Employers and legislators must decide whether to include e-cigarettes in smoking bans, according to NPR. Some public health experts say the devices should be included in workplace smoking bans because the health effects are unknown.

The federal government has not yet regulated e-cigarettes, but many cities and states are starting to include the devices in smoke-free laws.

Tim McAfee, Director of the Centers for Disease Control and Prevention’s Office on Smoking and Health, says it is difficult to study the health effects of e-cigarettes because there are so many different types. “We just don’t know what’s in them, and we don’t know how much of what’s in them would get out into the environment — but the assumption would be that it would,” he told NPR.

Some large companies, including Kraft and Walgreen, allow local managers to set rules about e-cigarettes. Exxon Mobil and General Motors allow e-cigarette use in designated smoking areas. CVS Caremark and Lowe’s ban them, while employees of Levi Strauss must go outside to use them. Smaller companies, where bosses can monitor whether e-cigarettes are bothering co-workers, are more likely to allow their use.

Sales of e-cigarettes in the United States are expected to triple this year, to $1.5 billion. Sales are expected to double annually through 2018. By 2028, sales of e-cigarettes could surpass conventional cigarettes. Large tobacco companies, including Altria, Reynolds American and Lorillard, have gotten into the e-cigarette market.