SAN FRANCISCO (CBS SF) — A third of all illicit massage businesses in the United States are located in California, according to a report released by the anti-human trafficking organization, Polaris Project.

Nationwide, the report found at least 9,000 illicit massage businesses — defined as massage parlors that front for commercial sex operations — spread out across all 50 states.

More than 3,300 of those illicit massage businesses are operating in California today.

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San Francisco has at least 181 illicit massage establishments, according to the report. The authors note that San Francisco’s Department of Public Health has brought the number of illicit massage businesses in San Francisco down significantly in  400 to 230 over the last three years.

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San Jose is home to at least 244 illicit massage businesses, according to the report.

But the California counties with the highest number of illicit massage businesses are Los Angeles, Orange and Santa Clara counties, the report found.

The Polaris project says indicators that the businesses is engaging in commercial sex, and potentially human trafficking, include: a locked front door, largely male clientele, covered windows, prices significantly below market-level and the appearance that women are living in the establishment.

The authors insist that “not all commercial sex venues are human trafficking sites — even if they involve massage,” and state that to be considered human trafficking a situation must include force, fraud or coercion.

Nationally, revenue generated from the illicit massage industry is estimated to be around $2.5 billion each year, according to the report.

The report found some illicit businesses are tied to legitimate businesses which are used to launder money earned from the illicit massage businesses.

The report provides a rare glimpse into the massive scale of commercial sex operations that are disguised — often only slightly — as massage businesses.

Perhaps even more importantly, the Polaris report sheds light on the prevalence of  human trafficking related to massage businesses.

In an analysis of more than 32,000 cases of human trafficking reported to the National Human Trafficking Hotline in 2017, trafficking related to massage businesses accounted for 2,949 cases, about nine percent, the report found.

Trafficking related to massage businesses was second in prevalence only to trafficking in the escort service industry, according to the Polaris project.

“There may be women who choose to sell sex either along with or under the guise of massage therapy, but evidence suggests that many of the thousands of women engaging in commercial sex in massage parlors are victims of human trafficking,” the authors of the report write.

The report outlines many legal recommendations to end human trafficking at massage businesses, including discouraging law enforcement agencies from arresting women at the illicit massage businesses and instead connecting them to a network of service providers who offer her ways to gain economic empowerment.

Anyone wishing to report suspected human trafficking in the United States should call the National Human Trafficking Hotline at 1-888-373-7888, go to the National Human Trafficking Hotline website, or call 911 if you or someone you know is in immediate danger.

By Hannah Albarazi – Follow her on Twitter: @hannahalbarazi.