By Julie Steenhuysen

CHICAGO (Reuters) - U.S. health officials said on Thursday there were now 530 confirmed and probable cases and seven deaths from severe lung-related illnesses tied to vaping, with no signs the outbreak is easing.

That is up from 380 cases reported a week ago as health officials link more illnesses and deaths to vaping. Three-fourths of the cases are male, and two-thirds are between the ages of 18 and 34.

The U.S. Food and Drug Administration is now investigating more than 150 products and substances and said it has activated its criminal investigations arm to explore the supply chain of vaping products and identify the cause of the outbreak. No individual vapers will be targeted, said Mitch Zeller, director of the FDA's Center for Tobacco Products.

Zeller said no single substance or compound, including tetrahydrocannabinol (THC), the high-inducing component of marijuana, or Vitamin E acetate, has been linked to all of the cases so far.

The Centers for Disease Control and Prevention (CDC) has confirmed seven people have died from vaping-related illness. The confirmed deaths were reported in California (2), Illinois, Indiana, Kansas, Minnesota and Oregon.

"We do expect others," Dr. Anne Schuchat, principal deputy director of the CDC, told reporters, referring to the number of deaths.

A Missouri man in his 40s died of a lung illness associated with the use of e-cigarettes earlier this week, the eighth vaping death in the United States and the first death in Missouri, state health officials announced on Thursday afternoon.

Last week, the CDC reduced the number of illnesses under investigation from 450 possible as states began assessing patients based on a narrower case definition issued in late August. At the time, the CDC said it expected cases to rise.

Illinois epidemiologist Dr. Jennifer Layden said on a conference call with reporters that the state has now reported 69 cases, up from 52 a week ago, and continues to get reports of new cases daily.

The state is asking residents to fill out a confidential online survey about their vaping habits to help pinpoint the differences between people who vaped and got sick, and those who did not.

Schuchat said no e-cigarette or vaping product, substance, additive or brand has been consistently identified in all of the cases, nor has any one product or substance been conclusively linked to lung injury in patients.

She repeated the CDC's advice that people should quit vaping if they can. Those who continue should monitor themselves for symptoms such as breathing issues, dry cough or chest pain, and in some cases diarrhoea, vomiting and fever, and should not hesitate to seek help from their doctors.

For people who use nicotine-containing e-cigarettes as a way to quit smoking cigarettes, Schuchat urged people not to return to cigarette smoking. Instead, she urged them to consider counselling or using FDA-approved smoking cessation products.

"The e-cigarette and vaping-related lung injuries are serious. People are dying. We ask you to take these recommendations seriously," she said.

A PIECE OF THE PUZZLE

Zeller said the FDA is analysing samples for a broad range of chemicals, including nicotine, THC and other marijuana ingredients, as well as opioids, cutting agents or diluents and other additives, pesticides, poisons and toxins.

Identifying compounds is only "one piece of the puzzle and will not necessarily answer questions about causality," he said.

He said the agency's criminal investigators have been on the case from the start, but emphasized that they are not pursuing any prosecutions linked from personal use of any controlled substances.

Vaping has only been around for a decade or so, and the sudden outbreak of acute cases has surprised scientists who have been studying the long-term effects of the practice.

"We weren't expecting a big clinical signal because they've only been really popular since 2010 or 2012," said Dr. Laura Crotty Alexander, a lung specialist at the University of California at San Diego who has been studying vaping’s effect on health since 2013.

"Usually it takes decades to cause really significant health effects," she said, noting that it took more than 50 years to detect a health signal from cigarette smoking.

Last week, the American Medical Association (AMA) urged Americans to stop vaping entirely until scientists better understand the cause of the illnesses and deaths.

"There are so many unknowns with this current epidemic, so again out of an abundance of caution, we recommend that everyone avoid the use of these products," AMA president Patrice Harris told Reuters.

The FDA is urging members of the public to submit detailed reports of any unexpected tobacco or e-cigarette-related health issues via its online safety reporting portal: www.safetyreporting.hhs.gov.

(Reporting by Julie Steenhuysen and Brendan O'Brien in Chicago and Gabriella Borter in New York; Editing by Sonya Hepinstall, Matthew Lewis and Tom Brown)