The US Centers for Disease Control and Prevention (CDC) has said that 84 health workers are being monitored after they were potentially exposed to anthrax.
A failure to follow proper safety practices caused the possible exposure, the agency explained in a statement, adding that it believed the risk of infection was slightly high and the number of reported cases could increase.
The number of researchers of those at risk of illness has climbed from the 75 initially reported last week.
The US health agency said the staff employees in a high-level biosecurity laboratory failed to inactivate the bacteria.
Anthrax bacteria can produce toxins in the body that cause severe illness or death, but it is not contagious, the CDC said.
“Workers, believing the samples were inactivated, were not wearing adequate personal protective equipment while handling the material,” the agency said.
“The unintentional exposure was discovered June 13 when the original bacterial plates were gathered for disposal and B. anthracis colonies [live bacteria] were found on the plates.”