Washington state could become the only U.S. state to raise its legal age for smoking tobacco to 21 from 18 under a measure proposed on Wednesday by its top law enforcement official.

The legislation, if enacted, would also put the legal age for purchasing and possessing tobacco and nicotine-vapor products on an equal footing with the state's minimum drinking age of 21, with the goal of restricting access to teens and lowering healthcare expenses.

The measure, however, would cost the state some $20 million a year in estimated tax revenue.

"Research shows the young adult brain, still developing between 18 and 21, is highly susceptible to nicotine addiction," Attorney General Bob Ferguson said in a statement.

"We must do more to protect our youth from tobacco's grip, and this bill is an important step toward keeping nicotine out of the hands of kids and young adults," he said.

The legislation would bring the cigarette-purchasing age in the Democratic-leaning Pacific Northwest state in line with the threshold for buying recreational marijuana products, which state voters opted to legalize in 2012 for residents at least 21 years old.

Cigarette smoking is the leading cause of preventable death in the United States, accounting for more than 480,000 deaths annually, or one of every five deaths overall, according to the U.S. Centers for Disease Control and Prevention.

Most U.S. states set the legal smoking age at 18, while a handful have set it higher at 19. Some cities and counties, including New York City and Hawaii County, have already raised the smoking age to 21.

The U.S. West as a region has a lower smoking prevalence rate than other parts of the country, at 14.2 percent, according to 2012 census data. The rate in the Midwest stands at 26 percent, the South at 19.7 percent and the Northeast at 16.5 percent, the CDC said.

Ferguson was joined by two Washington state lawmakers, Republican Senator Mark Miloscia and Democratic Representative Tina Orwall, who said they expect opposition from the tobacco industry as well as from some colleagues in the legislature.

Altria Group Inc, which owns Philip Morris USA and R.J. Reynolds Tobacco Company, did not immediately respond to requests for comment.

More than 90 percent of smokers began the habit in their teens, Ferguson said, citing federal data from 2012. He said smoking kills 8,300 Washington residents annually and contributes $2.8 billion in healthcare costs.