About 15 years ago, a respiratory condition known as “popcorn lung” emerged.

A number of workers at flavoring and microwave popcorn manufacturing plants developed bronchiolitis obliterans – an irreversible disease – after inhaling diacetyl. The chemical, a yellow-green liquid with a buttery flavor, is used in food and beverages such as popcorn, cookies and alcohol. An individual with popcorn lung experiences airway inflammation and scarring, leading to shortness of breath, coughing and wheezing. Some workers with severe cases were placed on waiting lists for lung transplants.

In 2000, NIOSH investigated the condition in former workers of a microwave popcorn facility in Jasper, MO. The agency conducted research and subsequently issued an alert to help protect potentially exposed employees who work with flavorings.

Focus on the disease shifted to consumers in 2007, when a Denver man was diagnosed with popcorn lung after inhaling microwave popcorn fumes from reportedly eating two or three bags a day for a decade.

Now, popcorn lung-related chemicals have resurfaced in a completely different product – electronic cigarettes.

A study from Harvard T.H. Chan School of Public Health in December found that more than 75 percent of the flavored e-cigarettes and refill liquids that researchers tested contained diacetyl. They also discovered diacetyl substitutes acetoin and 2,3-pentanedione in more than 90 percent and 45 percent of the samples, respectively. The study was published online Dec. 8 in the journal Environmental Health Perspectives.

To examine the flavors, researchers used a lab device that “inhaled” air through e-cigarettes for eight seconds before resting for 15 to 30 seconds. Tested e-cigarettes included flavorings such as candy, fruit and cocktails. Of the 51 tested flavors, 47 had at least one of the three chemicals. Acetoin and 2,3-pentanedione are characterized as “high priority” by the Flavor and Extract Manufacturers Association, meaning they may pose a respiratory hazard to workers.

“What is of concern is that the exposure pathway that led to illness in many workers – namely inhaling heated flavoring chemicals – is the same exposure pathway that is occurring for users of flavored e-cigarettes,” Joseph Allen, lead study author and assistant professor of exposure assessment science at the school, told me.

Currently, e-cigarettes are unregulated, although the Food and Drug Administration has proposed including them under its regulatory authority. Allen noted that workers who can potentially inhale these chemicals receive warnings about the hazards, while consumers of flavored e-cigarettes aren’t obtaining similar warnings. E-cigarettes can be appealing because they don’t produce tobacco smoke and come in popular flavors such as cotton candy and chocolate, but they contain nicotine and other potentially harmful ingredients, according to research.

“Our goal was to be sure that the history of workers with severe lung disease associated with inhaling flavoring chemicals reaches consumers of flavored e-cigarettes so that they can make informed decisions about the products they are using,” Allen said.

OSHA cautions that most types of flavorings haven’t been tested for respiratory toxicity, and workers may be exposed to flavorings in dusts, vapor or spray. The agency has no specific standard for worker exposure to diacetyl or butter flavoring, but it states that its general standards provide protection for workers exposed to these substances in all worksites. OSHA does offer a hazard communication guide for diacetyl.

In July, NIOSH issued best practices related to diacetyl and 2,3-pentanedione. The resources offers information on environmental monitoring, controlling exposures, engineering controls, administrative and work practice controls, and personal protective equipment.