US authorities have raised to 84 the number of government workers that have been potentially exposed to live anthrax.

It comes after researchers at a high-level biosecurity laboratory in Atlanta apparently sent live bacteria, instead of what they thought were harmless samples, to fellow scientists in two lower-security labs.

The message to the public is don’t panic.

“The public would be concerned because of publicity, but they should not worry that the disease could be transmitted to them because it’s not transmissible human to human,” said Dr Philip Brachman, a professor at the Emory University Rollins School of Public Health in Atlanta.

“You have to come in contact with animals or you have to inhale it. And a human who is ill with inhalational anthrax does not put out infectious aerosols.”

With no evidence of criminal activity, an investigation is underway into a breach in safety procedures. Vaccines and antibiotics have been offered to the health workers involved.

The incident has raised new concerns about the way laboratories around the world conduct research into the deadliest known pathogens, from anthrax to Ebola and avian flu.