United States health officials are urging people to stop using electronic cigarettes, known as e-cigarettes. The call to action came after such products were linked to health problems. Five deaths have also been reported.

　　The Centers for Disease Control and Prevention, or CDC, said on Friday that officials are investigating severe pulmonary disease cases linked to e-cigarette use. Pulmonary disease is a condition that restricts the flow of air to the lungs.

　　As of September 6, CDC officials had received reports of over 450 possible cases of lung disease tied to e-cigarette use from 33 states and one U.S. territory. The deaths were reported in California, Illinois, Indiana, Minnesota and Oregon.

　　E-cigarettes are a popular substitute for smoking products. The electronic devices heat liquid into vapor -- very small particles that users inhale. Many users vape with a liquid that includes flavoring, nicotine and other chemicals. Nicotine is found in many plants, including tobacco.

　　E-cigarettes or similar devices also can provide THC or other substances to users. THC is the natural chemical present in marijuana plants that gives users the mental state known as a "high."

　　Vaping products are said to be safer than normal cigarettes because they do not create the cancer-causing byproducts of burning tobacco.

　　Yet health officials say e-cigarette users have reported shortness of breath and other breathing problems, such as coughing and chest pain. Some have been sick to the stomach, felt very tired, suffered weight loss or developed high body temperatures.

　　In a statement, the CDC said, "the investigation has not identified any specific substance or e-cigarette product that is linked to all cases." However, many patients have reported using e-cigarettes that contain the chemical tetrahydrocannabinol, or THC.

　　THC-based vape pens are considered illegal under U.S. federal law, but they are legal in some states.

　　In New York, health officials are investigating a substance called Vitamin E acetate. It was found in 13 e-cigarettes collected from eight patients. Vitamin E acetate has been used to thicken marijuana vape juice. The mixture is considered dangerous if heated and inhaled.

　　Also on Friday, The New England Journal of Medicine published reports on e-cigarette-related illnesses in three states: Illinois, Wisconsin and Utah.

　　One report noted that most of the 53 illnesses reported in Illinois and Wisconsin were related to THC vaping products. But nearly 20 percent of the patients inhaled substances like nicotine from e-cigarettes. Most of the reported cases were of young men.

　　The report found that, "Since no single product or substance has been associated with the illness, persons should consider not using e-cigarettes while this investigation is ongoing."

　　Doctor David Christiani is with the Harvard T.H. Chan School of Public Health in Massachusetts. He noted, "Although more investigation is needed to determine the vaping agent or agents responsible, there is clearly an epidemic that begs for an urgent response."

　　Increased use of e-cigarettes

　　Reports of health problems appear to have increased quickly over the past month. Just a week earlier, the CDC reported the number of possible cases at 215.

　　With the latest increase, the CDC's Dana Meaney-Delman told reporters, "We're all wondering if this is new or just newly recognized."

　　A health official from Illinois told the Associated Press she did not know when such illnesses first began. But she added there has been a marked increase since last spring.

　　Last November, the CDC reported that "e-cigarette use increased considerably among U.S. middle and high school students during 2017–2018." Among high school students, it found e-cigarette use rose from 220,000 students in 2011 to 3.05 million in 2018.

　　Last week, Michigan became the first state to ban the sale of vaping products for six months. Governor Gretchen Whitmer told MSNBC, "As governor, I'm going to do it unilaterally until I can get the legislature to adopt a statute and write it into law."

　　She added, "This is too important."

　　On Monday, the Food and Drug Administration, or FDA gave a warning to JUUL Labs, maker of the best-selling e-cigarettes. The FDA accused the company of illegally marketing its products as being "safer than cigarettes" without the agency's approval.

　　In a statement, the FDA's acting commissioner said that, "JUUL has ignored the law, and very concerningly, has made some of these statements in school to our nation's youth."

　　A JUUL spokesman said the company is studying the FDA's comments and "will fully cooperate."

　　I'm Alice Bryant.

　　And I'm Bryan Lynn.

　　Hai Do wrote this story for Learning English with information from the CDC, FDA, the New England Journal of Medicine and the Associated Press. George Grow was the editor.