Federal investigators are conducting tests at about a dozen facilities across the country in the wake of a probe that exposed how a naturally occurring chemical endangers coffee workers.

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MILWAUKEE — A warning for coffee workers in roasting factories and corner cafes across the country: Keep your face away from the bins of roasted beans.

That’s the latest word from the U.S. Centers for Disease Control and Prevention (CDC), which is examining the health hazards faced by some 600,000 people nationwide who spend their days roasting, grinding, packaging and serving coffee.

The agency is conducting tests at about a dozen facilities across the country in the wake of a Milwaukee Journal Sentinel investigation that exposed how a naturally occurring chemical endangers coffee workers. The agency’s first test results, from a midsize roasting facility in Wisconsin, found extremely high levels of two lung-destroying chemicals in the roasting bins.

Investigators with the National Institute of Occupational Safety and Health (NIOSH), a research arm of the CDC, spent several days at Madison-based Just Coffee in July. Investigators tested personal air space and took air samples to measure the concentration of the chemicals diacetyl and 2,3-pentanedione. They also evaluated the company’s ventilation and other systems.

Diacetyl has been tied to the deadly lung disease bronchiolitis obliterans, more commonly known as “popcorn lung” for its association with the many illnesses suffered by microwave-popcorn workers in the early 2000s. The chemical’s molecular cousin, 2,3-pentanedione, has shown equal toxicity in animal studies.

Both chemicals have been made synthetically to give a buttery flavor to all kinds of foods and beverages and have been deemed safe to ingest in trace amounts. Inhaling the compounds, however — whether natural or synthetic — can prove deadly.

The two chemicals form when coffee beans are roasted, then are released into the air in greater concentrations when the beans are ground. Levels also build up as the beans “off gas” in the storage bins.

NIOSH Director John Howard said the issue is a priority for the agency. “There’s a large number of workers, and the harm is really severe,” he said.

Last year, the Journal Sentinel hired an industrial hygienist to sample the air in the Just Coffee plant and another Wisconsin roastery. Both agreed to allow the news organization to test for the chemicals.

Results showed levels at both facilities exceeded the government’s safety recommendations, in some cases by nearly four times. Executives at Just Coffee then asked NIOSH do a full health-hazard evaluation to get a better idea of the scope of the problem and understand how to protect workers.

NIOSH researchers found levels in three breathing-zone samples that exceeded the safety levels recommended by the CDC.

The CDC has proposed that workers not be exposed to more than 5 parts per billion of diacetyl over an eight-hour workday, during a 40-hour workweek. Results showed one Just Coffee roaster was exposed to more than 7 parts per billion.

Researchers noted typical exposure levels could be worse, depending on the time of the year. That’s because the testing was done when two large bay doors on each end of the production space were open and accessory fans were being used to increase air flow. Had the bay doors at Just Coffee been closed, the level of contaminants could have been higher.

Scientists familiar with diacetyl and 2,3-pentanedione say workers’ exposures to short blasts of high levels are also of significant concern. NIOSH found diacetyl concentrations inside the bins where beans are stored reached as high as 7,000 parts per billion. Thus the warning for workers to avoid sticking their heads in or hovering over the containers.

Rachel Bailey, a medical officer in NIOSH’s respiratory-health division, noted the results were the first among a dozen studies of coffee-processing facilities being done nationwide. Future results will focus more on specific tasks workers complete and look for links between medical data and exposure sources.