(Photo: AP)

CHICAGO, Nov. 12 (Xinhua) -- Chicago, the first big city in the United States to impose a tax on e-cigarettes, announced on Monday to take eight online retailers to court for illegally selling e-cigarettes to underage residents.

E-cigarettes are devices that heat a liquid into an aerosol the user inhales. The liquid usually has nicotine and flavoring in it, and other additives.

"Following the playbook used by Big Tobacco (companies), the manufacturers and sellers of e-cigarettes ... are attempting to lure youth into a harmful addiction with unfair marketing clearly aimed at young people," Chicago Mayor Rahm Emanuel told a press conference.

The eight accused companies, mostly based in Florida and California, sold e-cigarette products directly to Chicago residents under the age of 21 in the past month via the internet, during a joint sting operation conducted by Chicago consumer protection and law enforcement agencies.

Chicago public health official Julie Morita said the city will fight back and continue to focus on reducing youth access and exposure to tobacco products.

The Office of the Surgeon General (OSG), a federal public health agency, released a report in 2016 on e-cigarette use among youth and young adults in the United States.

The OSG report found that more high school students use e-cigarettes than regular cigarettes. The use of e-cigarettes is higher among high school students than adults.

"No matter how it's delivered, nicotine is addictive and harmful for youth and young adults," OSG points out on its website.

Brain development begins during the growth of the fetus in the womb and continues through childhood and to about age 25. Experts believe that nicotine exposure during adolescence and young adulthood can cause addiction and harm the developing brain.

E-cigarette users risk exposing their respiratory systems to other potentially harmful chemicals in e-cigarettes, according to OSG.