A breakdown in safety procedures has exposed 84 CDC staffers in Atlanta to live anthrax, the agency said.

"Based on most of the potential exposure scenarios, the risk of infection is very low," the agency said in a statement. Nonetheless, the CDC is taking "aggressive steps" to protect their health, including providing antibiotics for those involved.

There is no risk to others, outside experts told MedPage Today. "This is not a transmissible disease," said Gregory Poland, MD, of the Mayo Clinic in Rochester, Minn.

Like tetanus, anthrax causes illness by producing toxins in the body, Poland said, but doesn't then spread from person to person.

Also, immediate prophylaxis, with either antibiotics or a vaccine, is enough to prevent disease, commented Theresa Koehler, PhD, of the University of Texas Health Science Center at Houston.

"This is a very treatable disease," she said. "If exposure is known ... or it's caught very early, it responds very readily to a variety of antibiotics."

CDC press officer Benjamin Haynes told MedPage Today that the agency's occupational health clinic has seen 54 of the 84 exposed workers, although others might have seen their own physicians.

There is no evidence of disease or infection in any of the workers, he said.

Most of the 54 have been given oral antibiotics or anthrax vaccine or both, he said, while two refused antibiotics and 19 did not want the vaccine.

The exposure was discovered June 13, when plates containing Bacillus anthracis -- thought to be inactivated -- were found to contain live bacteria.

The agency said workers in a biosafety level 3 lab intended to inactivate B. anthracis specimens for study in other CDC labs with lower biosafety levels. The aim was to find new ways of means of detecting pathogens in environmental samples.

The CDC statement said the level 3 lab used a "procedure that did not adequately inactivate the samples."

Haynes said the workers used a combination of formic acid and acetonitrile to inactivate the samples, but then observed the results for just 24 hours, instead of the required 48, to ensure the colonies were inactive.

The samples were used for several days in three laboratories not equipped to handle live B. anthracis, where workers, thinking the samples were safe, did not use adequate personal protective equipment.

As well, investigators said two of the three labs might have aerosolized the spores, making it possible for workers to breathe them in.

That's worrisome, Koehler said. "The risk really depends on what form the spores are in," she said. "We're most concerned about inhalation anthrax."

On the other hand, Poland noted, anthrax spores on their own tend to clump together, making it hard to get them deep into the lungs. To engineer them into a weaponized form, he noted, they must be treated with substances that prevent the clumping.

In the absence of such a substance, he said, "the spores are likely to stick to things and not be inhalable."

If any of the workers had been heavily exposed to the aerosolized pathogen, he said, "you'd have seen symptomatic individuals by now."

But a smaller dose might take longer to have an effect, he added.

The CDC said the lab environments have been sampled and the labs and hallway areas were decontaminated, although it gave no details of what method was used.

Such events are relatively uncommon, Koehler said. The CDC itself maintains a registry of labs working with anthrax and there are strict procedures that are regularly monitored.

There's a "little bit of irony" that CDC labs fell down on the job, she said. "But given that humans make errors, sometimes so-called incidents occur."

The agency said "disciplinary action(s) will be taken as necessary" since the safety protocols weren't followed and will review its procedures again with all employees who work in the level 3 lab.