It was on my tongue in an invigorating way,” Costa said. “About two weeks after I started coming here I was having terrible allergies, a real bad headache. At the end of the salt treatment, the congestion and pressure had almost dissipated.”

Count Costa as yet another convert to “halotherapy.”

In ancient Greece, Hippocrates told patients with respiratory conditions to inhale salt water steam. Medieval monks treated patients in salt caves. And a 19th-century Polish doctor noted the low rates of respiratory illnesses among salt miners.

A 2006 New England Journal of Medicine study found that inhaling salt-infused vapor improved breathing for 24 patients with cystic fibrosis. Another small 2006 study in the European Journal of Allergy and Clinical Immunology said that people with asthma reported breathing easier after several weeks of regular halotherapy treatments.

Dr. Massoud Mahmoudi, a specialist in immunology and allergies with a private practice in Los Gatos and Los Altos, Calif., said the use of salt water to help clear sinus congestion is “well-established.” He can see how salt therapy could ease symptoms of some upper-respiratory ailments, but said patients should never stop taking medication without talking to their doctors.

In 2011, Spafinder Wellness, a company that tracks the international wellness industry, labeled halotherapy — “halo” is Greek for salt — one of the “top 10 global spa trends to watch.” Since then, the number of spas and hotels offering a salt “microclimate” has increased from a handful to more than 60 across the United States, including in Minnesota, said Beth McGroarty, Spafinder’s director of research.

Scott Wertkin and his wife, Jenni Dorfsman, of St. Paul, wanted a natural remedy after they both had sinus surgery and for their son who grew up with asthma. They tried halotherapy during a trip to Florida and “came back here and wanted to try it and it didn’t exist here.” So they founded the Salt Cave in Minneapolis in July 2012 and recently expanded to add another cave.

“People are talking about it because they are noticing a difference.”

He said they have had as many as 50 people in a single day come in for the 45-minute sessions. He said more people are trying it because they see “that it works.”

Gloria WahrenBrock, owner and founder of the Salt Room in Woodbury, agrees. She opened a year ago this week and said she has seen an uptick in clients, with as many as 40 people visiting in a single day.

“They see that it can be benefit for asthma, allergies, COPD [chronic obstructive pulmonary disease], any kind of respiratory illness,” she said. “We’re starting to see doctors refer people. They won’t write a RX but they say ‘Go and try it out.’ ”

She also became a halotherapy convert after trying it herself during a trip to Florida. She contracted Legionnaires’ disease in the Dominican Republic and struggled to get well. She said she tried one session “and got rid of her inhaler that day. Three more sessions and I was back out playing golf.”

Tradition, as well as some science, backs up salt therapy’s health benefits claim.

John Cumming, 48, of Hercules, Calif., said he has no choice but to rely on nonmedical alternatives to help manage his chronic obstructive pulmonary disease. He has built up tolerance to different antibiotics he’s used over the years to fight off infections that lead to extended bouts with bronchitis and pneumonia. “The salt is just a natural way of cleansing my lungs,” he said.